Top end problem

[flyboy]

2006 Yamiti UC250 w Honda clone engine. Had to change a bad muffler and got too lean but ran 74mph on GPS. Changed jets and found a 125 was a little much, went to a 40 and 120. Adjusted like I would a Harley. Started at 21/4 turns out and went in until it staggered and went out for best idle plus 1/8 turn. Now low and midrange seem OK. It goes to a little over 60mph and stops there. Plug looks good and am using an electric pump. Does not act lean or rich. Also have changed the fuel lines and filter. Have 800 miles on it now.. Got me beat. any opinions from the experts would help.
Bob (TX)

[ashley]

My first guess would be the jet size not being large enough, which is probably not the case. Maybe a valve adjustment?

[cloudaz]

Maybe a timing adjustment is now needed

[jontam]

Do you have a tachometer on your Yamati? If so what are your rpm's at 60 now?