The exhaust leaks could very well cause this problem, I had the same thing on my 83 and that was the problem! As far as the compression goes buy or borrow a tester and check them even if theyre down around 80psi if theyre EVEN, it should still run ok, under that, hone the cylinder and change the rings. Because the bike is so old, you really should do them anyway. Change the exhaust and do a quick compression check. If it still does it, clean and readjust the carbs, and check ALL the lines, fuel, vacuum and even vent hoses, air locks will mess with the way it runs