Main
Page | Log In | LogOut | Register
My Profile | Active Users | Search

CorvetteforumC3
CorvettesQ-Jet Problems and Tips

lars
Senior
Member

2588 posts [100%]
Lafayette
Colorado

Q-Jet Problems and
Tips

6:04
PM 11/11/2002

Over the years, I've rebuilt and tested several
hundred Q-Jets; I've done over 140 just in the last year.
Doing this, I've seen a lot of problems, issues, and plain
weird things on these carbs, so I thought I'd share a sampling
of the things I've found wrong with these carbs with all of
you. As you rebuild your own carb, some of these issues might
be things for you to watch for.

As always, if any of you have trouble with your Q-Jets, or
if you need any assistance, feel free to drop me a note....

So here we go.... Q-Jet Problems I have seen (and that I
can remember right now...):

Missing Fuel
Filter
Results in: Lack of a fuel filter will allow dirt
and debris to get into the carb. This often unseats the
needle, resulting in severe carb flooding. I have also seen
this cause severe sediment build-up in the float bowl, with
eventual plugging of metering orifices, resulting in poor idle
and other performance problems such as jammed power
piston.
Comments: A lot of Q-Jets have had their filters
removed. People seem to think that the little filter in the
carb must be very restrictive to fuel flow, so they toss it
and install an in-line filter. Eventually, somebody removes
the in-line filter, but never puts the in-carb filter back in.
The in-carb filter is actually very good, and does not produce
a flow restriction.

Missing Fuel Filter Spring
Results in: A missing filter
spring will allow dirt to completely bypass the filter.
results in same problems as a missing fuel
filter.
Comments: These springs are commonly lost during
filter changes. Many people don’t see a purpose in the spring,
so they leave them out. I’ve also heard people say that the
spring makes the filter “block off” fuel flow, so they remove
it. The spring is essential for proper filter operation.

Stripped Fuel Inlet Threads
Results in: Inability to
tighten the inlet fitting with resulting fuel dripping/leaking
onto the intake manifold.
Comments: Very common on the
pre-’75 carbs with the smaller diameter inlet fittings. I’ve
seen people try to epoxy the fittings in place. This never
works. There are also over-sized self-tapping fittings
available that can solve the problem temporarily, but these
fittings will also strip out. There are O-Ring type adapters
that can be used to seal up the inlet system, but these
fittings do not allow use of the in-carb fuel filter. The only
way to properly repair this problem is to have the inlet
machined and tapped for a HeliCoil thread insert.

Damaged or Broken Fuel Inlet Fitting
Results in: Fuel
leaking at carb inlet. Fitting cannot be tightened enough to
stop leakage.
Comments: It is actually possible for the big
1” Hex fitting at the carb inlet to break internally. The
cone-shaped inner seal surface breaks loose from the wall of
the fitting, making the fitting appear to be a 2-piece
assembly. It is also common for the cone-shaped inner seaing
surface to be gouged, scratched, or damaged, making it
impossible for the flared tube end to seal against it. Replace
this fitting if there is any damage.

Missing Inlet Fitting Seal
Results in: Fuel leaking at
carb inlet. Fitting cannot be tightened enough to stop
leakage.
Comments: The early Q-Jets use a black,
rubber-coated metal seal ring up against the hex nut part of
the big inlet fitting as a fitting seal. Later Q-Jets use a
white plastic seal ring at the end of the fitting threads as a
fitting seal. These seals – not the fitting threads – seal the
fitting. There is no point in using Teflon Pipe Tape on the
threads to seal the fitting if these seals are missing.

Stripped Inlet Seat Threads
Results in: A loose/stripped
seat in the float bowl will allow fuel to leak by the
needle/seat assembly. Mild to severe flooding can result. Poor
idle. Poor hot-start.
Comments: Degradation and stripping
of the seat threads is common in the older Q-Jets – especially
the ones that have been commercially rebuilt and sandblasted.
I often see carbs with the seat epoxied into the stripped out
float bowl. Epoxy will not hold the seat in the bowl for very
long. The problem can only be fixed by machining and tapping
the seat inlet threads for a HeliCoil insert.

Damaged/Leaking Needle/Seat
Results in: Mild to severe
flooding and rich-running conditions. Poor idle. Idle mixture
screws will be non-responsive. Hard to hot-start.
Comments:
It doesn’t take much to cause enough damage to a needle/seat
assembly to make it leak: A piece of debris passing through or
damaged caused during a previous rebuild process can easily
make the assembly leak. Replace the assembly anytime the carb
exhibits flooding or fuel control problems.

Needle Retaining Clip Incorrectly Installed to Float
Arm
Results in: Flooding, poor idle, poor hot-start, fuel
puddling in bottom of intake after shut-down.
Comments:
This is one of the most common assembly problems I see on
Q-Jets. GM actually issued a Service Bulletin on this subject
back in the late ‘60s because GM technicians were doing the
same mistake. There is a small retaining wire on the needle.
The float arm has two holes in it where the needle interfaces.
People think the retainer goes through the float arm holes.
Fact is, the retainer slips over the rear edge of the float
arm, and must not be installed through the holes. Installing
the retainer through the float arm holes results in the needle
and/or float jamming.

Float Level Too Low
Results in: Hesitations, sluggish
performance, poor idle mixture screw response, surging at
cruise, “flat” feeling going to wide open
throttle.
Comments: Different year Q-Jets have different
float level requirements. The early carbs all have a factory
spec of ¼”. The late carbs have much lower specs at about
.420”. Setting up an early carb to the late spec is a common
mistake, and will cause performance problems. Personally, I
prefer setting up the early carbs to a float level of .275” to
.375”. I run the late model carbs at .400” - .410”.

Float Level Too High
Results in: Flooding, fuel coming
out of accelerator pump shaft hole, leaking air horn gasket,
fuel discharging out main discharge nozzles at idle, fuel
dribbling down venturi after engine shut-down, poor hot-start,
hesitations off idle, poor fuel economy.
Comments: See
comments above for Float Level Too Low. Often, late model
carbs are set up to early model specs. This results in poor
performance on the post-’75 carbs.

