low compression

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billo6663

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Joined
Apr 5, 2009
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47
Age
38
Location
River Valley
hi guys
well i bought this srx 2 years ago checked the compression and it only had 100psi(checked with 3 different comp. tester) i took the motor apart and found out there was spx or spi pistons in it.
So i put in brand new yamaha pistons and rings in there and tested the compresion again and same thing i still have 99 to 101 compresion
Now ive been running like this for 2 years never had a problem (knock on wood)
and i raced 3 different srx around town and i take them all bout 4 sled lenghts
i only have a hauck growler in there stock clutching
would some porting be the cause of low compression
p.s i tryed 5 compresion tester all together same reading
 
Make sure your testing with the throttle wide open and if belt is dragging on primary, to remove so you get good pulls. Elevation makes a difference too.
 
^ X2 Also if an aftter market (thicker) base gasket was usedthis will lowercompresion as well. If it aint broke dont fix it! Have you run a leak down test? Gives a more acurate "state of the union" so to speak.
 
Devilin AblueDress! said:
^ X2 Also if an aftter market (thicker) base gasket was usedthis will lowercompresion as well. If it aint broke dont fix it! Have you run a leak down test? Gives a more acurate "state of the union" so to speak.
Click to expand...

X2 what base gasket did you use?
 
After market piston kit more then likely got installed with after market thicker base gasket. I bet came as a kit. Replace with Yamaha base gasket
 
tomseal6 said:
After market piston kit more then likely got installed with after market thicker base gasket. I bet came as a kit. Replace with Yamaha base gasket
Click to expand...

WAS aftermarket pistons when he got it, replaced with yamaha oem. Went from 100 to 99-101, Never mentioned if he re-used base gaskets from PO or replaced as well.
 
billo6663 said:
used a kit for gasket and rings are broken in, ive been running for 2 years like this.
Click to expand...

Was kit Yamaha oem or "other". If it was oem yammi then base/head gaskets are stockers and has no bearing on compression. If they were "other" than probally thicker which will lower compression.
 
With aftermarket being thicker it raises cyclinder and head. Leaving a void or bigger than needed combustion chamber. Thats why i am sure you have read about guys peeling a layer of gaskets off less space = more compression. This is where the "squish test" comes into play. Fine line between to much and too little and too little is VERY BAD! If you are happy with its perforamance than by all means ride the snot out of it! Not going to hurt anything riding it IF the low compression caused by this. I would run a leak down test, it will tell you if rings are holding or blowing by. When my sxr burnt down last year it had 122-125 compression but center cylinder wouldnt hold ANY pressure!
 
Not looking to hijack but I have a similar question. What do you guys consider the low number for compression for a motor to still be "healthy" I checked my mountain max 700 today and it was 140/135/137 pto/ctr/mag. It was a long hose tester and the throttle was closed. I know leak down is also important but just looking at compression numbers how does my sled look. its a 2002 with 5400 miles on it. I'm pretty sure an old guy owned it before me because the bars were rolled down low for highway riding.
 
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