I don't want to be argumentative, either, but I read your earlier response as being you cleaned out the heat riser passages in the intake? head? both? when you took the intake manifold off. That says nothing about the functioning of the heat riser valve in the passenger side exhaust manifold. They are supposed to be hit with solvent every so often, but that generally doesn't get done and it is not uncommon for them to be frozen in position. If it is frozen closed (like the one in my 56 Plymouth was), then virtually all the right side exhaust will be forced up through the heat riser passage into the intake, down into the driver's side heat riser passage and out the driver's side exhaust. It won't just do that when cold. It will do it all the time. (Removing the heat riser valve in my 56 was worth 8 tenths in the quarter.)
You need to at least check the heat riser. Easy to do, just unbolt the right side pipe from the manifold and take a look see.