Incorrect Float Installed
Results in: Flooding, fuel
coming out of accelerator pump shaft hole, leaking air horn
gasket, fuel discharging out main discharge nozzles at idle,
fuel dribbling down venturi after engine shut-down, poor
hot-start, hesitations off idle, poor fuel
economy.
Comments: Over the years, several different floats
were used on the Q-Jets. These floats had different geometry
with differing float arm lengths and different float lengths.
Some floats can be interchanged, but this will result in
binding, sticking and improper needle control. I also see a
lot of aftermarket brass floats used in Q-Jets. The original
carbs came from the factory with NitroFill floats. Brass
floats do not behave the same. Be sure to install a correct
NitroFill float when you work on these carbs. Echlin makes an
exact reproduction and correct replacement float.

Fuel-Logged Float
Results in: Flooding, fuel coming out
of accelerator pump shaft hole, leaking air horn gasket, fuel
discharging out main discharge nozzles at idle, fuel dribbling
down venturi after engine shut-down, poor hot-start,
hesitations off idle, poor fuel economy.
Comments: Older
NitroFill floats can actually become fuel soaked and start to
float lower than intended. This has the same effect as raising
the float level. Replace the float any time you rebuild the
carb.

Float Binding Against Power Piston Tower
Results in:
Flooding, fuel coming out of accelerator pump shaft hole,
leaking air horn gasket, fuel discharging out main discharge
nozzles at idle, fuel dribbling down venturi after engine
shut-down, poor hot-start, hesitations off idle, poor fuel
economy.
Comments: During rebuilding, it is easy to bump
and bend the arms on the float. If the arms are squeezed
together, the float arms can rub against the power piston
tower, and this will restrict float movement. In severe
conditions, it will prevent the needle from ever seating,
resulting in severe flooding. It is also possible to bend the
float arms such that the float itself rubs against the walls
of the float bowl. This causes the same problem.

Incorrect Main Jets
Results in: Hesitations, sags, poor
performance, ineffective idle mixture screws, poor idle,
surging, poor mileage, poor vacuum, sooting out the tailpipes,
poor starting characteristics, stumbles, poor WOT
power.
Comments: “Creatively jetted” Q-Jets are more the
norm than the exception. Be sure you know the stock and
correct jetting configuration for your carb before you ever
begin chasing problems. Always start with the correct stock
jetting setup before you start tuning, and keep changes
conservative: The boys and girls who designed these things in
Detroit put a lot of work into the jetting configuration on
these carbs, and they actually knew what they were doing…

Incorrect Primary Metering Rods
Results in: Poor idle,
rich idle, ineffective idle mixture screws, surging at cruise,
poor mileage, poor cold-running characteristics, hesitations
off idle and/or at cruise, poor manifold vacuum with resultant
ancillary problems.
Comments: See comments for main jetting
above.

Incorrect Secondary Metering Rods
Results in: Poor WOT
performance, sags or flat spots going into the secondary side,
car “falling on its face” somewhere in the secondary range,
smoke at WOT.
Comments: Since the secondary rods are so
easy to change, they are frequently used as the “primary
tuning tool” on a Q-Jet. This results in some pretty strange
rods being installed. Make sure you know which rods are stock
for your application, and make changes in small increments
from there.

Incorrect Power Piston Spring
Results in: Poor idle,
rich idle, ineffective idle mixture screws, hesitations and
stumbles.
Comments: At some time, there must have been a
popular article published that told people to cut their power
piston springs to hop up their Q-Jet: I see more cut springs
that you can imagine. A cut, or soft, spring will keep the
power piston seated in the full lean position during
part-throttle power, resulting in a sag and poor throttle
“feel” during part-throttle acceleration. In contrast, if a
stiff spring has been installed, the piston will never seat,
and the carb will be running in a full-rich condition even at
idle. This causes incredible tuning problems for idle and
off-idle performance, and the idle mixture screws will have
little effect. New power piston springs are available in packs
of 10. If you suspect the spring to be non-original, stick in
a new one to eliminate this as a problem.

Jammed Power Piston
Results in: Poor idle, rich idle,
ineffective idle mixture screws, hesitations and
stumbles.
Comments: The power piston can be jammed in
either the full lean or full rich position, producing the same
symptoms as an incorrect spring. You can test the power piston
with the engine “off” by inserting a thin, long screwdriver
down the bowl vent in the air horn. You should be able to
depress the power piston and feel it pop back up. A jammed
piston is usually caused by dirt entering the bowl, but can
also be caused by carbon sooting up through the intake
manifold from bad valves or valve timing problems.

Bent Primary Metering Rod Hanger Arms
Results in:
Flooding, poor idle, hesitations, poor throttle response, poor
fuel economy, ineffective idle mixture screws, sooting out the
tailpipe, plug fouling.
Comments: If the primary metering
rods were not engaged properly into the main jets before the
air horn was installed to the fuel bowl, the rods will bend
and this will also bend the piston hanger arms. If the arms
are not bent back to their correct and original position, the
primary metering rods will be pulled up out of the jets into
the full-rich position, if they engage in the jets at all.

Bent Primary Metering Rods
Results in: Flooding, poor
idle, hesitations, poor throttle response, poor fuel economy,
ineffective idle mixture screws, sooting out the tailpipe,
plug fouling.
Comments: If the primary metering rods were
not engaged properly into the main jets before the air horn
was installed to the fuel bowl, the rods will bend and fail to
engage into the main jets. This causes a severe rich
condition.

Cut Power Piston Lower Stop Pin
Results in: Hesitations,
stumbles, poor idle, surging at cruise, ineffective idle
mixture screws, hot running.
Comments: Many (not all)
Q-Jets have a lower stop pin in the power piston. This pin
rests against a cam in the throttle plate, and determines the
lowest (leanest) position for the power piston. Cutting this
pin results in the primary metering rods seating too deeply
into the main jets, severely restricting fuel flow. Again,
there must have been an article published at some time telling
people to do this…

Incorrectly Set Power Piston Stop Height
Results in:
Poor idle, rich idle, ineffective idle mixture screws,
hesitations and stumbles.
Comments: Post-’75 Q-Jets have an
adjustable power piston stop that determines the lowest
(leanest) position of the piston. Raising or lowering this
stop outside its limits will adversely affect all performance
parameters of the power enrichment curcuit.

