Compression testing..

[gdean7]

Trying to test the compression on my 1985 Force 125 motor. The Clymers manual is very discreet, and i'm not really sure what it means when it says ground the spark plug wires. Can anyone tell me, simply, how to do a compression check? Sorry for the dumb question, but for some reason I don't understand the manual all too well. Thanks!

 

[DJ]

Re: Compression testing..

Basically, "grounding the plug wires" means putting the spark plugs in the boots and making sure that the electrodes are touching something metal, like the block.<br /><br />Once that is done, open the throttle and turn the engine over. At least five rev's per cylinder, being checked.<br /><br />That should give you an accurate reading/cylinder.<br /><br />A "good" reading would be all cylinders with 5-10 psi, of each other.<br /><br />If not, it's time to look further.<br /><br />Let us know.

 

[eurolarva]

Re: Compression testing..

Also when doing the compression test you need to have all the plugs out with the tips grounded, not just the one you are testing. It is best if you have the motor in water (test tank) or using water muffs. You want the motor warmed up to get a better test.<br /><br />You can buy a test jig or build one. I take wire and braid it. If you have 4 plugs you will need 4 wires. Braid or twist one end of the wires and find a screw on the motor that is at ground potential and tie one end to this. The other end of the braid twist the wires on the spark plug threads. It is best to keep the spark tips as far away from the open cylinders as possible to avoid the possibility of fire. When the motor cranks some fuel can come out of the holes where the spark plugs have been removed.