My buddy was having the same problem you are having after he wreaked his quad. He looked at everything and could not figure it out well the other day he bumped it just right and the coil feel apart. He replaced and it runs fine now. He guesses in the wreak he hit the coil causing it to malfunctionSounds like an ignition problem- tight valves wont cause it to consistently stall after 25 minutes of riding- they will either break, and destroy your engine or will wear out the valve guide and you'll get lots of back firing, popping, loss of HP, chatter & poor compression -depending on the severity.
Just like TRX2fiveO states, a cracked or bad CDI / coil will cut out on you as soon as it heats up, just like in the old fords- when they used to mount the coil on the intake manifold- car would stall- poor design. Some bikes / atv's stuff that coil just above the cylinder head where it can heat up-
I know I was just agreeing with what you said because My buddy told me basically the same thing Blaze described and you posted the same response I would have.tomgodd- I'm not having any problems with my atv- I was responding to blazaroma about his stalling problem....
Hello all, I am new, so go easy on me. I have an 87 trx250x, that starts and runs great for about 20 mins. Then, it dies, like it runs out of gas, or stops getting spark. When I first start it, it will puff blue smoke, then clears out. Any ideas?