Incorrectly Adjusted Secondary Rod Hanger Height
Results
in: Lean secondary performance, flat spot going into the
secondaries, poor WOT power.
Comments: It is common for the
secondary rod hanger to not pull the rods up out of the
metering holes adequately. This produces a lean condition with
associated poor power. The distance from the top of the choke
rear airhorn wall to the center of the rod holes with the
secondary airvalve in the wide open position should be 41/64”.

Incorrectly Adjusted Secondary Airvalve
Results in: Car
falls on its face when the gas pedal is pushed to the floor,
sag going into the secondaries, flat spot going into the
secondaries, jerk or delay going into the
secondaries.
Comments: Very common problem. The secondary
airvalve is adjusted using the slotted head screw at the
secondary airvalve lever, and this is released using the allen
head screw up underneath the airhorn. Spring windup should be
½ to ¾ turn.

Jammed secondary airvalve
Results in: Poor WOT power and
performance, car falls on its face going into the secondaries,
flooding going into the secondaries.
Comments: It is a
common problem on the Q-Jet for the two back screws in the
airhorn to have been tightened so much that the airhorn is
distorted in this area. This distortion will jam the secondary
airvalves, preventing them from opening. When the secondary
throttle plates are mechanically opened (pushing the gas pedal
to WOT) and the airvalve is jammed closed, manifold vacuum
will suck fuel right out of the secondary discharge nozzles
with no airflow going through the secondary side. To fix this,
the carb must be removed, and the displaced metal must be
filed away. Often, the secondary airvalve plates must be
loosened and re-aligned to assure a bind-free operation.

Incorrect, Missing or Damaged Secondary Airvalve
Rod
Results in: Car falls on its face when the gas pedal is
pushed to the floor, sag going into the secondaries, flat spot
going into the secondaries, jerk or delay going into the
secondaries, flooding during cold-start, erratic fast
idle.
Comments: The secondary airvalve rod is one of the
most misunderstood parts on a Q-Jet carb. Owners see that the
rod is holding the secondary airvalve closed when the engine
is running, so the rod is either removed or bent. The next guy
who builds the carb installs an incorrect rod, which really
screws up the secondary airvalve operation. On early Q-Jets,
this rod also operates the choke vacuum break system, so
removal or bending of the rod prevents the choke from cracking
open during cold-start. This causes flooding and rich running
in cold weather operations.

Cracked or Warped Air Horn
Results in: Fuel leaking
around air horn gasket, poor idle, ineffective idle mixture
screws, erratic idle, fuel discharging out main discharge
nozzles at idle, hesitations & stumbles, poor throttle
response.
Comments: Very common on older Q-Jets: The two
forward carb hold-down bolts have been tightened so tight that
the entire top of the carb is warped or cracked. Once this
happens, the airhorn no longer seals properly to the float
bowl of the carb. This causes leaks in the idle transfer fuel
circuit between the airhorn and the bowl, effectively
eliminating the entire idle circuit in the carb. It also
causes the gasket to be ineffective in sealing the rest of the
airhorn to the bowl, resulting in fuel leakage around the top
of the carb. No fix for this – buy a new carb.

Wrong Air Horn Gasket
Results in: Poor idle, ineffective
idle mixture screws, erratic idle, fuel discharging out main
discharge nozzles at idle, hesitations & stumbles, poor
throttle response.
Comments: Use of the incorrect airhorn
gasket can result in blockage of idle bleed air passage and
blockage of idle fuel passages between the float bowl and the
airhorn. There are several styles of airhorn gaskets, and many
of them look very similar. It’s a good idea to lay the airhorn
gasket you’re going to use onto the float bowl and visually
verify that all the holes line up.

Plugged accelerator pump transfer holes in air
horn
Results in: Off-idle stumble or hesitation, hard
cold-start.
Comments: There is a tiny transfer hole that
runs horizontally in the air horn, just inside of the actual
accelerator pump discharge orifice. It is common for this to
be plugged with some type of debris, especially in carbs that
have been sitting around for a while. If this passage is
plugged, there will not be any accelerator pump shot.

Jammed accelerator pump check ball
Results in: Off-idle
stumble or hesitation, hard cold-start.
Comments: This is a
common problem in carbs that have been allowed to “dry out”
for a while. Sediment in the bottom of the fuel bowl will lock
up the check ball as if it were set in concrete. This will
prevent any accelerator pump discharge.

Accelerator Pump Rod Installed in Wrong Hole in Pump
Arm
Results in: Off-idle stumble or
hesitation.
Comments: There are two holes in the
accelerator pump lever arm for the lever arm rod to engage
into: Inner and Outer. The outer hole produces a leaner pump
shot, and can cause a lean stumble on engines requiring a
robust pump shot. Corvettes and performance cars always used
the rich, inner pump arm hole.

Jammed accelerator pump
Results in: Off-idle stumble or
hesitation, hard cold-start.
Comments: This is becoming a
common problem. The alcohol additives in modern fuels are not
compatible with the materials used in many accelerator pumps
(even some of the pumps in brand new carb kits). This causes
the pump plungers to swell up and to seize in the pump bore.
The spring on the pump shaft still allows the shaft to move up
and down, making it look as if the pump is functioning. But
the pump itself can be seized up solid in the bore with the
shaft working just fine. To fix this, you must install a pump
that is specifically compatible with alcohol.

Worn accelerator pump
Results in: Off-idle stumble or
hesitation, hard cold-start.
Comments: Common on older
carbs and on carbs that have been allowed to dry out (vehicles
stored without being started through the entire winter
season). The rubber plunger material will dry out and shrivel
up, making the accelerator pump completely ineffective. The
pump must be replaced.

