Open valves will act like a vacuum leak, yet you will not find them with the normal ways of checking. Any time you hear popping from the carb, you have something miss firing or back firing in the intake side of the engine. A very lean condition can also cause a miss fire, but usually only under a load.
Recheck the valve adjustment,
pull out all six plugs
rotate the engine till both intake and exhaust valves are closed on the first cylinder,
if you align the timing marks, that would be even better,
adjust both valves per manual settings,
rotate engine till the next two valves are closed and adjust them
do all six this way, do not jump around, go cylinder by cylinder.
Double check top dead center, timing marks, and where your rotor is pointing.
use piece of wire or wood down number one cylinder and rotate by hand till the piston reaches the top. At this point, the timing mark, and the rotor with #1 wire wire should all line up.
Attach your vacuum gauge, plug any port you see on the manifold or carb, and fire it up.
Let us know what the gauge says.
joe
