125-200cc 90 hours on wr165 still like new compression

Did you have your head machined for more compression? My stock compression was only 170 PSI on my 2008.

My 2008 cr125 and after doing 144 had 175 psi. My first 165 on 2008 cr had 205 psi but had to run race or could detonate put under big load but did have better grunt. My 2013 wr 125 had about 170 psi stock after changing to wr 165 has been 195psi and stayed there for 90 hours. Can run less race gas with lower compression and never heard it detonate.

It is a factory head and I did have to run race gas or it would detonate unless jetted a little rich. I've run race gas in my wr250 since the beginning (after trying one tank of 93 pump gas) and feel it is part of the reason for the long life of the top end.. Amsoil and spot on jetting being the other reasons.

Not sure what Wallybean did to make 165 rip but looks like some porting and playing with base gasket to get the right squish or what ever they call it.

Wow, I wonder why yours had that much compression. I thought all of the 250/300's were below 180 PSI.

The 300s have 160-180 unless you buy the race head. I just now checked my compression with 1 ride since new top end and it's 202psi. I do know that the base gasket that mine had was the second to the thinnest available from Halls/OEM and I put the same thickness back in. Maybe you could gain a few psi with a thinner base gasket.