Missing idle bleed restrictors
Results in: Poor idle,
ineffective idle mixture screws, off-idle hesitations, very
high idle required to keep engine from dying when placed in
“drive,” fuel discharging out main discharge nozzles at
idle.
Comments: Different Q-Jets have different idle bleed
calibrations. Part of this calibration is the installation of
some brass restrictor orifices in the venturi area/booster
ring area. Many commercial rebuilders will actually remove the
orifices during rebuild, causing too much air to be pulled
into the idle fuel circuit. The engine will then only idle if
the idle speed is run up into the transition & cruise
metering circuit. This is a difficult problem to diagnose and
correct, since some Q-Jets did not use the restrictors, and
replacement restrictors are not serviced separately.

Emulsion tubes fallen out of air horn and laying in float
bowl
Results in: Poor idle, hesitations, odd &
inefficient performance.
Comments: It is very common on
older and high-mileage Q-Jets for the air horn emulsion tubes
to fall out and end up in the bottom of the float bowl. These
tubes assist in the emulsifying of the fuel mixture, and will
cause a notable decrease in carb performance when they fall
out. The can be re-installed into the air horn and tapped into
their fully-seated position with a small plastic mallet.

Plugged idle air bleed transfer holes in float
bowl
Results in: Poor idle, idle mixture screws
ineffective, fuel discharging out main discharge nozzles at
idle, erratic idle, high idle required to keep engine running
(engine dies when idle speed is decreased).
Comments: The
Q-Jet has supplementary idle air bleed holes in the throttle
plate just below the primary butterfly plates. These bleed
holes assure that the engine is allowed to pull enough air at
idle to keep it running, while keeping the throttle plates
closed far enough to keep fuel discharging through the idle
metering circuit. The bleed holes act like the “drilling holes
in the butterflies” trick that so many people advocate. The
air for these holes is transferred from the venturi area of
the carb through two small holes in the outer plenums of the
float bowl. For some reason, it is very common for commercial
carb builders to plug these transfer holes with aluminum plugs
or rivets. This dramatically changes the idle characteristics
of the carb, and frequently destroys the entire idle and
cruise mixture control. If you see these plugs installed,
drill them out and set up your carb the way it should be.

Leaking well plugs
Results in: Hard starting after the
car has been sitting for a day or two. Flooding and black
smoke when hot-starting.
Comments: This is a grossly
over-rated problem. It is not by far as common as some
articles and publications would have you believe. The Q-Jet
uses soft metal plugs to seal off the production drill
passages in the bottom of the float bowl. The passages are
drilled and plugged under the main metering jets and under the
secondary metering orifices, usually referred to as the
primary and secondary well plugs. The primary well plugs are
visible from the bottom of a fully assembled carb. The
secondary well plugs are only visible and accessible once the
throttle plate is removed from the float bowl. If these plugs
leak, the float bowl will run empty after sitting overnight,
and all the fuel will end up inside the intake manifold. You
can test for this condition by simply removing the carb,
filling the float bowl with fuel, and observing the plugs to
see if they leak. It would be a rare condition if you actually
have a leaker. But if the primary plugs leak, the only
permanent solution to the problem is to drill the plugs out,
tap the passages for a #10-32 screw, and install a ¼” long
allen-head countersunk screw with some good epoxy into each of
the two passages. I see carbs with the plugs coated with
epoxy: Simply smearing some epoxy onto the well plugs will not
seal them permanently. You have to drill, tap, and plug them
with the epoxied screws if you want the repair to last. The
secondary well plugs can be sealed by using the seal gasket
supplied in all of the Echlin brand carb kits.

Wrong throttle plate gasket
Results in: Poor idle,
erratic idle, ineffective idle mixture screws, symptoms of a
vacuum leak.
Comments: There are several different designs
for the throttle plate gasket, and they do not interchange.
Two common problems occur: The first problem was addressed in
a GM Service Bulletin around 1971. There is a difference in
the open area in the gaskets just forward of the centerline of
the primary throttle holes. Some later carbs use a gasket with
a larger open hole in this area. If this gasket is used on an
earlier carb, you will end up with a massive, undetectable
vacuum leak. The other problems with these gaskets occur due
to the idle fuel and vacuum bleed holes not lining up from one
design to the next. Use of the incorrect gasket can result in
blocked idle fuel and blocked vacuum signals. Always lay the
gasket onto the float bowl and onto the throttle plate to
check the hole alignments.

Loose throttle plate
Results in: Erratic idle, poor idle
mixture screw response, off-idle hesitations, symptoms of a
vacuum leak.
Comments: Since the throttle plate attach
screws are located in the bottom of the carb, you cannot tell
that the screws are loose until the carb is removed from the
engine. It is quite common for the screws to be loose,
producing not only a vacuum leak around the base of the fuel
bowl, but violating the seal for the idle fuel transfer
passages. This makes adjustment of the idle mixture and idle
speed almost impossible.

Wrong idle mixture screws
Results in: Poor idle screw
mixture response, poor idle, erratic idle.
Comments: There
have been several styles of idle mixture screws used in the
Q-Jets over the years. The screws differ in the taper of their
tips: Some screws have a steep taper, while others are very
long and slender. Commercial rebuilders typically remove the
nice factory screws and install aftermarket one-size-fits-all
steep-taper screws. These steep-taper screws often do not work
at all in many Q-Jets. Most of these steep-taper screws are
brass. All factory Q-Jet idle mixture screws are steel. If you
have a set of brass idle mixture screws in your carb, trash
them and find a correct set of factory screws.

Seized/rusted idle mixture screws
Results in: Adjustment
of idle mixture not possible.
Comments: A lot of the cars
used in the Midwestern and Coastal States have severe
corrosion problems, as we know. The steel mixture screws in
the aluminum throttle plate promote dissimilar metals
corrosion, and can often seize solid into the throttle plate.
Attempts to force them out will usually result in the screws
snapping off in the throttle plate, rendering the carb
useless. To remove rusted and seized mixture screws, the
throttle plate must be removed from the carb, and the screws
must be carefully heated while “rocking” them back and forth
to loosen them up.

