Howhit 150cc Sputters at High RPM
 

Problem: Tomberlin Crossfire 150 Howhit 150cc sputters/slaps/dies when the throttle is at high RPM. It engages and rises fine, but at around 75% full speed, it starts to crackle and pop. It will drop some RPM, and then clime back up to sputter at the same spot.

Troubleshooting steps taken:
Put on a new carb
adjusted the valve springs to .004 intake and exhaust.
Tried brand new CDI
Tried both warm and cold (zero change, no improvement at all)
Adjusted the low speed mixture adjustment

Next steps:
swap spark plug
Adjust valves again? Willing to accept I messed this up.
Maybe reseat the intake manifold? When replacing with the carb, it was tough to get seated. Had to use tightening the bolts to get the surfaces to meet. Maybe this isn't sealed and causing mixture issues at high rpm?

To me this sounds like the timing gets messed up at High rpm, but am at a loss. Any insight would be appreciated!

Aaaaaand already an update. I was suspecting also air/fuel mixture. It now has a "cold air" intake vs the stock box. Was thinking maybe it wasn't getting enough fuel at high rpm.

I wrapped my hand around the cone and revved it. I def got higher rpm. When I kept the throttle steady, and let go of the cone, it immediately bogged down and did the same thing.

Going to try upjetting next.