Compression hot? cold? 30hp Johnson

[BoatBuoy]

I'm working on a 1956 30hp that I acquired recently. It's not quite ready to run yet though. I wet the cylinders good w/TCW3 about a week before checking compression. <br /><br />It has 140 lb on each cylinder, cold, and somewhat oily. <br /><br />What might I expect to see for compression numbers after running and warming up, i.e. how much does compression vary from cold/oily to warm/dry?

 

[muskrat]

Re: Compression hot? cold? 30hp Johnson

Could go 5 or 10 lbs higher or lower.140 lbs on a 56 ok..

 

[wayne h]

Re: Compression hot? cold? 30hp Johnson

are u checking at WOT? 140 is not bad at all might wanna decarb it and see what happens

 

[Bradster941]

Re: Compression hot? cold? 30hp Johnson

"I'm working on a 1956 30hp that I acquired recently. It's not quite ready to run yet though."<br /><br />At this point, we know nothing other then the TC3 is making a good seal.<br /><br />You must get it running before a compression test can be made now that you have coated the cylinders with oil.

 

[BoatBuoy]

Re: Compression hot? cold? 30hp Johnson

That's what I was thinking. 140 lb. seemed a little high, but not impossible, for a 50 yr. old motor.

 

[rodbolt]

Re: Compression hot? cold? 30hp Johnson

on a two stroke hot or cold really does not matter, what your looking for is the spread of the numbers not the numbers<br /> if the numbers are bogus cold they dont get any less bogus warm.oil in the cyl for testing compression is a waste of time usually done by car mechanics that have no experience or understanding of a two stroke. if a two stroke has low compression its one of 2 things, a piston/ring or a head gasket. oil in the cyl wont find either. it will make a mess and blow 20 ft when ya crank it tough and it will damage gauges when they fill with oil but adding engine oil to retest is usless on a two stroke and mostly so on a 4 stroke, its to inaccurate and does nothing.<br />by the time it sat a week all the oil has either run past the rings or out the intake/exhaust ports. a bit of lubrication on a motor that sat wont hurt but it wont help bad compression. if both cyls are at 140 I would proceed with the repairs .

 

[BoatBuoy]

Re: Compression hot? cold? 30hp Johnson

Actually, I oiled the cylinders good not so much to check compression, but 'cause the engine hadn't been run in several years. Just out of curiosity, I did the compression check to see if I'd bought a bag of bones. Just curious about how the oiling might affect compression numbers. Thanks for the replies.

 

[lark2004]

Re: Compression hot? cold? 30hp Johnson

If the rings aren't sealing properly, the oil can sometimes hide this.<br /><br />Personally, I find it a bit hard to believe that you have a real 140psi on motor that old. Normally these motors in good condition show anywhere from 90 to 120 psi.<br /><br />The real thing to watch out for, is that the figures you get, are not anymore than 10% apart from each other.<br /><br />Does the motor run? If so, warm it up first, then check the compression.

 

[rodbolt]

Re: Compression hot? cold? 30hp Johnson

why bother warming it up? if the sealing is ok its ok. 140 on those old 3 ring motors was about normal.<br /> I have seen them all over between 130 to 180 in the 50's 60's and 70. first 225 jonnyrude I ever tested was 80 on all six. I even borrowed another tester and got the same results. run the same test on a 72 50 jonny and you will see 170-180. but the whole key is are the numbers within 10 psi. anymore indicates a fault.<br /> with a two stroke the actual number is not critical, the spread is.<br /> if one hole is low and you add oil all you do is make a mess. it has no trouble shooting benifit. however you can blow oil 20 ft or more on the next test .

 

[BoatBuoy]

Re: Compression hot? cold? 30hp Johnson

Motor's not ready to run yet. I've yet to re-sealing the lower unit (awaiting manual), soak and re-build carbs, and pull flywheel to check under there. Then I'll be ready to lite it up.

 

[dafox99]

Re: Compression hot? cold? 30hp Johnson

On a somewhat side note, shouldn't the compression be limited by the compression ratio? Meaning, if you have an 9:1 ratio, the most you should see is 9 X 15 (at sea level) = 135 psi. Yes? I don't know the comp ratio on these motors, but it might make sense to check that out. Would a compression of 140 psi indicate a compression ratio of a little over 9? I would think a compression reading of 180 would indicate a ratio of 12:1. Wouldn't that motor ping like mad on regular gas?