Seized/rusted fast idle screw & cam
Results in:
Adjustment of fast idle not possible.
Comments: The same
cars with the rusted and seized idle mixture screws will quite
often also have rusted & seized fast idle screws and idle
cams. Once again, any attempt to force the fast idle screw
will usually result in the screw snapping off in the fast idle
lever. To remove rusted and seized fast idle screws, the
throttle plate must be removed from the carb, and the screw
must be carefully heated while “rocking” it back and forth to
loosen it up. To loosen the cam, the throttle shaft cam screw
must be carefully removed, and the spring & linkage pieces
must be carefully pried loose, cleaned up, and re-assembled.

Primary throttles adjusted to not open fully
Results in:
Poor throttle response, poor WOT performance.
Comments: It
is amazing how common it is for the throttle linkage on the
carb to be grossly misadjusted. When rebuilding a carb, always
operate the throttle linkage and make sure the primary
throttles open fully. They should open the the exact vertical
position. Anything less will prevent full airflow through the
carb. The position of the blades is adjusted by bending the
throttle stop linkage.

Primary throttles adjusted to open over-center
Results
in: Poor WOT performance.
Comments: Even more common that
throttles that do not open fully are throttles that have been
adjusted to open over-center. I guess people figure that te
car will run faster if you can open the throttle even more.
Fact is, once the throttle blades go over-center, they are
restriction airflow just as bad as a throttle that does not
open fully. Check for an over-center condition anytime you
rebuild you carb, and bend the linkage so that the throttles
stop at the vertical positioon.

Secondary throttles adjusted to not open fully
Results
in: Poor WOT performance.
Comments: The secondary throttle
shaft is actuated by a link off the primary shaft that hits a
lever on the secondary throttle shaft. It is common for this
to be misadjusted so that the secondary throttle blades do not
fully open. Some factory cars & carbs were intentionally
set up to limit secondary throttle opening (like
1st-generation 400-powered Firebirds) in order to limit
horsepower for one reason or another. Check for this whenever
the carb is disassembled and adjust it by bending the contact
tang on the primary throttle shaft.

Secondary throttles adjusted to open over-center
Results
in: Bog or hesitation going into the secondaries, poor WOT
performance.
More common that secondaries that do not open
fully are secondaries that open too far. Q-Jet secondary
throttle plates should NOT open to the full vertical position
or beyond. The secondary throttle plates should open to a
position where the angle of the throttle plates points and
aligns towards the lower edge of the secondary airflow baffle
located in the secondary venturi bore. When the secondary
throttle plates are opened beyond this point, turbulence in
the secondary side actually decreases airflow. Also, if the
secondary throttle plates are adjusted to open over-center,
they will, as a result, also open too soon. This will cause a
stumble or hesitation going into the secondaries. Adjust and
align the opening by bending the primary throttle contact
tang.

Secondary throttles not closing – not aligned in
bores
Results in: Fuel discharging out the secondary
discharge nozzles at idle & cruise, poor idle, flooding,
black smoke, off-idle stumble.
Comments: If the secondary
throttle plates do not fully close, or if they are misaligned
in their bores, engine vacuum will be applied to the secondary
venturi area. With engine vacuum in the veturi area, and the
upper airvalves closed, fuel will be siphoned out of the
secondary discharge nozzles. This will cause a very rich idle
and cruise condition. The problem can be caused by the
secondary throttle linkage being bent/misadjusted, or by the
secondary butterflies being misaligned in the throttle plate
bores.

Secondary throttle linkage springs missing or incorrectly
installed
Results in: Secondary throttles not closing (see
above), secondary throttles inoperative.
Comments: There is
no reason to disassemble and remove the secondary throttle
shafts, throttle plates, and the associated springs during a
rebuild, but some people do it anyway. During re-assembly,
these parts get left out or incorrectly assembled, resulting
in the secondary throttle plates not closing fully or no
longer actuating at all. These problems can be hard to
identify unless you are very familiar with the Q-Jet or unless
you have another carb to compare against.

Missing secondary airflow baffle
Results in: Poor
secondary WOT performance, sags or hesitations at
WOT.
Comments: The baffle installed inside the secondary
venturi area actually creates the venturi effect required to
discharge the fuel properly out of the secondary discharge
nozzles. Deleting the baffle causes turbulence in the
secondary venturies that will completely mess up your
secondary metering. Make sure the baffle is correctly in
position. It is frequently missing.

Choke plate misaligned
Results in: Poor cold-running
characteristics, sticking choke, engine stalling when cold,
engine not coming down off fast idle unless gas pedal is
hit.
Comments: If the choke plate is misaligned in the
airhorn, the choke can stick and bind in a part-closed
position. It cam also fail to close completely during initial
cold-start. Check to make sure the plate fits squarely and
tightly in the air horn. If not, crack the two screws loose
and wiggle the plate around until it fits right.

Choke pulloff seized or ruptured
Results in: Poor
cold-run characteristics, flooding when cold, stalling when
cold, poor fast idle control, sag or hesitation going into the
secondaries.
Comments: This is one of the most common
maladies on older Q-Jets. When the pulloff fails, not only do
you loose proper control over the choke, but you also loose
opening rate control over the secondary airvalve. Always check
the pulloff by attaching a long piece of vacuum hose to it and
sucking on it. The pulloff should smoothly retract, and it
should smoothly extend when the suction is released.

Choke pulloff incorrectly adjusted
Results in: Poor
cold-run characteristics, stalling when cold, flooding when
cold, engine won’t stay running after cold
start-up.
Comments: The primary purpose of the choke
pulloff is to crack the choke open just a tad upon initial
cold-start. If the choke is not cracked open, the engine will
flood. If the choke is cracked open to far, the engine will
lean out and stall. When correctly adjusted, the choke pulloff
will open the choke ¼” as measured from the forward lower edge
of the choke plate to the airhorn wall.

Fast idle screw incorrectly adjusted
Results in: Initial
cold-startfast idle too high or too low.
Comments: The fast
idle screw is hidden so well that many people don’t know it
even exists. The screw is located under the choke linkage on
the passenger side of the car. The screw head faces forward.
Fast idle speed should be adjusted to about 1200 rpm on a cold
engine.

