Any Ideas?

what carb is on it ?
some times the powervalve is right on the vacuum limit and the valve can close shutting the fuel off on the p/v circuit.
ideally you want the p/v closed at idle and the mixture screws opened up, so it is the idle mixture scres that feed the idle fuel. you may have the butterflies open too far which start to draw fuel through the main fuel circuit as well.
you can drill holes in the butterflies to fix that.
also check if the vac advance in the dizzy is moving , ie pull it off.

The carb is a 625 Demon. The power valve is a 6.5, way below what my idle vacuum is. I have checked for vacuum leaks and cannot find any. I think that the key is that this problem starts only when the engine is warmed up. When it starts to stumble, I apply throttle and all is OK. The engine also revs up strong if throttle is applied. It just won't hold idle. Is there something in the carb's idle circuit that can cause this when warmed up?

The car will naturally run richer as the engine heats up and the air is less dense. Could you be running into a mixture issue at hot idle? Is the choke plate opening all the way when the motor warms up or not closing all the way when the engine is cold? If not it could be running too rich or lean at hot idle. Could be the idle mixture just needs to be adjusted a bit more. You also may have to adjust the idle slighlty higher when the engine is cold to compensate for the idle decrease at ht idle. Hope you figure it out. Im a fuel injection guy so forgive me if my carb tuning is not 100%.

Blaine

No choke on the carb, never needed one here in Hawaii. My climate here is very forgiving, I've never had idle settings cause a problem like this. In addition, I have adjusted the mixture every which way with same results. Maybe its time for a carb rebuild?

Dennis,

It definitely sounds to be a carb or fuel related problem somewhere.

You have a trio of parts there, I'd probably swap one of the Pantera carbs onto the Goose and see how she runs to verify that its carb related.

Problem with the goose is that many carburetors don't fit due to the jackshaft. My current carb is a Demon which sits on a 1" spacer and is turned around backwards because the vacuum pot will not clear the jack shaft. The Edelbrock and Holley Avenger off of my other 2 cats don't fit on the goose no matter what orientation. While trouble shooting the 'goose prior to removing the engine, I put the Demon on my '73 and it ran well (only problem is I didn't run it for long . . . just to see if it ran) I tend to think that its the carb too but its funny how it runs great until the car is warmed up. I think a carb overhaul is in order.

with the demon are you adjusting the idle with the idle screw on the throttle shaft ? or on the screw in the centre under the air cleaner stud ?

This Demon is the older model without the idle eeze.

ok, did you have to wind the idle screw up a fair way to get it to idle ? ie is it pulling fuel from the intermediate circuit ?
that can do the same effect.

It initially idles well until the motor warms up, say 15 minutes or so. Then I have to turn up the idle speed adjustment for it to stay running. I'm going to overhaul the carb and see what happens, its overdue for an overhaul anyway.

A friend of mine is visiting family. His cousin is a machinist and engine builder. He and some racer guys were talking about car stuff and my friend brought up my problem. His cousin told him that my problem is likely an internal vacuum leak. His cousin had the exact same problem as I am having and it turned out to be the intake gasket leaking on the oil valley side, small enough not to cause oil fouling of the plugs. His car like mine would run well for about 15 minutes then refuse to idle at low rpm speeds. I have my carburetor apart and am waiting for my overhaul kit ( I figure that its due for an overhaul anyway even if it isn’t the problem). If that doesn't solve the problem, I guess that the intake comes off for a reseal. I never thought of an internal vacuum leak but I guess that its as possible as an external one at the intake or carburetor base.