While rebuilding a ninjette I picked up that had been stored uncovered in the backyard for 10 years, one of the many issues I found was that the end of the plug wires at the coil were nasty. I trimmed about 1/4" off each and that made the difference. Also the resisters in the plug boot were corroded, so I removed and cleaned them. You can check these things with an ohm meter. All you need is in the ninja FAQ https://faq.ninja250.org/wiki/Ninja250_Howto I used a timing light to diagnose a coil that was cutting out. It wasn't the coil though, it was a worn through wire under the tank. Likely that air filter has disintegrated - a couple years and they turn to powder. There is a tiny fuel filter in the gas line where it connects to the clutch side carb. You can't see it but it's in the line and if it's clogged, would prevent revving. Speaking of fuel, a crankcase full of gas would also cause it to not run right. Open the filler and take a whiff if you can't tell from the sight glass. Did you buy it from a guy named Justin by chance? I knew I remembered reading this issue once before... https://www.ninjette.org/forums/showthread.php?t=116053 There is really little to these bikes, so I'm confident you will find the issue quickly. Valves are super easy to do and oddly, I didn't find an issue with the space. Check out the timing chain just because you're in there... likely not an issue but if it's not in alignment, I can see it running sort of but not properly.