Open the main needle to make the engine run slopy rich.
GET A TACHOMETER
start over on adjusting the engine... and find peak rpm, then richen the mix for a 400 rpm drop.

Double-check for any pinholes in the fuel lines, including the clunk line inside the fuel tank...

It could also be an overload propellor... Too much prop will make 4-strokes do wierd stuff.

Did you goof and offset the valve tming when you had things taken apart?

If you are running a tuned pipe... you may have the pressure tap in a location that pulls a vacuum at higher throttle settings... that will make the engine either surge or kill it at high throttle. (I found that out the hard way...)

(I'll bet on the first suggestion.... If its lean... it surges.)