Rough idle when warmed up
 

I'm trying to figure out why my engine starts to stumble and runs rough when it's warmed up. When I start it cold, it sounds great. Sharp engine pulses, sounds tight. Then, after it warms up, it starts to run like crap. Stumbling, shaking, low vacuum. I checked everything and I can't figure it out. Any suggestions?

Sorry, forgot to add that it's a fairly tame 400 out of a '77 Firebird. Everything seems stock. Mild cam, HFT lifters. '70 rebuilt Quadrajet. I put an HEI distributor in it, that's about it.

A easy quick thing to check is this.

Once it gets hot enough to start to run ruff pull the 12 volts to the HEI and then crank the motor and listen for a steady speed of cranking thru all the cylinders.

If you hear the cranking rate speed up then you have a valve hanging open once that temp is reached.

fuel lines getting hot?

Choke fully open after it's warned up?

I'd be checking at all vacuum lines and vacuum motors.

EGR valve still hooked up?

Thanks for the suggestions so far.
So, I can definitely eliminate a vacuum leak. I disconnected and plugged everything.
The fuel line gets warm but not overly hot
There’s no EGR valve on this engine.
The choke is operating properly

If a valve was hanging open after warm up, would that be indicative of a bad lifter? Should I try a top end additive as a jumping off point?

Correct voltage to the HEI?
Lean idle/lowspeed circuit?

A misfire can sometimes be masked at higher RPMs. Running issues often come out best at idle - try pulling plugs one by one to see if the idle changes or not to narrow down a cylinder.

Or maybe your idle speed is just too low for your cam?

Make doubly sure that the carb base gasket fits the carb AND the intake, since you have a 70 carb on it..

When the motor is acting up you might also look down into the carbs primary side when idling to look for the problem of fuel dribbling in out of the boosters from too high a fuel level.

I would try all the above suggestions. A couple more to add. When it's warm and idling lousy, close the choke blade a little at a time and give the engine a chance to react. If as you begin to close the choke blade, the engine seems to smooth out and run better, the engine is LEAN for some reason. If it just runs worse and worse as you close the blade, the engine is running RICH and you need to track that down. The above assumes the ignition is working properly, the base timing has been checked and the vacuum advance is working. Did this situation happen all at once one day? Or did it get worse over time? Another quick carburetor check is to see if the idle mixture screws react to adjustment. Turn each in 1 full turn, then back out 2 full turns. Give the engine a chance to react. If no reaction or only one side reacts, you have isolated the problem.

Hey, Mgarblik,
My condition is exactly as you have described. I closed the choke blade a bit and then more, and it ran better and better. Not 100%, though. The vacuum is less than 15 inches, and in drive it dives to considerably less. I had to advance the timing and set the idle at around 1000.
Importantly, adjusting the idle screws makes no difference as you mentioned one way or the other no matter the turns. What's my problem? Carb?

it's lean. you sure you don't have a vapor issue?

You're running too lean. The problem may be below the carb, like an intake manifold leak. You can check it by spraying brake cleaner/carb cleaner all around the intake manifold gasket surfaces with the engine running. If the engine speeds up, you found the leak. Also check the base of the carb this way and around throttle shaft on both ends of the carb. Sometimes worn throttle shaft bushing leak so bad the engine won't idle.

Checking and adjusting engine timing with the idle above 900 RPM's can be deceptive as the distributor weights can start adding timing above that RPM if not a smidge below that.

If its till got the original EGR intake (not a bad piece...) make sure the EGR valve isn't part of the problem. Check the PCV valve.

If you have not had the intake off, it might be good to remove it, to verify the gaskets are not a problem...

Did this start recently? Does it start running like crap as soon as the choke fully opens, or only after the engine is fully heat-soaked?

Yes, the guys beat me to the response. Do what they suggested first, as that is easy. Spraying brake clean around all the potential vacuum leaks is the place to start. Around the carb base, EGR valve if equipped, intake runners, all vacuum hoses, even the brake booster if equipped. Look for changes in the idle while testing. Make absolutely sure plug wires and any source of spark is in good shape. You don't want a fire while spraying. If no change is found, my guess is the idle circuit is plugged or causing an issue. If you have to turn the idle speed screw clockwise allot to get the idle speed, it's likely your into main metering circuit or a combination of the main circuit and transfer slot. As mentioned, when idling, you should not see ANY fuel dribbling out of the center venturi cluster. All fuel should be entering below the throttle plate. If you see dribbling, it's time for some carburetor work. Start with the easy stuff. A vacuum leak is likely. Good luck with it.

Closing off the choke and getting improved idle is a double edged sword.

Because it might only be a lean condition on one side of the primary’s.

You richen up the lean side by closing off the choke and then you might be making the good side idle too rich which will still give you a bum idle.

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So, on that note about the EGR valve, I do not have one installed. I did purchase one. It's got the two take off ports, one to the carb (?), and one to the vacuum advance on the distributor.
The problem is that I couldn't get the plug out of the manifold to install it. Impossible in the car. See where I just plugged the vacuum advance into a port right on the manifold. I know that this is not best, but I figured I'd need it to drive the car.
Anyways, since it looks like I may pull the manifold, It will be much easier to remove that plug on a bench.
I'll have to check again which manifold gaskets I bought. I've got this combination of the '68 manifold on the 6X heads, block year unknown WA. Carb gasket as shown with the '70 carb.