Compression Test

[Paynerd]

I have a Mercury 1985 35hp motor. I recently did a compresion test on both cylindars. The first cylindar was 55lbs and the second one was 50lbs. I read all of the time that the most important thing is that both cylindars are within 20lbs of each other. My question is should I be concerned how low the compression is? Should i do something about it? and what? Thanks

 

[jigmister]

Re: Compression Test

need to find out what the numbers should be. they are close enough for sure. 50 seems pretty low.

 

[Paynerd]

Re: Compression Test

The manual says an "acceptable pressure" is 110lbs or higher.

 

[Part-time]

Re: Compression Test

did you crank it over WOT?
try putting a little bit of oil in the sparkplug hole and see if it comes up.
and are you sure your tester isn't faulty, I doubt a motor would even start with numbers that low.

 

[paultjohnson]

Re: Compression Test

How does it run? The fact that they are so close and that low makes me think your test procedure or equipment is at fault, Ive done alot of comp tests on 2 stroke snowmobiles and Ive never seen both equally, reallly low, usually 1 hole give up the ghost before the other. How did you perform your test? Engine warm, cold, throttle held wide open ? How long did you crank the engine ? Till the needle stopped rising?

 

[Paynerd]

Re: Compression Test

alright, i'm an idiot. I warmed it up, but did not have WOT when I performed the test. Thank you both for your help.

 

[paultjohnson]

Re: Compression Test

alright, i'm an idiot. I warmed it up, but did not have WOT when I performed the test. Thank you both for your help.

How did it come out...? BTW the wide open throttle isnt gonna buy you a whole lot of PSI. On snomos it might buy you 5-10% but you arent gonna go from 50-60 to 110. Or I should say i would be amazed if ya did.. I would still suspect a faulty comp tester, in addition to you being an idiot... JUST KIDDIN'- JUST KIDDIN' !!!

 

[JB]

Re: Compression Test

You might have some gasket leakage between cylinders, but not likely.

My take is that your engine is fine and your equipment and technique are weak. They are good enough to show both cylinders within 10%, which is good enough for me.

For example: Rope or electric start? When was it last run? How many turns to peak each cylinder? If electric start, what is battery condition? When did you last service battery connections? Etc, etc.

It is not realistic to expect on your 25 year old engine, at home, with DIY equipment and inexperienced wrench to get the ideal results the manual talks about for a professional shop and a new or rebuilt engine.

 

[MASHHADI]

Re: Compression Test

HI EVERY ONE

EDIT: What you have done is a hijack, MASHHADI. It is very rude to take over someone else's thread.

Please obey our rules and start your own thread. Thanks.