Q-Jet 17057274 idles too fast

[Cliff Ruggles]

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

[beertracker]

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

[429bbf]

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

[beertracker]

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

[beertracker]

Forgot to mention the carb is drilled for by pass air.  bt

[TommyK]

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? 

[beertracker]

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

[TommyK]

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.

[Shark Racer]

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.

[beertracker]

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

[429bbf]

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

[beertracker]

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 

[Cliff Ruggles]

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