Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: beertracker on May 26, 2010, 04:56:07 PM
-
Problem:
Q-Jet 17057274 idles at 1500 rpm. It's on a 75 Pontiac 400 with stock 6x-8 heads, stock cam, stock dual exhaust, auto trans.
It has idled too fast every since I bought the car and I cannot slow the idle down by adjusting the idle mixture screws either in or out or the idle speed screw. The fast idle cam is in the down position so it's on slow idle. There is an air gap between the throttle stop and throttle stop screw so the primary throttle is fully closed. If I manually open the secondary air flap just a small amount the engine will slow down but as soon as I let go of the air flap the engine speeds up. The carb does not have a bushing kit installed
I installed a rebuild kit from Cliff and this did not solve the fast idle problem. I believe the kit had either .037 or .038 idle tubes. During the rebuild I did not touch the primary or secondary throttle shafts or plates. I carefully set the new float to 1/4" at the toe. I used the green spring for power pump return. Primary rods and secondary rods were carefully inserted in to their respective jets.
After I get the fast idle problem solved I am going to install milled (.040") Pontiac 6x-4 heads, Summit headers and a Summit 2802 camshaft.
I need some ideas what to do next to solve the fast idle problem.
Thanks,
BT
-
When the cars off, are the primary throttle flaps closing all the way,
and is the throttle stop sitting on the idle screw? Could be the vacuum pulling it open
because an incorrect adjustment. If there is a gap on the throttle stop when it is off then just look and see if something is binding and preventing it from closing. Either its binding, or being held open to far because incorrect adjustment.
-
Remove the baseplate and hold it up to a light. All the throttle plates should be fully closed.
The air has to be coming from someplace, either leaking around the secondary throttle plates, or a big vacuum leak someplace?.....Cliff
-
The "idles too fast problem" has been with engine running without an air filter installed in the garage. Is this ok? Would carb idle speed change depending if a air filter is used or not?
Thanks,
BT
-
Not really, idling that fast is air getting in under the throttle plates.....Cliff
-
What's the ignition timing at idle? Vacuum advance connected to manifold or ported vacuum?
-
Distributor advance is connected to ported (above throttle plates) vacuum. I will have to check on ignition timing. Could a bad EGR valve cause this problem? I haven't changed the EGR but the ball shakes freely.
BT
-
I just noticed the primary throttle plates have two V cuts in the outer edge of the plate. I am wondering is this factory correct or did some one take a triangle file to the plate. The cuts about 1\16" deep will allow air to pass even at closed throttle.
BT
-
Distributor advance is connected to ported (above throttle plates) vacuum. I will have to check on ignition timing. Could a bad EGR valve cause this problem? I haven't changed the EGR but the ball shakes freely.
BT
Sounds like you're describing the PCV valve, not the EGR valve. Yes, a defective PCV could cause a large vacuum leak that results in fast idle.
Having the throttle blades modified by cutting/drilling is probably the underlying problem, though.
-
Hello,(I ve been MIA for a while but now I am back)
I agree with Cliff, air is bypassing the throttle somewhere. Be it at the PCV or the throttle base plate [warped or shafts worn]. Also make sure the distributor is not advanced too much at idle and uses the correct vac source. (sounds like you got that covered). Bypass air [in the form of passages in the carb body or drilled/notched throttle plates] is only needed when adjusting the throttle plate opening does not suffice. Throttle plates should not need to be opened more than .040 of the transfer slot exposed.
More than this will cause the dreaded nozzle drip. Make sure choke or fast idle mechanism is not hanging up or incorrectly adjusted. It does not take too much air woth the q jet to vary the idle (mine idles a bit too high sometimes but I know it is because of air leaking by the shafts.)
A stock Pontiac should be able to idle at 800 and maybe even less.
-
A couple of posts above I mentioned V cuts in the throttle plates. After closer examination it appears these are factory made. They cover the throttle shaft and are not a source of air passage.
I installed bushings and the fast idle continues however all play in the throttle shaft is gone.
Before and after the bushing work I noticed a tiny air gap about 1/32" uniformly spaced around the throttle plate and carb bore. I centered the plates within the bore so the tiny air gap is uniform this is with the throttle fully closed. Is this air gap normal?
With throttle fully closed and idle speed screw even with carb casting there is a small air gap of 1/16 to 1/8" of an inch. This idle speed screw is not touching the throttle and not adding speed to the engine.
Thanks,
BT
-
All of the throttle plates should be tightly closed. If not, something on the linkage end is holding them open.
This accounts for the lack of control of the idle speed with the curb idle screw when the carb is in service.
It's actually a common problem, and requires bending a few items on the throttle linkage to get the plates to fully close, and open to exactly 90 degrees at full throttle.
There should be very little light visible around all the throttle plates when closed, if not, it still needs adjustments before being placed in service.....Cliff
-
It's actually a common problem, and requires bending a few items on the throttle linkage to get the plates to fully close, and open to exactly 90 degrees at full throttle.
what are these few items that need to be bent?
-
^ I think my Qjet has that problem.
