Cranking compression on RB head mod

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pw455
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Cranking compression on RB head mod

Post by pw455 »

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.

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Pete
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Re: Cranking compression on RB head mod

Post by Jaguar »

175psi is usually the max for dirt bikes.
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pw455
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Re: Cranking compression on RB head mod

Post by pw455 »

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.

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Pete
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Slick_Nick
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Re: Cranking compression on RB head mod

Post by Slick_Nick »

You have the RB head mod.... Higher compression.
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Re: Cranking compression on RB head mod

Post by fuzzy »

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.
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Re: Cranking compression on RB head mod

Post by dfeckel »

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.
David Eckel
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