hi all seized my mc21 up again but this time don't no why (bottom cylinder same one) she had been running really well then first ride out in the sun this year and nipped up two miles down the road all warmed up didn't even get on to main jet?

ok then don't really no a lot about nsr250s but im ok with 2strokes my question is, my top barrel and piston are fine its a B barrel but i have a spear A barrel with the correct head for it.

can i run the B top and A bottom together ?

also david silver guy on the phone said i need to run B pistons with an A barrel !!! is that right sounds mad to me what is the differents between A and B

thanks