Ok so after seeing many ways of this being done I thought I would look for a new answer.
Should the motor be hot or cold?
Do you need to push in the kill switch?
What is acceptable range if the compression is not exactly the same?
Should you hold the throttle wide open?
If sled is hard to start when warmed up, could that be a low compression factor?
Should you pull all plugs , or just the one for the cylinder your testing?
Should the motor be hot or cold?
Do you need to push in the kill switch?
What is acceptable range if the compression is not exactly the same?
Should you hold the throttle wide open?
If sled is hard to start when warmed up, could that be a low compression factor?
Should you pull all plugs , or just the one for the cylinder your testing?
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super1c
Super Moderator
Many variables. Hot, cold. Should it be wet, dry? The main thing is do the cylinders match within 5-10% depending in who ya talk to. Key is do the test and when repeating always do it the same. I pull all plugs as it makes no difference. You have to pull this thing 30 times so I figure easier to pull the better. Then I tape the throttle wide open. Engine cold and pull 10 times and check psi. Then repeat. My vipers are all the same every year and I record this. If I start to have problems I can do a check and it the numbers are off I got a problem.
The primary reason for doing a compression test is to check the condition of each cylinder relative to the others (provided CR is the same for each). As long as the veribles you listed are the same while testing each cylinder, it doesn't much matter. If your testing a sled that is suspect for a burned piston, you wouldn't want to warm it up and cause further damage. Wet vs. dry is a test used on a 4 stroke without big honkin' holes in the cylinder that a 2 stroke has and is used to check the condition of the ring seal. Open vs. closed throttle doesn't matter either. Pulling all the plugs helps pull it over faster and is less taxing on your shoulder.
IMO - probably the most important thing to remember is to ensure the key is off or the kill switch is down to avoid damage to the coils... or put the plugs back into the cap and ground them to give the spark a place to go.
IMO - probably the most important thing to remember is to ensure the key is off or the kill switch is down to avoid damage to the coils... or put the plugs back into the cap and ground them to give the spark a place to go.
mile9c1
New member
I do it cold, pull all plugs, leave throttle closed, and check for spark at the same time (put the plugs back in the caps and leave the ignition on). It's just a sanity check, not a science.
steveg_nh
New member
I do it cold, throttle wide open, all wires off plugs (so it cant' start plus kill switch down for safety sake), plugs in. I have electric start so cranking takes no energy from me. Plugs in and out, I got the same ratings. I did get a lower reading on my PTO side (125 vs 138 on the other two) but was told that's cause my sled has something called Yamaha staggered compression and it's fine...
roudyroy1
Active member
cold and dry. the wet test can be used to indicate the condition of the rings, do a dry test, then put a little oil in each cylinder and do it again, if the result is very different you may have leaking /wore rings. i read this out of an old johnson evinrude service manual, i would think that it would work on sleds as well.
dbldybangofst
New member
I always do cold test 1st. like stated before incase of major damage (don't want to make worse). plugs out , turned off , throttle open. record #s
THEN if close to what they should be do a hot test. Reason for hot test you ask? Because in the past I have had cold tests close and hot tests with a wider variance. What I found when this happened was broken / damaged ring. Because rings expand at a different amount than the piston / cylinder SOMETIMES you wont see it on a cold test. Good luck
THEN if close to what they should be do a hot test. Reason for hot test you ask? Because in the past I have had cold tests close and hot tests with a wider variance. What I found when this happened was broken / damaged ring. Because rings expand at a different amount than the piston / cylinder SOMETIMES you wont see it on a cold test. Good luck
WedigSRX
Member
Followed up with a hot test is not a bad idea, I recently was working on a little fan cooled polaris 500. I did a compression test and both clinders were within 5lbs.
I ran the sled and did not make it across the driveway before the thing lost all power, are inspecting the intake the skirt was broken on the piston. I tore it down and their was scoring on the cylinder wall and pieces of piston everywhere in the lower end.
I still cannot believe the bad side had a 120psi cold........
I ran the sled and did not make it across the driveway before the thing lost all power, are inspecting the intake the skirt was broken on the piston. I tore it down and their was scoring on the cylinder wall and pieces of piston everywhere in the lower end.
I still cannot believe the bad side had a 120psi cold........