correct comp test procedure

[crazy charlie]

I thought i knew the proper procedure for a compression test but i have recently seen a few different ways while looking for a new "used" motor.<br />the most recent was an 82 140hp johnson.the guy removed all plugs and connected the wires back on the plugs and pushed them aside as he cranked the motor for a good 5 seconds.The 82 motor read 150 comp in all cyls.It seemed aweful high for an old motor.If it had tilt ,it would have been hard for me to pass up,but I didnt want to replace my old motor with another old motor and get into a project putting my tilt on this motor.Honestly I dont think I can do it myself and would wind up costing me more than it was worth.What is the correct procedure to check compression????Charlie

 

[JB]

Re: correct comp test procedure

Howdy, CC.<br /><br />Most methods work. Hot, cold, throttle open throttle closed, other plugs in, other plugs out.<br /><br />What is important is that measurements from cylinder to cylinder are taken in exactly the same way, for 5 or more compression strokes, and produce psi measurements withing 10-15% of each other.<br /><br />I have seen the same OMC V4 give measurements of 95, 125 and 135 from all four (+/- 3-4psi), using three different "rules". Same conclusion each time: good engine.

 

[bubbakat]

Re: correct comp test procedure

I get 138 to 141 on all four of mine and it is 1975 86 hp. I ck with all plugs out and 8 to 10 turns

 

[jim dozier]

Re: correct comp test procedure

On a 4-stroke you should use full throttle to provided air to the cylinders, on a 2-stroke it doesn't matter, the air is available from the exhaust port on every rotation if its not firing. Pulling all the plugs justs lets the engine turn a little faster and makes the test go a little quicker. Its best to ground the plug wires in some fashion or instead use a switch on the starter solenoid with the ignition off to turn the engine so the spark doesn't do something weird in the ignition because it has no place to go. Even if you don't follow these rules, but you do each cylinder the same you will probably get usuable results.

 

[wakebite]

Re: correct comp test procedure

Most Compression gauges are not very accurate. What you are looking for Is a reading above 100 psi and no more than a 10% variation from High to low. There are a few engines out there that have 1 or 2 cylinders lower than others...Check the factory SV manual....