All the sleeves do is position the manifold in relation to the head. They don't seal vacuum- - - -that's what the gasket is for. If you're talking about the head pipe between the exhaust manifold and the muffler not sealing, it has NOTHING to do with how the engine runs. Find and fix the vacuum leak you created during assembly, and it will run properly with or without the exhaust system connected. I use an unlit Propane torch moved around all the mating surfaces between the carburetor, intake manifold, and head to find vacuum leaks. The engine will speed up or smooth out when the gas gets sucked into the leak. Acetylene works for leak testing, also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!