If the manifold gasket MAY not be good you should check it and replace if necessary.
I usually put a very thin film of grease on both sides so that the gasket does not
self-destruct on removal.
Who rebushed your carbs? Does the engine idle smoothly when first warmed up?
Finally, unless you have starting difficulties I would recommend that you not leave it
idling for too long because among other things, at least in hot weather, it doesn't cool well
and also it wastes gas.
What year is your car? (not ALL!)