Hard to say. What is the general health of the engine? approximate miles? Have you run a compression/ leakdown test? Does it use oil?
The timing could be somewhat late (retarded) and or the advance mechanism could be rusted/ sticky and not advancing enough. Ya might want to check that out
Simple: Have you checked that the throttle, with pedal to the floor, is ACTUALLY fully opening the carb butterfly? How about the exhaust, any chance the heat riser is plugged?
Does it miss at RPM or just seem to flatten out?
VALVE adjustment:
"Some people" insist that adjusting valves while "running" is the only way. I say "doodley squat"
I use what is known as the EOIC method, which works on any 4 stroke engine with poppet valves from a Briggs to a P&W turbo compound radial
I use the feeler "you want" and one or two thousand's larger, as a "no go." Warm the engine up, lay out your tools, so you can work fast to remove the valve cover and do the adjustment. Look things over closely so you know you can get the cover off. One quick thing you can do is pull the no1 plug wire off at the plug AND at the cap to "mark" no1, then pull the cap and wires off as an assembly to keep oil off and get it out of the way.
So EOIC, what does it mean, how do you use it? First, you can go "down the head" front to back, and after you get "good" at this, you can go in firing order, which will reduce the amount of turning the engine over. So for now, just start at the front, and work to the rear
Bump the engine until..............
the EXHAUST valve just starts to OPEN, and adjust the intake valve on that cylinder
then bump the engine until.......
the INTAKE valve opens, then nearly CLOSES, and adjust the exhaust valve on that cylinder
Take care to slide your feeler(s) in straight, and you should be able to insert the gap you want without much force at all, and the "no go" +.001 or .002 not at all
BE SURE your adjustment wrench is not applying force to the rocker which will affect the adjustment.
See? easy.
