I have a 1988 kx 250 which at end of last summer I installed a new piston & rings (stock bore) I checked bore and was within tolerance. My problem is at full throttle its sputtering. I know the main jet controls this but cant understand why it would need changed as its same bore size and nothing else has been changed on bike. I tried changing the jet needle clip position but didnt help much. I put it back at #3 position for now. I am getting an exhuast leak (oily around flange) but other than that I dunno.When I put in the new piston I cleaned the exhaust valves out completely unless they might have gummed up since then. The bike is very cold blooded to start but runs fine at everything but full throttle once running. Any help would be appreciated. Thanks