I got a 93 model polaris 350 2 stroke with no compression. I put a used stock sized cylinder with a new piston in it... has 120-125 psi. It starts very nicely BUT when it does it just reves way out like I was holding it at full throttle. The throttle is not stuck open and the reads are flush with the cage. Any ideas why this is happening? I have had the idle screw and air screw twisted all the way out and all the way in and set at 2 1/2 turns out, and still no change. any ideas would be greatly appreciated. Thank you.

 

Usually running way too lean on jetting, crank seal sucking air,or an air leak around the intake manifold or base gasket can cause this to happen.125 psi is still low on compression with a new piston. Plus they can run super hot and start running lean real quick if you didn't bleed the head of any trapped air.