Exh to small???

[Jman271]

:think: 04 CR 250
Hmmmm, took apart top end, LOTS of black carbon smudge in the transfers and down into the cases. Looks mostly like it was in the front transfers only though,,,,my initial thoughts is that the gases do not have enough area to evacuate and are spilling back into the transfers, WDYT? I have to do a port map and check all dimensions to see what I have for TA, but this is my first guess,,,,

 

[uts]

Is ya bike stock. If it is I doubt the exhaust is too small.
Maybe ya top end is really worn and exhaust gas is blowing by the rings and into the case, then being pushed back through the transfers with fresh fuel charge. There is a point just as the piston starts travelling back up from BDC when there is slight pressure in combustion chamber and the transfers are still open. Maybe gas is being pushed back down then. I am no expert that is for sure.

 

[marcusgunby]

Recent CRs tend to be overported as std so i would guess the ex port isnt too small, many tuners lower the base by as much as 1.5mm.