I have a 86 trx 250 r last summer the bike was running good but this year
took it out to ride it let it warm up then ripped it about 1/8 mile and melted the piston so got new piston had 175 main jet in it and now I`m up to a 190 and it still is lean checked the reeds and gasket everything looks good any ideas and why did it change from been alright for 4 years and now been lean even with a bigger jet

 

Left crank seal may be leaking, replace the seal and have a leak test done on the engine to make sure it's sealed up good

 

As allready stated, do the leak down.

 

what does it take to do a leak down test

 

what do you have for engine mods and intake do you have pipe,reeds,with just the few things i have on mine my main jet is only 158 for 70 degrees every 10 degrees warmer i go up 1 size so even if it were a 100 degrees here i would still only be at a 162
you should not need a main that BIG.
and like they said check for those leaks VERY IMPORTANT
dont run crap gas- take your carb off clean it- what kind of 2 stroke oil and what ratio is your mix man you got so many open variables you need to be more clear

 

THE ONLY THING THAT I`M SAYING IS FOR THE LAST FIVE YEARS I`VE BEEN MIXING THE OIL THE SAME AND IT HAS THE SAME PIPE ,CARB,INTAKE ,REEDS AIR,CLEANER AND CYCLINDER AND NEVER A PROBLEM AND NOW THIS SUMMER
ITS RUNNING TO LEAN AND I DIDNT NO WHY SO FIRST THING I`M DOING IS REPLACING THE CRANK SEAL ON THE LEFT SIDE AND SEE IF THAT HELPS

 

The leak down test involves sealing up the intake and exhaust ports at the head and pressure testing the engine, it should hold pressure. You may want to consult a dealer or engine builder to have this done, it's not difficult you just need the equipment to perform the task otherwise you risk burning another piston. If there is oil in the stator cover the crank seal is leaking.

 

THANK YOU FOR YOUR ADVICE AND HELP

 

Your welcome.....

 

when you put your new piston in it, did you face the arrow towards the exhaust port?-may seem minor-but a friend did it to his kx250 and he had no idea what the problem was, till he took it to the dealer and they told him his mistake