Carb is "racing" at idle. Is it not compatable?

[TheDude]

The rebuilt Holley 2bbl carb I got is installed. The engine starts right away but the engine idle speed is very high. Even if I back the idle screw out all the way so it is not even touching anything, the idle speed does not drop. I have checked the timing. Every time I change the tuning of the carburetor the timing changes. This did not happen with the old Holley 2bbl I had installed. No matter how I tuned that one, the timing was consistent. It could be a vacuum leak. I removed the carb and positioned the gasket and carburetor as best I could and tightened it down plenty. Is it possible that the carb is not compatible with the distributor? As far as I know, it is a stock 1968 I6 distributor, with the exception of a Pertronix electronic igniton.

Thanks,

Justin

 

[TheDude]

I can see that the primary throttle plate is not closing all the way, even with the idle screw backed out completely. I will have to remove the carburetor again...

 

[Ronn]

Are you checking the choke cam screw?

 

[mustang6]

Sounds to me like something is wrong with this carb and raw fuel is getting dumped in the engine. I think you hit on that a bit earlier when you described fuel coming out the vent tube and IIRC you plugged that tube. Fuel should not be getting that high in the bowl and even though you have kept it from coming out the vent I believe it is still making it's way into the engine. You can't do any tuning that means anything until this fundamental problem is fixed, shown by the fact that adjustig the idle mixture screw is doing nothing. I'd be calling the place you got that carb from to see if opeing it will void your warranty. If so, I'd return it for another.

 

[TheDude]

The throttle plate is fine. It is in the same position as the throttle plate on the old 2bbl.

Are you checking the choke cam screw?

No, what does that do?

You can't do any tuning that means anything until this fundamental problem is fixed, shown by the fact that adjustig the idle mixture screw is doing nothing. I'd be calling the place you got that carb from to see if opeing it will void your warranty. If so, I'd return it for another.

Do you mean opening the actual carburetor? Can this be fixed easily?

 

[Anonymous]

I have read that some 2brl carbs are bigger than others and don't fit the manifold correctly causing them from closing all the way. Trucks came with these biger carbs.
Speedy

 

[Ronn]

Raises your idle for cold starts. Sometimes choke is out of adjustment and this screw which is located near the throttle screw is resting againts a stepped cam which closes choke plate and raises throttle. It is possible to have the choke plate wide open but the screw all the way in and therefore idle is way high. Causing the carb to suck fuel through the cruise circuit and not the idle circuit which could explain why the air fuel adjust screw does nothing because it only works best at normal idle speeds.

 

[CobraSix]

There should be 1 or 2 other settings. Check the choke throttle setting, back it out. If it's having the issue when it's warmed up, there is also (at least on most Holley's) a butterfly adjustment screw. Remember, the idle screw only controls the fuel mix, not the butterfly.

Slade

 

[TheDude]

I backed the choke screw out but I did not notice any difference at all.

 

[CobraSix]

There should be another screw right at the base of the choke mechanism, but not part of it. Though, depending on the carb, it may be on the other side or not attached to the choke. I think it's open too much and that will cause your idle mix screw to have no effect.

Slade

 

[Anonymous]

Did the plates operate freely and not bind before the carb was installed?
Sometimes if the carb is rebuilt, the throttle shaft is removed or re-bushed rquiring the removal of the throttle plates. Proper alignment and installation of the plates on the shaft is necessary for the plates to close completely and not bind in their motion.

You mitgt want to also disconnect some choke linkage from the throttle plate arms just to make sure that the choke linkage is not binding the motion of the throttle plates. Depending on the carb, these linkages have to be bent to get proper adjustment.
Doug

 

[Anonymous]

You mentioned once that the throttle plate was not closing all the way--then you kind of blew that off. If the base gasket is smaller than the carb throat it will hold it open, also if the linkage for the throttle is slightly different it may have to be adjusted to allow the plate to close completely. With the throttle adjust screw backed off the stop, the throttle plate should seal off the bottom of the carb throat completely. Try disconnecting the linkage and see if it closes farther.

 

[Anonymous]

Good point about the gasket Danwagon.
I forgot to mention that if your plates are not closing fully, the carb is running off of the intermediate fuel circuit or transitioning to it. Therefore, adjusting the fuel mixture screw will have no effect because the fuel mixture screw controls idle fuel. Re-set the fuel mixture screw to where it should be. I think the shop manual refers to the throttle plate stop adjusting screw as the curb idle speed adjustment screw. As you mentioned, you aleady have it backed out. You need to find out why the plates are not closing.
Doug