Choke linkage/intermediate shaft system incorrectly
assembled or missing pieces
Results in: Inoperable choke,
engine not coming down off fast idle, sticky
choke.
Comments: The choke and fast idle linkage on a Q-Jet
can be a mystery of odd parts once the carb is fully
disassembled. It is easy to get some of the pieces incorrectly
assembled or installed in a bind. When this happens, the choke
system will not operate properly. Best to take a look at
another carb and do a little comparison if you’re not
intimately familiar with the linkage system.

Broken choke housing
Results in: Poor choke performance,
inoperable choke, vacuum leak.
Comments: The 1975 and newer
Q-Jets use a cast aluminum choke housing for either a hot air
choke or for an electric choke. Many carb kits contain new
choke hosing screws, and these screws are often too big fr the
intended application. Installation of the aftermarket screws
will crack the choke housing, and can make it impossible to
properly adjust the choke cover. The hot air chokes have a
vacuum source from the inside of the choke cover to draw hot
air through the choke system. If the housing is cracked from
the oversized screws, and the choke cover is not tightly
installed, the vacuum source will not pull the hot air through
the system properly, and the choke will remain “on” too long.
Broken housings can be welded.

Missing choke intermediate shaft seals
Results in:
Sticky choke operation due to dirt contamination.
Comments:
The 1975 and newer Q-Jets use two shaft seals on the choke
intermediate shaft: One seal is installed inside the choke
housing, and the other seal is installed in the carb float
bowl where the shaft goes through the side of the bowl.
Failure to install the seals can result in a sticky choke
system.

Missing secondary lockout lever
Results in: Bogging, sag
or stumble when going to WOT with a cold engine. Engine damage
from going to WOT on a cold engine.
Comments: The secondary
lockout lever, located on the passenger side of the carb just
forward of the secondary throttle shaft, is intended to
prevent the secondaries from opening when the engine is cold.
Placing the engine under maximum load before reaching normal
operating temperature can result in engine damage. The lever
is retracted by the choke linkage as the choke opens up. Many
people are afraid that the lockout lever is preventing the
secondaries from opening, so they remove the lever. This is
fine on a racecar, but not advisable on a street car. Make
sure your choke is operating, and make sure the lever is
adjusted correctly to perform its intended function. When
properly adjusted it will retract when the engine has warmed
up, and the secondaries will function as intended.

Incorrectly adjusted secondary lockout lever
Results in:
Inoperable secondaries.
Comments: It is possible to have
the lockout lever and the secondary throttle shaft pin
adjusted so that the secondary throttles remain locked out
even after engine warm-up. Check the lockout lever once the
choke is wide open and assure that the lever does not
interfere with throttle opening.

Missing idle vent parts
Results in: Dirt & debris
entering the float bowl.
Comments: The early Q-Jets use an
idle vent valve on the forward, upper part of the carb. The
idle vent valve consists of two stainless steel “reed” pieces,
a rubber seal, an actuation rod to the accelerator pump lever
arm, and a sheet metal “doghouse” to cover all the parts. Over
the years, many carbs have come up missing some or all of
these parts. This not only results in a carb that looks
incorrect and incomplete, but the resulting hole in the top of
the carb is a perfect source for contamination of the carb
float bowl.

Incorrectly adjusted idle vent
Results in: Engine
stalling/flooding when hot idling.
Comments: The idle vent
valve is to be adjusted so that the vent is cracked open when
the throttle is at idle, and it should close when the throttle
is moved off-idle. If the vent does not open at all at idle,
the engine may display poor hot idle characteristics and
stalling when hot. If the valve is adjusted so it never closes
at cruise, dirt and debris can enter the float bowl.

Wrong accelerator pump linkage installed
Results in:
Off-idle hesitations, stumbles, flat spot when
accelerating.
Comments: Different model Q-Jets use
different length accelerator pump actuation rods. Many of
these carbs have been pieced together by rebuilders using
various different parts. If the incorrect pump rod has been
installed, the accel pump may be inoperable for the initial
part of its travel, or the pump may bottom out in the pump
bore too early in the throttle travel. Poor throttle response
and/or hesitations will result.

Wrong accelerator pump installed
Results in: Off-idle
hesitations, stumbles, flat spot when
accelerating.
Comments: Just like the pump actuation rod,
different Q-Jet models use different length accelerator pumps.
Use of an incorrect length pump will change the geometry of
the accelerator pump linkage, and can result in an inadequate
or delayed pump shot. After rebuilding a carb, always visually
inspect the function of the accelerator pump to assure that
the pump is discharging fuel into the primary venturis upon
the slightest movement of the throttle (do this with the carb
primed and the engine off).

_______

Lars in Denver
'85 Coupe, no MAF
screen, K&N, Energy Suspension, Bilstein, glass top,
200,000 miles & daily driven... hard.
'71 GTO Judge 455
H.O. 4-spd (12.6 on slicks at sea level)
'68 GTO Convert
455 4-spd - the CorvetteForum Q-Jet Testbed
'74 Ram Air
Formula 455
'80 Ford Pinto (with factory 2-tone paint it's
a collector!)

Xracer
Senior
Member

1438 posts [100%]
Rochester NY

Re: Q-Jet Problems
and Tips (lars)

6:11
PM 11/11/2002

Holy Smokes Batman! I will be printing this
out.

Joe
_______

Click here for my Corvette **FOR SALE**
Web Page

Click here to E-Mail Me.

Proud Member of the The
Rochester Corvette Club

silvervetteman
Senior Member

2105 posts [100%]
red light city of
Orlando Floriduh

Re: Q-Jet Problems
and Tips (lars)

6:13
PM 11/11/2002

Hi Lars... Just an little note... your
statement.. "Cut Power Piston Lower Stop Pin
Results in:
Hesitations, stumbles, poor idle, surging at cruise,
ineffective idle mixture screws, hot running.
Comments:
Many (not all) Q-Jets have a lower stop pin in the power
piston. This pin rests against a cam in the throttle plate,
and determines the lowest (leanest) position for the power
piston. Cutting this pin results in the primary metering rods
seating too deeply into the main jets, severely restricting
fuel flow. Again, there must have been an article published at
some time telling people to do this".... Every Q-jet rebuilt
by holley has this done to it. Terrible throttle response at
anything other than WOT... Go figure.
_______

69
LeMans blue coupe/brite blue int.
427/390 Matching
#
M20, PS, PB, AC, PW,Very Original,Daily
driver

JB
Senior
Member

1381 posts [100%]
Somewhere in Texas

Re: Q-Jet Problems
and Tips (silvervetteman)

7:20
PM 11/11/2002

What silvervetteman said. When he told me
this I popped the top on my Holley built qjet & sure
enough it had been clipped off. Replaced it with a piston from
a donor carb and instantly cured my driveability problem.

