rough idle at cold start, warms up runs great

[74hurstolds]

I have an '87 Monte SS with a 305 Chevy motor, carburated. When it is warm out (summer) the car will start up cold no problem, but as soon as it gets to be <50 degrees outside the car won't idle on it's own when the engine is cold without my foot on the gas. It runs choppy and the exhaust is really rich. After about 4-5 minutes the car will fast idle with no problems and will eventually warm up and run like a champ.

I've adjusted the fast idle screw a little bit eventually returning it back to it's original location and that didn't solve anything except causing the car to idle at a higher/lower RPM once warmed up. I thought it may be the heat riser or EFE valve sticking but I've heard that when they are stuck they will stick in the open postion. I have replaced the intake manifold gasket last year as well as installed a brand new converter. Both made no difference.

 

[gto78]

Running too rich when cold sounds to me like the choke is closing too far, or not opening after it starts up. It's been a long time since I've had a choke on my car, but I think right after you start up the engine the choke should open a little bit. Then it will slowly open the rest of the way for about 5-10 min as the engine gets warm.

Someone here may still have an original choke setup on their car to confirm it. The choke housing should have 3 screws that you can loosen, then rotate the black part of the choke to adjust how far it closes for the outside temperature your at. I think if you look at it while it's cold out, tap the gas pedal to get the choke to pop closed, then see if it's completely closed or 3/4 closed etc. I always adjusted it to close about 1/2 to 3/4 closed at the most.

 

[74hurstolds]

I forgot to mention the choke. I did try adjusting it a few times (before marking where it was originally) and I think what did work was loosening it all the way and just letting it be fully open. The only problem is that I had to keep my foot on the gas the whole time it's warming up. I'm thinking that the bimetal spring may be bad but then again maybe not because it does fast idle fine after a few minutes with the choke closed. I'll give the choke another adjustment in the morning and try 1/2 - 3/4 closed. Thanks for the input!

 

[350_85cutty]

I have the same prob with my Holley 600! gotta keep my foot on the gas til it gets warm or it'll die out on me, but when warm it does run like a champ in cold weather...

 

[DRIVEN]

Check your choke pull-off. Your choke plate should be all the way closed with a cold engine and not running. The (vacuum) choke pull-off will crack the choke open, usually about 3/16", when the car is started. Also, spray some Carb-Kleen on your step-up linkage. It doesn't take much residue to affect the fast idle cam.

 

[74hurstolds]

Thanks everyone. I did some adjustments today and it worked. Here's what I did before starting the motor...

Depressed the accelorator (on the carb) all the way down to make the choke plate close entirely.
Moved the cam with my index finger so that it was engaged on the initial step (highest idle setting)
Adjusted the tension on the choke plate so that it opens about 3/16"

Started the car up and it ran perfect, was very supprised. I then loosened the fast idle screw about 1/6 of a turn because I did noticed some binding when moving the cam to the inital position with my finger. After work if it fires up okay then I know that it worked because I am skeptical on that the issue may be with the fast idle cam not going to the highest setting when cold upon depressing the accelorator all the way. It was about 40 out this morning too, not sure if I'll need to readjust again for colder weather. I'll follow up later tonight.