how do you check compression???

[Newfusion900]

I want to check the compression on a motor i bought and i am not sure if i am doing it right. i threaded the compression tester into the head and pulled the cord and can only get a reading of 20 on each side. what do i need to do to check it properly

 

[Timbersled]

Make sure if you have a tether that the tether is disconnected, ignition key and engine stop switch are in the off position. Remove the spark plugs. Install the tester in one cylinder. Hold the throttle wide open to get a full charge of air in the combustion chamber and pull a half dozen times. Check the reading. Do the same for the other cylinder. NOTE: Make sure the schrader valve in the end of the compression tester is tight and not bleeding off. Jason

 

[Newfusion900]

Does the motor have to be installed in the sled or can this be done on the bench???

 

[BlueFusion]

Doesn't really matter where the engine is at, sled or bench, what matters is that you are spinning it over fast enough to equal a good pull on the rope with the throttles open. The equivalent of at least four good pulls...... Can be hard to hold them still on the bench though! Good luck!

Ps, do check the valve in the end of the tester, I had one come loose one time,

 

[retiredpop]

Do you have another engine around to see what it reads with your tester or better yet another compression tester? Is the rubber o-ring in place on the thread adaptor?

 

[jsledder]

I think it would be tough to get a good result while the engine is on the bench, unless you have some good way to hold it in place while getting enough pull 3 or 4 times for a good reading.

 

[Newfusion900]

The seals on the tester are good, its brand new. I only pulled it once or twice on each side so I think that is where I am going wrong. I will try again tomorrow morning when my dad is around to help hold the motor and try again. I can always compare the readings to my 05 900 that was just rebuilt. Thanks for all the help guys I will let you know what I find out tomorrow. Regardless the motor is getting a new top end anyway I just wanted to know what shape this motor I just bought is in.

 

[Newfusion900]

Well i finally got it. 130 mag side and 135 on the pto side. maybe it won't get a top end and I'll just run it.