I recently swapped a CRF450X motor into a RS125 road racing chassis. Call it... an RSF450! The bike is super fast but I am experiencing an intermittent stutter / miss in the higher RPM range (above 6-7K) when at steady throttle. First of all, here's a couple pics of the bike for reference: Here are the specs that may be relevant to the issue: Motor: - '14 CRF450X base motor (low miles, near new condition) - '08 CRF450R head, valves, and cam (brand new) - JE Pro Series 12.5:1 piston (#261965) Intake / Carb: - '14 CRF450X carb w/ JD jet kit - #170 main jet - #45 pilot jet - NO AIRBOX (just a clamp on cone filter) Exhaust: - Custom SS system with 1-3/4" header into universal glasspack w/ 2" exit I'm running the unmodified CRF450X wiring harness and all OE electronics. If you watch the video below starting around 0:20 you can hear the miss. I'm holding it steady right at about 7K RPM as I go by. Things I've already tried: - replaced spark plug - inspected spark plug (looks perfect, #170 main jet seems good) - bumped pilot jet down from #48 to #45 (fuel screw now in optimal adjustment range) - raised needle clip one groove - lowered needle clip one groove - swapped out coil and wire with another known good unit - swapped out CDI with another known good unit - checked pulse generator and flywheel was clean of debris - checked all electrical connectors - checked cam timing - checked valve lash - checked tank vent None of the above changes really made any difference. Any ideas on what else might be causing this 'stutter'? It runs great and pulls like a raped ape otherwise. It's only when cruising at steady throttle in the upper RPM range (above 6K) where I'm experiencing this issue. You can hear it and feel it (there is a momentary loss of power).