Hello, I have a 1991 trx 250x. As my son was riding it the other day, he said it made a funny noise and now it misses very bad after about quarter throttle. It misses almost like it is hitting the rev limiter. I thought it jumped time. Took the valve cover off, found top dead center and timing is correct. The valves and cam are in excellent shape. No valves are bent. We have another 250x, so we began swapping parts to see what the problem could be. CDI swap did not fix it. Coil Swap did not fix it. Put new grounds on to see if it was a bad ground. Swapped carb just in case. Checked all wiring to no avail. We have spark. It will start easy and idle great. We have good compression. We put in a new spark plug. Then I thought maybe the flywheel key sheared and that changed the timing. Flywheel and key intact. There is a little side to side play in the crank, but I could not see how that would cause the symptoms we are having. I am at a dead end. Thought maybe pulse sender, but they rarely fail and I have spark. Bike runs like a new one until quarter throttle exact same symptoms every time. I am lost.