How do I properly check Compression?

[leishmaa]

I would like to check the compression on my YZ125 and have never done it before. I read that I need to keep the throttle wide open when performing the check. Do I need to have the exhaust in place?

The Clymer manual tells me that I should be getting 100 to 125 psi. I can get up to 190 if I keep kicking, but I don't know if I should be kicking the bike over multiple times or just once when checking compression.

Basically, what is the step by step procedure. My ultimate goal is to check if I need a new top end.

Thanks for your time

 

[darringer]

Throttle wide open. Kill switch pushed. Keep kicking until the gauge stops moving. Either warm or cold engine( the difference in readings is marginal). Leave the exhaust connected. Compression on a 125 should be above 150 at least. Don't rely on compression testing to check top end woes. Tear down the top end and measure the cylinder, piston and ring(s), ring end gap, and check the condition of the rod and rod bearings. Even when these parts are worn out, your compression can still be quite high, giving you a false sense of security.

 

[RM_guy]

Make sure your gas is turned off too so you don't flood it.

 

[taksx]

yes what darringer said! :cool:

 

[jwren00]

where do you get a compression testing tool? Is there one specifically made for testing 2 stroke dirtbikes?

 

[RM_guy]

You can get the screw in type at most any auto supply store (AutoZone, etc) The screw in type are the best becsue you know you have a good seal. They make one that you have to push into the spark plug hole but they are pretty awkward to use. You'll probably have to remove the tank to install it but I found with some effort you can screw it in with the tank in place.

 

[leishmaa]

I got my compression gauge at "mcmaster carr". They have a web site. It's an industrial supply store located in southern california. The gauge is the screw in type. Delivered the next day for a total of $40.00.

 

[Jaybird]

RM,
I always leave the fuel on when checking. The reason we open the throttle up is to allow a full flow of fuel to the friction area, allowing the normal amount of oil to seal.

Running the same test with a different ratio of oil can net you a different reading.
Which segues into why running a high ratio of mix (~18-26:1) can be very beneficial for high rev two strokes. Not only for the better ring seal, but for increased protection of the bottom end...but I digress.

 

[RM_guy]

You have a point but there is plenty of oil on the cylinder wall to get a good seal. One time I flooded it by leaving the gas on so I've always turned it off. :ohmy"