milling the cillinder head on a 1990 yz125

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Sep 10, 2003
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#1
I have a 1990 yz125 I gust put a new piston in it but it still has no compresion so im going to mill the head so I want to no if it is a good idea and do I have to use race gas
thanks <<<rhino87>>>
 
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#4
What is your ring end gap ? What condition is the cyl ? Did you de glaze the cyl ? Did you hold the throttle wide open while performing the compression test ?Dont just cut the head without getting specs first. I would humbly reccomend asking EG for specs. Be sure to provide the info on where you want the power to be in the rpm range and what type of gas you plan on running. Then be prepared to rejet. The relationship of the squish band is very important.
 
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#5
I dont no the piston gap/brand new cilliender/no I did not hold the throttle open/wher is eg located and do thay have a web site.
Im going to do a compression with the throttle open this time.
thanks <<<rhino87>>>
 
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#6
Ok Im back i did the compresstion test with the throttle all the way open that helped a lot but is still 3 1/2 psi less then what the book sed
thanks rhino87
 
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#8
What condition was the plating in? Did you make sure your cylinder was not out of round when you did your top end?(if your not familiar or equipped with preecision mesuring tools, bring the cylinder and head to your local mechanic) If it's never been replated, chances are the cylinder is so worn that it's oval in shape instead of being round. If the cylinder was scored at all, that would cause the compression to go down. Do you have the right piston for your bike?(yamahas have a system that goes by letter a,b,c or d wich designate the piston size. a cylinder marked b will not work with a piston marked a, c or d.) Is the powervalve functionning properly? make sure it's not seized in the open position. That too can make your compression reading low(ever wonder why dirtbikes have 2 compression ratios at high and low rpm's?). Take a look at these other things before going out and mill the head.

If the bike has never been modified, and it's not getting good compression, something is definitly wrong. And milling the head won't solve the problem.
 

Studboy

Thinks he can ride
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#9
3 1/2 PSI is negligable. Very many things could affect it that much. Altitude, oil ratio, etc.
If it is only 3 1/2 PSI short of factory specs, I wouldn't even worry about it.