compression testing

[jfarmer]

Can someone tell me the correct way to check the compression on a 2 stroke? Should it be cold with no fuel running through it, or warm with fuel running, or checked while engine is running?<br /><br />I also need to know what the compression should be on a 1978 mec 50hp 4 cyl.<br /><br />Thanks

 

[north40]

Re: compression testing

Compression should be checked with engine at operating temp with throttle in wide open position. Anything over 100 psi would be acceptable but the most important being no more than 10-15 psi difference in any cylinder.

 

[Alumarine]

Re: compression testing

NOT with engine running.<br />Let it turn over 4 to 5 times.

 

[north40]

Re: compression testing

Right

 

[Moody Blue]

Re: compression testing

Remove all spark plugs and ground them or better yet disable the ignition system. You don't want the motor firing up on you accidently or acting like a flame thrower out the plug holes.

 

[jfarmer]

Re: compression testing

I am getting 65# on all 4, but I did not have the throttle open. I will try that to see if there is a difference.<br /><br />Sounds like I am due for a rebuild.<br /><br />Thanks for the info.

 

[Alumarine]

Re: compression testing

Just a note. I've seen more bad compression guages than I have seen bad engines.<br />You might want to try another guage as a comparison.

 

[Moody Blue]

Re: compression testing

Before condemming it to a rebuild, try suirting a couple drops of oil into each cyl then try the compression test again. See if there is any difference. You could also do a decarb with a can of seafoam and try to remove all the built-up carbon from inside the motor. Helps in many cases.

 

[jfarmer]

Re: compression testing

I will check another gauge. I tried again after warming it up and got the same result.<br /><br />This motor has been in use for 9 years that I know of without any major repair. Earlier this year I had to pull the powerhead and replace the upper mounts. After putting it together I decarbed it. I just started using it regularly this year, but it seems that it does not have the power it used to have. So I am believing the compression may be the cause of the loss of power. It still runs good for a 27 year old motor. Just takes forever to plane and reach WOT rpms with more than 1 person in the boat.<br /><br />I am a little leary now of running too far with it now. Don't know how long it will continue to run with this low compression.<br /><br />Thanks again for the help.