I have a 88' Kawasaki Mojave 250. When I set it on timing and try to kick the kickstart down both intake and exhaust valves open when piston is returning to TDC. I've got it in a trade so I can't really tell you the history behind it but all I know is that it's bored and ported, 13:1 compression, everything in the head looks new. I first thought the timing could be wrong but I followed the rest of forums and look up pics and set it the same way and the valves hit so any info would be much accepted thanks!

 

There could be many, many causes of this.


#1. Wrong valve timing. (Not likely if both valves hit)
#2. Too tall of a piston for cam lift.
#3. Too big of a cam.
#4. Too thin of a head gasket.


A 13 to 1 piston is big compression. A lot of times the piston has to be fly cut for the valves to clear. The only way to see what is going on is tear the thing down and do some measuring.


Another thing. 13 to 1 is going to take all out race gas. Race gas is 10 bones a gallon now. If it was me I would pull the 13 and put in a decent piston. But that's just me.