Q-Jet 17057274 idles too fast

[beertracker]

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

[Toronado]

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.

[Cliff Ruggles]

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

[beertracker]

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

[Cliff Ruggles]

Not really, idling that fast is air getting in under the throttle plates.....Cliff

[Schurkey]

What's the ignition timing at idle?  Vacuum advance connected to manifold or ported vacuum?

[beertracker]

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

[beertracker]

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

[Schurkey]

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.

[omaha]

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.

[beertracker]

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

[Cliff Ruggles]

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

[rhino33]

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?

[slicer87]

^ I think my Qjet has that problem.

[beertracker]

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