No qjets had brass floats? Dang. Looks like I'm going to be
popping the top off my carb again and replacing the brass
float Holley put in there with the right one. Holley really
should stick to their own carbs!

JB
_______

1969 Monza Red Roadster, 427/390 L-36,
4-speed, A/C.
Original radio (it works!), original spare
tire, 3rd owner.

'79ProwlerOrange
Senior Member

1730 posts [100%]
Lifes a Garden... dig it

Re: Q-Jet Problems
and Tips (silvervetteman)

7:43
PM 11/11/2002

That is the longest post ive ever seen!
ill be printing that out too.

also i call you some time this week to see about the CCC

_______

Engine: 347 brake horsepower.
9.99:1 comp. ratio,
Forged Eagle pistons-Rods, forged crank, shaved heads,
overbored=4.030, 327/350 horse cam, re-jetted Q-jet.

The '79 is for sale! im selling it to get
StingrayRacing's '72, if he sells before im able to buy, the
'79 is getting a 427BB & ROD 6-Speed!!! (within 10 years
)

American Boy
Senior
Member

249 posts [100%]
Quebec

Re: Q-Jet Problems
and Tips (JB)

7:56
PM 11/11/2002

Great info as usual. With all your info, I became the Q-jet
guru of our Québec Corvette club. Thanks.

I was missing the spec on the choke pull-off setting,
should crack open to 1/4". I'll check that one since on cold
start, it has a tendency to stall the first time I start it.

D'Oh...my car is in storage for the winter... I'll check it later.

Stephan
_______

1980 Stock L48, 186 rwhp, Automatic
Fresh
paint and running great.

MNJack
Senior
Member

8142 posts [100%]
Summit, NJ NJ

Re: Q-Jet Problems
and Tips ('79ProwlerOrange)

7:57
PM 11/11/2002

Interesting about the floats! When I rebuilt my
Qjet it ran like crap. I took out the new brass float and put
the old plastic float back in and it ran much better.
Unfortunately the fuel inlet was leaking. Have you ever seen
bubba coat the inlet with epoxy thinking he could tap some new
threads into the epoxy? Can I send you an example?? MJ
_______

68 International blue
roadster
427 L36 4spd white top
Summit

Big Fish
Senior
Member

241 posts [100%]
St. Charles IL

Re: Q-Jet Problems
and Tips (lars)

8:21
PM 11/11/2002

Great article Lars! I hope this gets saved in
the "FAQ" pages. I am saving it just in case, but I am
confident my carb problems have been solved for now. Thanks
for all the great things you do for this forum!
_______

75 coupe, frame off
restoration almost finished or never
finished?

69shark
Cruise-In I
Veteran
Cruise-In II Veteran

2688 posts [100%]
Granby CT

Re: Q-Jet Problems
and Tips (lars)

8:27
PM 11/11/2002

Lars, is there a tech tip on sealing up the
bottom of the Qjet, I have a slow leak in mine as I need to
spin the engine over alittle after it sits for a week or so to
pump fuel into it. I think this guy from joisey told me abot
this, right Mark . If you have a link or can recommend a
manual or book.
_______

Tom
Gaul
Granby, Ct.
1969
Corvette Registry

NRA Lifetime Member
69-Burgundy
Conv.
93-Bright Aqua Metalic
Conv13.646@103mph
Send Email Here--> [email-masked]

lars
Senior
Member

2588 posts [100%]
Lafayette
Colorado

Re: Q-Jet Problems
and Tips (69shark)

8:52
PM 11/11/2002

69shark -
The tech tip on sealing the bottom
of the Q-Jets is included in my post above under the topic
heading "Leaking Well Plugs." You need to drill them, tap
them, and epoxy screw-in plugs into the wells if you want the
repair to last. But before you do this, pull the carb off the
engine, fill up the bowl, and take a look at the plugs for
evidence of leakage: I'll bet you they don't leak. The most
common cause for the fuel to drain out of the fuel bowl on a
Q-Jet is a leaky check valve in the fuel pump. This allows the
fuel in the fuel line to leak back through the fuel pump after
engine shutdown, and this will eventually siphon the fuel out
of the float bowl. You can solve this problem in one of three
ways:
1. Install a fuel filter with a checkvalve. These
Q-Jet inlet filters with checkvalves are available from
NAPA.
2. Install a seat that does not have the fuel slots
in its base. There are "solid" seats available that do not
have the slotted fuel inlet holes around the perimeter. This
prevents any fuel below the upper ridge of the seat from
draining back through the fuel inlet in the carb and keeps the
fuel level up after shutdown.
3. Replace the fuel pump with
a good quality pump.

MNJack -
Yeah, I've seen lots of epoxied carbs, with
drilling, tapping and machine work done to the epoxy repair.
Funny how that just doesn't seem to work very well....

Stephan -
When you do the choke adjustment, make sure
the choke is in the fully closed position. You can even apply
a little light finger pressure on the choke intermediate shaft
to simulate the choke closing. When you suck on the hose to
crack the choke open, apply very light finger pressure to the
choke itself to simulate the force of the air on the lower
edge of the choke. With this light force on the choke (which
pushes it towards the "open" position), and the choke pulloff
fully retracted, the gap should be 1/4".