-
I have been working on other things and today I got back to this problem. Still idles too fast. I found a vacuum leak in the power brake booster but plugging this did not help. All vacuum lines have been removed and plugged at the source. Could I have a internal vacuum leak in the Q-jet? If so how would I debug this?
The other leak possibilites I can think of would be manifold runner, manifold to carb base or manifold to head. I have a new PCV gromit so I don't think a PCV leak the is the cause. Running out of ideas.
bt
-
All 4 throttle plates should be fully closed and very little light showing around them with the idle speed screw all the way backed out.
Correct this problem first, then you know if it's still running too fast (rpm's), then you have a vacuum leak someplace.....Cliff
-
The four throttle plates are fully closed and very little light around them with the idle speed screw all the way out. New PCV, gromit and hose did not help. I suppose the vacuum leak could be internal to the carb, carb base gasket, manifold gasket next to head, rusted through manifold or valley pan?
Carb has primary bushings & your rebuild kit installed. I don't believe the secondary shaft is leaking air but how can I test; water & soap spray bottle or propane test?
Is there something I should be looking at in your book?
Thanks, bt
-
something has me thinking . you said in your first post that if you open the sec. flaps a little bit the engine slows down.correct me if im wrong but you would think that this would allow it to run a little faster if the lower plates were slightly open. if the plates are completly closed i dont see where it would change anything unless you were leaking on the lower bowl gasket.you dont have a spare carb to throw on to help solve the problem? iv got spares for people with problems i have them throw one on and if it fixes the problem then i overhaul there carb . if not they have to look elseware like timing etc.f.w.i.w. dean
-
429bbf, thanks for the suggestion. I will look into a leaking bowl gasket as you mention. Unfortunately I don't have a spare carb to try. I may have to get one at the next swap meet.
I did measure the vacuum and it's 18-20 " Hg so low vacuum is not a problem. Another idea, does this sound like too strong a power piston spring? bt
-
Forgot to mention the carb is drilled for by pass air. bt
-
I don't think you ever stated what your initial timing is. Have you checked the timing pointer position by placing the #1 piston on TDC and verifying 0 on the balancer?
-
Ignition base timing is set to 12 deg. BTDC. It's a Pontiac 400 and on a Pontiac forum I was told to set it to 12 deg. I can change it to 0 deg and see what happens. bt
-
Reducing the timing will almost certainly change the idle speed. The question is whether or not the 12 degrees you are reading on the balancer is actually 12 degrees. If you would take a few moments to pop the spark plug out, get the no. 1 piston as close as you reasonably can to TDC on the compression stroke and verify 0 on the balance it will either at best uncover a potential cause of your high idle or at least eliminate one variable from the equation.
-
Forgot to mention the carb is drilled for by pass air. bt
How much bypass air is there in the baseplate? This could very well be the cause if you went hog wild on it.
-
I don't think you ever stated what your initial timing is. Have you checked the timing pointer position by placing the #1 piston on TDC and verifying 0 on the balancer?
Yes, just performed this step and the timing pointer is at 0 on the balancer with #1 piston at TDC compression. I also slowed dist timing down to 0 and the rpm dropped by 25 to 1275 rpm. Very insignifiicant.
Since my hot air choke isn't working I blocked off the air passage between the choke housing and carburetor. This made the engine harder to start, backfire out the tailpipe once and through the carburetor once. After the engine warmed up it was idling too fast just like before. I plan to back this change out.
I was wrong about my base plate being drilled for bypass air, it's not. Does my problem seem like it could be solved by adding bypass air? I haven't ruled out a vacuum leak causing this problem. Or what should my next step be?
In Cliff's book he talks about how to install idle bypass air and modifying the idle system. Should these be done seperately or can they be done at the same time? If they are done seperately they can be tested individually before moving to the next one.
bt
-
bt. im having a brain cramp im still thinking about your first post. (when i open the sec.flaps it slow down.) i think one of two things . is it possible that the lower sec flaps are open a little bit and the upper are closed and the vacum is pulling fuel out the sec. tubes causing the engine to idle fast?try this take the sec.needles off the hanger and leave them down in the holes. fire up the engine and open up the top flaps and see what happens. also on the primaries .(that carb should have a front apt)screw it all the way down and take a straw or something small and push the needles all the way down with the engine running.one last thing not to be a smart (a) if this has had this problem does it have all the matching parts (airhorn -fuelbowl-baseplate. )and if so what about the right gaskets.let us know what you find. dean
-
Cliff called it! :) Problem solved. The secondary throttle plates were slightly open at idle letting air directly in the intake manifold. I solved the problem by bending the secondary throttle shaft spring so it exerts more force at closure. I just tested this out and the engine idles at 720 rpm. Much better than the 1400 rpm idle before.
I discovered the partially open secondary throttle plate the last time I removed the carb. Strange I didn't see this previously when I removed the carb.
Thanks to all who contributed,
bt :)
-
The throttle plates should be fully seated at idle. Make sure there is some play in the linkage that applies them, or they will not fully seat each time they close.....Cliff