ok i have a 45mm crank and a 52mm airsal bore with 28mm koso carb "main jet 135 and plot jet 50" and a Blue cdi box and coil . ok When i first built the motor did a hole on top of the piston then i got a other new piston and it did that same thing again it Keep blowin a hole on top of the piston it not hittin the head or and thing.. what cause that to happen ????

 

You are running very very lean. Need to richen up the main jet a few sizes. Or you have a leak somewhere between the carb and the engine and its letting in air leaning the fuel to air mixture.

 

i don see no leak no where... Can't be the timming CDI BOX?

 

<div class="FTQUOTE"><begin quote>Originally posted by: dominicano3o5

i don see no leak no where... Can't be the timming CDI BOX?</end quote></div>

Maybe if the timing was advanced a lot it would ping enough to hole a piston but I would think you would hear it ping and let off.

My son burnt a hole in 2 pistons on his WR 250. Both times it was while he was in the dunes running wide open. first time he blew a head gasget just as it leaned out, 2nd time it was because of a blown base gasget.

 

It could be the timing

 

So i should just used my stock Cdi box and coil...

 

What did the spark plug look like?

 

round the hole on the piston it look gold and White.. on the Sp it White

 

White is lean, still it could be caused by the timing being off. Have you checked the timing?

 

Can it be That the piston dose not have the holes on the side of the piston for the Intake??