Compression #'s taken cold...How Meaningful?

[SeaKaye12]

Hi,<br />Well, I did a compression test today on my Merc 850 (1977) that has been sitting un-used for 27 years.<br /><br />The numbers were as follows: (taken cold; throttle closed, after a good squirt of 2-cycle oil in each cylinder...<br /><br />#1. 168<br />#2. 170<br />#3. 166<br />#4. 170<br /><br />I've always believed that compression should be taken at operating temperature. However,the engine is not start-able because of wiring harness issues.<br /><br />My question then...the numbers look good to me...even but high... I'm wondering how much the oil in the cylinders is affecting them. How much credence should I place in a test taken in this manner?<br /><br />Thanks, Chuck

 

[Dunaruna]

Re: Compression #'s taken cold...How Meaningful?

Originally posted by SeaKaye12:<br /> How much credence should I place in a test taken in this manner?<br /><br />Thanks, Chuck

A lot. The bottom line is that if it won't start, a cold comp test is the ONLY comp test you can do.<br /><br />In your case, it's more important that the readings are close to each other, rather than the actual readings. But why with the throttle closed?

 

[timmathis]

Re: Compression #'s taken cold...How Meaningful?

You are lucky that thing will turn over after 27 years! Are you sure that somebody is not pulling your leg?

 

[SeaKaye12]

Re: Compression #'s taken cold...How Meaningful?

No leg pulling Tim; I've watched it set for most of those 27 years; it was a friends boat.<br /><br />I kept asking him when were we gonna take it out; he always said "soon"...<br /><br />Last month he called and said "if you want it...come take it..."<br /><br />In terms of hours of use...it's a brand new engine. But; rubber parts and the wiring harness are pretty much gone.<br /><br />As for why the test was done with the throttle closed; well....the shift/throttle cable seems awfully stiff...probably rusty inside...so it's difficult to advance right now.<br /><br />I was eager to do the comp test to see if all the other issues are worth addressing or not.<br /><br />Thanks , Chuck

 

[Rovernaut]

Re: Compression #'s taken cold...How Meaningful?

Isn't squirting oil into cylinders a way of checking compression for stuck worn rings?<br />People check compression and if the figures are low, they squirt oil into the cylinders and if the compression then rises it means worn rings. By putting oil in first it would not be an accurate reading.<br /> my thoughts

 

[SeaKaye12]

Re: Compression #'s taken cold...How Meaningful?

Yep Rover...that was my concern too.<br /><br />But my numbers seem high...most people here seem to post numbers closer to 135; although I don't remember anyone (including the manual) saying what a good engine should produce. It's always more a matter of consistency.<br /><br />So, I'm hoping that with the oil...and numbers in the 170 range...maybe without the extra oli they would still be in a normal range?<br /><br />What is normal and by how much would the oil tend to raise them up?<br /><br />Thanks all for reading, Chuck

 

[Rovernaut]

Re: Compression #'s taken cold...How Meaningful?

Chuck, your right when you comment about lack of info on what are acceptable comp' figures.<br />My classic 50 shows each cyl @105psi with cold cranking with carbs fully open. I thought this was low, but they are all consistant.<br />Variables might come into it like how good and accurate your Test equipment is. Mine is only an el cheapo<br />I think if mine showed 3 at say 105psi ans one was @ say 80psi then I would worry.<br />I want to take a reading agian after I put a can of poertune through it and see if compression figures increase.<br />Regards,<br /> Mars