Quadrajet Choke Operation

[Bruce Wingate]

I am trying to figure out how the choke / fast idle work on a Quadrajet carb. Chevy 350 SBC, no EGR/emissions. Carb has electric choke which works.

I set idle and high idle the other day to 750 and 1200. Today I started the engine cold: one pump of the accelerator pedal and a short spin of the starter. Choke plate was closed and slowly opened, but the RPMs never dropped (it actually went up a bit). I'm used to manual bike chokes and the choke on my VW Bug. The bug choke system has a fast idle cam that slowly clicks down and the rpms drop. I expected the quadrajet to work the same - am I wrong or does the fast idle cam need some sort of adjusting.

BTW: if I blip the throttle, the idle drops down to curb idle

 

[Alaska 79 K-10]

There is a stepped cam on the Q-Jet too. But as you found out, it doesn’t pull itself to the next step as it warms up, you must blip the throttle as you did.

 

[SirRobyn0]

I am trying to figure out how the choke / fast idle work on a Quadrajet carb. Chevy 350 SBC, no EGR/emissions. Carb has electric choke which works.

I set idle and high idle the other day to 750 and 1200. Today I started the engine cold: one pump of the accelerator pedal and a short spin of the starter. Choke plate was closed and slowly opened, but the RPMs never dropped (it actually went up a bit). I'm used to manual bike chokes and the choke on my VW Bug. The bug choke system has a fast idle cam that slowly clicks down and the rpms drop. I expected the quadrajet to work the same - am I wrong or does the fast idle cam need some sort of adjusting.

BTW: if I blip the throttle, the idle drops down to curb idle

Yes, Bruce this is normal operation. Just like bryan said there is a stepped cam, the idle will not drop on it's own unless you touch the gas a little. If everything is operating as it should, you should be able to start driving the truck in 10 - 30 seconds after starting it and the idle comes down on it's own as your pressing the petal, in normal driving.

 

[Bruce Wingate]

Thanks all. I just expected that it would transition down to curb idle over time

 

[SirRobyn0]

Thanks all. I just expected that it would transition down to curb idle over time

Just for a little further info, if your interested. This picture is pretty similar to what the Q-jet fast idle cam looks like.

You must be registered for see images attach

On the left side of the picture, those notches are the steps. The end of the fast idle screw starts out at the highest step as the choke opens if the accelerator is being pressed like when your driving the the cam moves to the lower steps. So at any point if you take your foot off the gas it'll idle at reasonable speed. If adjusted and fine tuned enough fast idle will be barely above curb idle, when it's cold and you'll be hard pressed to tell when your in the drivers seat if the choke has opened because it'll be darn near seemless. A car or truck with a manual choke has a smooth cam, so you can push and pull the choke cable at will. The reason the automatic choke is stepped is to ensure at any given choke position it hits a certain level, since you can't tweak it at will like with a manual. It's actually a pretty slick system.

 

[Bruce Wingate]

@SirRobyn0 - great pic.

The Weber carb on my bug has a very similar notched cam, but you can watch the cam click down as the carb warms up (I forget, but I am pretty sure it is an electric choke).

 

[mtbadbob]

The throttle plate spring pressure keeps the throttle stuck on the fast idle notch until the throttle plate is opened and the weight of the cam can drop it down when gravity takes over, results of blipping the accelerator pedal. Not sure how the Bug would work any other way with the lock notches on the choke arm??