HS 142 pulls hard to mid power then bogs and stutters

[Timcocycle]

Hello all. I just built my HS four stroke and took it out for the maiden voyage.
The only mod I did was take off the black box muffler and put the generic chrome
2 stroke pipe on. The bike idles just fine and accelerates hard up until 3/4 throttle then
Starts losing power and stuttering. I can hold the the throttle maxed and it will go a constant 23 but no more. Gas flows like a river to the carb so no flow issue there. Anybody have an idea of what is going on? It'll idle all day even after bogging around for ten miles. Stock plug as well. 89 octane gas. Valves set at .01 maybe a little less. Any help would be Much appreciated.

 

[Agreen]

Sounds like the carb is too lean on the main jet.

 

[KCvale]

Sounds like valves to me but one easy thing you want to do anyway is a NGK 7544 (CR7HIX) Iridium spark plug, no matter what else may be an issue, that's the plug you want for the best performance on the HS engines.