MS660 Decompressor valve pops too soon

[albert]

why not?
the fresh mix is being forced in and the exhaust gases are being forced out by that.
the only place with negative pressure is the crankcase

Maybe put a cheapo valve in your saw and rip the button off, tape it so it does fall in until you start cutting or reving see what happens.

 

[pioneerguy600]

why not?
the fresh mix is being forced in and the exhaust gases are being forced out by that.
the only place with negative pressure is the crankcase

Yes plenty compression/pressure with the exhaust port open.