Center Rochester 2v carb (66 442 Tri carb) idle dies once the engine warms up. When the engine is cold it starts right up and idles normally until it warms up again. Any idea what’s going on here?

 

"When the engine is cold it starts right up and idles normally until it warms up again" Thats because the chokes on and its rich. When it dies its too lean.
Start simple.
Vacuum leaks?
Carb tight on intake?
Whens the last time it was rebuilt?
Do you have a vacuum gauge?

 

Here's a shot in the dark. When its cold, the choke is on and the carb is providing a rich enough AFR to idle normally. After it warms up, the choke is wide open and the AFR leans out for some reason. One possibility is that the throttle shaft is worn and is leaking air, which is not enough to lean-out the AFR when the choke is closed, but when open it is. I think a vacuum leak may produce similar symptoms.

 

Try warming the engine to full temp, revving the engine to 2500-3K RPM and force the choke blade shut by hand and then release the choke blade before it stalls. The engine will starve for air and run very rich, let it clear and repeat this 3-5 times. I'm guessing that a passage in the carb is blocked and this will often open it up.

 

I initially thought of possible blockage but the engine idles smoothly when cold so I don't think it's an idle circuit problem, more likely a choke problem. Where the normally open choke (when warm) may be closing instead of remaining open for some reason.

 

What is the throttle shaft?

 

I checked all the vacuum hoses and they appear to be secure. I do have the vacuum gauge on the console - I hadn't thought of checking that. From some other archived posts, similar diagnoses have been linked to the brake booster vacuum as a leak source. I will check that but I would think that would affect the brakes too and that is not the case.

 

Remove the air cleaner, fully open the choke, note the position of the choke linkage with the blade fully open, reinstall the air cleaner, get the car warm and see if the choke blade is open by looking at the linkage. Don't remove the air cleaner to look at the blade position because removing it may allow the choke blade to open...been there.

 

That's the shaft that the carb butterfly plates are attached to. A worn shaft or bushing where this shaft goes into the carb can cause a vacuum leak, whose degree of severity might vary depending on throttle position.

Reading the other posts, I concur that a choke plate that is stuck shut can cause these symptoms by running too rich when warmed-up. The cold/warm aspect of these symptoms do point to the choke.

 

Check the PCV valve too. I have had more than one old car that wouldn't idle because of a bad PCV valve.

 

I rebuilt all three carbs in either 2012 or 2013. It always cranked easily cold with a few pumps and with the touch of the key once cranked during that day.

Did the issue just begin after the motor install? I know the issue wasn't present beforehand.

 

Have you tried adjusting the idle speed screw and the mixture screws? When the choke is on, the idle speed is controlled by the fast idle step cam on the passenger side of the carburetor. Once the choke is off, the throttle closes to the "curb idle" position, which is on the driver's side and also is affected by the mixture screws.

 

I think I discovered the problem. I happened to notice the vacuum line coming from the front of the carb was slightly pinched by a zip tie holding it to a cable to clear the intake manifold.I freed the line and replaced with new line. Car idles as normal now. I think the vacuum was possibly collapsing this old soft rubber hose which had deteriorated.

Thanks for all the good suggestions.