Hello and thank you in advance for any help,this is a great forum with an incredible amount of knowledge. I recently rebuilt the top end of my 96 polairs xpress 300 due to low compression. I sent the cylinder to summit offroad in florida where it was bored .040 over and fitted with a new namura piston and rings. I assembled the engine and disabled oil pump and ran a mixture of 32.1 during break in as instructed by machine shop. The bike ran great but after only about 4-5 hrs of use it died and wouldn't restart. After performing some basic checks I checked compression and only have 90 psi. Any ideas as to what could have caused such premature failure. I performed the break in procedure as recommended by machine shop as well. Only thing I haven't checked yet is the crankcase seal, however if the crankcase seal blew out wouldn't that cause much lower compression than 90 psi?? Again thanks for any help!

 

Crank seal leaking can cause one to run lean/suck air and can cause piston seizures. You have to have good top end compression along with bottom end compression for any two stroke to run right.You can still have good compression on the top end,but if a seal or base gasket,head gasket or case is leaking air,an engine can run lean on mixture and a new top end won't last. That's why I refused to guarantee just a top end repair on two strokes. Also it may not be just a seal,but a lower rod bearing if really worn can spit bearing pieces back up into the cylinder and snag a ring and score a cylinder wall. That's why I've tried to get people to really check for any up and down play at all in rods,plus roll the crank shaft around and feel for any rough feeling crank bearings. You have to figure a bottom end works just as hard as a top end does,but most people over look this. You're not the first or the last one to have this happen to.