Silver & JB -
Yes, Holley rebuilding is one of the
biggest offenders when it comes to cutting off the power
piston stops. I have no clue why they do this. It destroys the
idle and cruise performance of the carb completely. I have a
whole box full of cut power pistons - I'm gonna' save 'em
someday when I figure out a good fix for
'em...
_______

Lars in Denver
'85 Coupe, no MAF
screen, K&N, Energy Suspension, Bilstein, glass top,
200,000 miles & daily driven... hard.
'71 GTO Judge 455
H.O. 4-spd (12.6 on slicks at sea level)
'68 GTO Convert
455 4-spd - the CorvetteForum Q-Jet Testbed
'74 Ram Air
Formula 455
'80 Ford Pinto (with factory 2-tone paint it's
a collector!)

69shark
Cruise-In I
Veteran
Cruise-In II Veteran

2688 posts [100%]
Granby CT

Re: Q-Jet Problems
and Tips (lars)

9:01
PM 11/11/2002

Thanks I'll check into that, I'm replacing the
intake and will have the carb off then
_______

Tom
Gaul
Granby, Ct.
1969
Corvette Registry

NRA Lifetime Member
69-Burgundy
Conv.
93-Bright Aqua Metalic
Conv13.646@103mph
Send Email Here--> [email-masked]

Captain Morgan
Cruise-In III Veteran
Knocked Troy out of
the bracket racing

4449 posts [100%]
Either at school or at
work What time is it

Re: Q-Jet Problems
and Tips (lars)

11:03
PM 11/11/2002

Wow. Thank you for the time spent typing all
that up. I have copied and saved it for future reference, as
with all of your papers.

clem zahrobsky
Cruise-In I Veteran
Cruise-In II
Veteran

3988 posts [100%]
delmont pa

Re: Q-Jet Problems
and Tips (lars)

11:34
PM 11/11/2002

lars, on big HP,big cubic inch engines you may
need the "windowed" needle seat to pass enought fuel to keep
the bowl full at WOT. also the "windowed" needle seat cuts
down on the air bubbles in the fuel. just my 2 cents worth.

quote:

69shark -
The tech tip on sealing the bottom of the
Q-Jets is included in my post above under the topic heading
"Leaking Well Plugs." You need to drill them, tap them, and
epoxy screw-in plugs into the wells if you want the repair
to last. But before you do this, pull the carb off the
engine, fill up the bowl, and take a look at the plugs for
evidence of leakage: I'll bet you they don't leak. The most
common cause for the fuel to drain out of the fuel bowl on a
Q-Jet is a leaky check valve in the fuel pump. This allows
the fuel in the fuel line to leak back through the fuel pump
after engine shutdown, and this will eventually siphon the
fuel out of the float bowl. You can solve this problem in
one of three ways:
1. Install a fuel filter with a
checkvalve. These Q-Jet inlet filters with checkvalves are
available from NAPA.
2. Install a seat that does not have
the fuel slots in its base. There are "solid" seats
available that do not have the slotted fuel inlet holes
around the perimeter. This prevents any fuel below the upper
ridge of the seat from draining back through the fuel inlet
in the carb and keeps the fuel level up after
shutdown.
3. Replace the fuel pump with a good quality
pump.

MNJack -
Yeah, I've seen lots of epoxied carbs, with
drilling, tapping and machine work done to the epoxy repair.
Funny how that just doesn't seem to work very well....

Stephan -
When you do the choke adjustment, make sure
the choke is in the fully closed position. You can even
apply a little light finger pressure on the choke
intermediate shaft to simulate the choke closing. When you
suck on the hose to crack the choke open, apply very light
finger pressure to the choke itself to simulate the force of
the air on the lower edge of the choke. With this light
force on the choke (which pushes it towards the "open"
position), and the choke pulloff fully retracted, the gap
should be 1/4".

Silver & JB -
Yes, Holley rebuilding is one of the
biggest offenders when it comes to cutting off the power
piston stops. I have no clue why they do this. It destroys
the idle and cruise performance of the carb completely. I
have a whole box full of cut power pistons - I'm gonna' save
'em someday when I figure out a good fix for 'em...

_______

a.k.a. MOTORMAN, 2002 torch
red coupe, proud corvette owner since 1959
NCM
member#5583
NCM ambassador from Corvette Club Of Western
Pa
9TH-VETT vanity
plate

» Return to C3
Corvettes

Forum Jump 50th
Anniversary Events Audio/Electronics Autocrossing
& Roadracing Babes Breathless Performance C1 &
C2 Corvettes C1 & C2 Parts
C1s & C2s for Sale/Wanted C3 Corvettes C3 Parts
for Sale/Wanted C3s for
Sale/Wanted C4 Forced
Induction/Nitrous C4 General
Discussion C4 Parts for
Sale/Wanted C4 Scan & Tune
C4 Tech/Performance C4s
for Sale/Wanted C5 Forced
Induction/Nitrous C5 General
Discussion C5 Parts for
Sale/Wanted C5 Scan & Tune
C5 Tech/Performance C5s
for Sale/Wanted C6 General
Discussion Car Care Products
Corvette Accessories Corvettes of Houston Cruise
Control Auto Die Casts &
Collectibles Drag Racing
Engine Mods Exit 28
Motorsports F-Body General
Discussion Factory Correct
Resto Funny Off Topic Threads
Great Lakes Help
Forum Ken's Newsletters Kill Stories LAPD
Moderator's Corner Mountain - Prairie National
Events New Design
Problems/Questions Northeast
Off Topic Other
Cars Other Country Events
Pacific Politics &
Controversy Regional Coordinator's
Forum Southeast Southwest T Byrne
Motorsports Testing West Coast Corvettes Wheels
& Tires Xtreme Motorsports
Z06 Discussion Zip
Products Section ZR1
Discussion

Powered by zeroforum 2.0.5. ©2002
zeroforum.com.

C5
FAQ
CF C5
Registry
CF C4
Registry
Performance
Reg
Neighbor
Locator
Corvette
Events
Event
Galleries
CF Name
Tags
Product Reviews
Tech Tips
Home Pages
Corvette Portal
Corvette
Clubs
Vanity
Plates
Race
Sites/Tracks
Vendor Sites

Forum FAQ

Moderator
Bios
Advertising

Wuname