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I finally got my bike back together after doing my first top-end: replated cylinder, wiseco piston and RB head mod (and carb.) I figured I'd check the cranking compression before starting it for the first time just to see where things are and was surprised to see it at 168psi. This is a cold motor, with a fresh, never-run top end. It was 120psi before I pulled it apart to do the top end.

So, is 168psi too high? Manual shows a max of around 150psi. I expected it to be on the high side after break-in, but not before. Just want to make sure I didn't do something wrong as this was my first top-end.

This may sound strange, but it also sounds like it has more pop when I'm just kicking it over for the compression check. I used to barely feel it when I kicked it over before. I did crank the motor over before putting the head back on and it was smooth as butter.

Thanks,
Pete
2001 kdx220

175psi is usually the max for dirt bikes.

Jaguar wrote:175psi is usually the max for dirt bikes.

Well, that has me a little worried since the bike is cold and not broken in yet. Granted, the rings are probably seating fairly well with the 2-stroke oil I coated everything with. I think I'll go ahead and start it and just keep my eye on it during the break-in.

Thanks,
Pete

You have the RB head mod.... Higher compression.

That was the whole point of what you paid for. 200psi is not uncommon, but likely 110oct or methanol would be the fuel to match.

The beauty of the RB mod is that it not only increases compression, but it dials in the "squish" band. A properly designed squish band will increase the turbulence in the compressed fuel/air charge, as well as concentrate it closer to the spark plug, allowing it to burn more uniformly and decrease the chance for pre-ignition at the new, higher compression. I don't think you have anything to worry about with your new higher compression. Just run pump premium and you should be fine.