Possible high idle sticking?

[turpentyne]

I recently bought a '73 Chevy C20 with a 454 engine.

I haven't begun to dig into the mechanicals, as the truck seems to be generally running fine. But a few days into driving it, I've noticed an odd thing that seems out of the norm - at least from the last '73 I owned ( a C10 350 4x4 )

In the mornings, it's a little rough to start the truck. Once it's started, I have to tap the pedal a little to keep it from dying, then it'll go into high idle.

But then it seems to stick at too high an idle, and doesn't come down when it should. I know the engine needs warm up time,  but I live in sunny Arizona. Working the gas pedal doesn't bring the idle back down. It's about 5 minutes before it goes away.

To illustrate my point, If I put it in gear, I can coast down the road at about 15 - 20 mph without touching the gas. Finally after about five minutes, I can rev the engine and it'll drop to normal and everything goes fine from there.

[rich weyand]

Adjust the electric choke per the carburetor manufacturers instructions.

Also, before you start it, you should push the throttle once to the floor and release.  Depending on the temperature, this will set the throttle to high idle before you turn the key, rather than you have to tap the pedal to reset it once it is started.

[Jason S]

First, what carburetor do you have?  It sounds like some form of automatic choke, but there a few varieties.

Working off memory:

An original Quadrajet setup for your truck should've had a choke operated by a thermostatic coil spring in a little metal box on the passenger side of the carb. 

A later Q-jet and many replacement aftermarket carbs could have an electric choke.

Additionally, some later Q-jets have a hot air tube that feeds the thermostatic spring.


This in addition to what Rich said about pushing the throttle to the floor to set the choke. Sometimes it's adjustment, sometimes it's worn cams on the throttle or defective thermostatic springs.

[turpentyne]

I'll be taking a closer look at the carburetor setup tomorrow. I haven't had a chance to order a manual for this yet. Are there any online resources, until I'm able to order a book?

the last couple days I've pressed the pedal down to set the idle before starting, as you've suggested. No difference - it's still sticking in high idle.

Looks like I might need a valve job too, I'm getting a good puff of grey smoke every time it starts up.

[rich weyand]

When in doubt, go to the parts book.  Looks like you probably have the mechanical choke spring on the manifold.  Something there is holding too much choke, for too long.

The simplest fix might be to modify the carb with an electric choke.  Not sure if the casting holes are the same though.

Anybody know?

[Jason S]

Turpentyne:  Can you post a closeup photo or two of your carburetor? Especially the passenger side of the carb?

[bd]

Whoa!  Could be as simple as an adjustment or faulty choke pull-off (aka, vacuum break).  If you still have the factory Quadrajet, look at the M4MC carburetor in Section 6C of the 1979 Service Manual, beginning on the bottom of page 695 (6C-62) for troubleshooting and carburetor adjustment before reconfiguring the carburetor and perhaps introducing new problems.

The Rochester Quadrajet M4MC carburetor uses a hot air choke.  It draws heated air through a 1/4" metal tube that loops through an exhaust port running underneath the intake manifold.  The tube should heat fairly quickly once the engine starts (too hot to touch without severely burning your fingers).  If the tube takes a long time to heat, there maybe a restriction in the crossover passage that passes through the bottom of the intake manifold from the right cylinder head to the left cylinder head.

Edit:  Train wreck!  See below.   

[Jason S]

Whoa!  Could be as simple as an adjustment or faulty choke pull-off (aka, vacuum break).  If you still have the factory Quadrajet, look at the M4MC carburetor in Section 6C of the 1979 Service Manual,

True. 

Unless there is something I missed from a different thread it's a 1973 C-20.

If it still has the original or even the correct replacement carb, that ought to be the 4MV with the divorced choke. However, I think some photos of the what carburetor is currently in place should definitely clear things up.

[bd]

Thanks Jason!  My eyes were playing tricks....  Apologies....

I agree.  Pics will help clarify the circumstances.

But, since I previously opened the jar...     ...if it is the original style carburetor...
refer to the 1973 Service Manual, beginning on page 488 (6M-64).  Turpentyne, "divorced choke" just means the choke thermostat is fastened directly to the intake manifold instead of being mounted on the carburetor.  Nonetheless, the same principle applies regarding heating of the choke thermostat and opening of the choke plate.  The intake manifold surface below the choke thermostat should heat relatively quickly as long as the exhaust passages are open and flowing beneath the intake manifold. 

The choke vacuum break (pull-off) is designed to crack the carburetor choke plate open and reposition the fast idle cam as soon as the engine starts.  Generally, the cold high idle will decrease a few hundred RPM following a light tap of the accelerator pedal after engine start-up.  Occasionally, the fast idle cam and choke linkage will gum up and stick, and need to be sprayed with carb cleaner to free them.

[turpentyne]

i'll post pics this afternoon. I'm fairly sure everything is original on this truck.

Just needed a break after wrestling with the driver's side window rollers all morning. I went to the coffee shop to relax a bit.

Oh, and to clarify: Yes, this is a 1973 C20 with a 454.

[turpentyne]

I've got some photos for you now. It's a Rochester Quadrajet...

Here's the passenger side:


Is This the "divorced choke" that was mentioned? (green arrow):


And for extra reference, the driver side, and the name stamping...

[Jason S]

Is This the "divorced choke" that was mentioned? (green arrow):

Yep. Inside the metal box is a thermostatic clockspring that will expand as the manifold heats up. 

Also in the first photo you posted, the vacuum chamber closest to the fuel line is the choke vacuum break (a.k.a. pull-off).