02 Nighthawk Compression issues
Have an 02 Nighthawk 250 with 7k miles in my shop right now. It's being quite uncooperative. I'm hoping, given the similarities with the Rebel, someone here can help shed some light.
Starting from cold, bike runs great. Once warmed up, idle becomes rough and will stall. The longer it runs, the faster the rpm tanks when you release the throttle. At full operating temp, it becomes impossible to restart, regardless of choke/throttle position. This is the case no matter where the idle adjustment screw is and using a free flow auxiliary fuel tank.
I've also tested all applicable electrical components for faults, both cold and after running. Charging is solid, spark is strong at all points.
Compression was a fair bit under spec. ~130psi each cylinder (manual for the year puts spec at 200psi or so). Leak down was 1-3% at 100psi. Set valve lash to .08mm. Compression came up to ~150. Did a run test in case specs were misprinted. Same issue. Tried a range of lash measurements. Only way I could get compression up was at 2mm(and god, do I wish that was a typo), and even then it was only 170. Oh, and did verify cam timing was correct.
Pulled the engine. Removed the head and started giving my calipers a workout. Cam lobes were good. Valve stems were good and straight. Springs were in tolerance. Seats had a little carbon buildup, but nothing major. Only real issues were valve seals having hardened a little, and some drag when removing the exhaust valves. Both likely due to the bike sitting for too long.
Replaced the seals. Drag seemed to be gone after removal, so guessing part of the cause was oil debris. Lapped the valves. Assembled and set lash to .08mm. Compression was still 150, same running issues. Did a leak down cold and hot tonight. 0-1% both ways.
Dropped the engine again. Applied heat to the head to see if it would cause any of the valves to drag. Was only using a heat gun to minimize the risk of warping anything. Still moved freely.
I'm kinda stumped. Also tired, so probably missing details, but I think I touched on all the major points. If you have read this far, thanks. If you have ideas (especially in terms of common issues I may be overlooking) I am all ears.