Originally Posted by 2door
I concur. What you have is the classic symptoms of excess air entering somewhere it isn't supposed to. Intake manifold to cylinder gasket, carburetor to intake manifold, crankshaft seal.
Yup, I think 2door is right. An easy way to check, (if it will idle long enough, If not, set the idle up for a minute.) is to spray some water on the intake connections. A little water wont hurt the engine at all, and you will be able to tell immediately if there's a leak. It MIGHT kill the engine. At the very least you will detect a change in RPM's. I really dont know how you would check for air leaks at the crank seal.