do a leak down test on the cyl's...that will let ya know if the head gaskets ok and the valves are seating correctly. If you get any hissing noise from the carb the intake valve has issues....and from the exhaust then obvously the exhaust vales the problem.....if you dont et any hissing but the leak down is excessive then head gasket is most likely shot. If all that checks out.....then i personally would yank the exhaust and inspect all gaskets especially around the risers...they leak all the time. If that checks out then that leaves the intake gasket.....and since you nead to take that off and break engine gaskets to check i mentioned it last. If everything checks out then you may have bigger problems...like a crack in the block or head somewhere.....or even reversion if you canged the cam to something other then stock.......good luck!