Moderated Discussion Areas
ContinuousWave: Whaler Performance
|Author||Topic: Compression Testing|
posted 05-29-2002 01:51 PM ET (US)
I'm looking at used 60/70hp 3 cylinder OMC (2 stroke) motors. When doing a compression test, what kind of numbers indicate good/fair/poor condition?
posted 05-29-2002 04:50 PM ET (US)
Forget Fair and Poor.
All cylinders over 100psi and within 10% of each other is good. Smaller differences between cylinders is better. Anything else is bad.
Red sky at night. . .
posted 05-30-2002 10:23 AM ET (US)
I have looked at 4 different used OMC 60hp this week. Every one of them was a Lake Powell houseboat take-off. They are installed in pairs - when one dies the other is for sale. All appear neglected and abused, probably high hours too.
|Tom W Clark||
posted 05-30-2002 11:29 AM ET (US)
The compression numbers themselves mean little. It is the variation between the cylinders that matters. Generally as long as they are within 25% of one another they are considered normal, but the closer the better.
posted 05-30-2002 02:50 PM ET (US)
If the houseboat chartering on Lake Powell is anything like most of the other houseboat operations I have seen, you probably ought to look at those engines with a jaundiced eye.
For some reason, people who have virtually no boating experience feel perfectly at ease in taking command of a houseboat. Since most of the houseboats are underpowered and have very substantial windage, they are quite difficult to control. Combine the inexperienced pilot with a bulky boat, and you have a forumula for lots of engine abuse like frantic shifting, lugging, poor fuel mixtures, etc.
If they are bargain priced, the engines might be attractive, but I would think they've probably been through some hell.
posted 05-30-2002 06:18 PM ET (US)
my thoughts exactly, thanks
posted 06-03-2002 03:45 PM ET (US)
Never buy anything that is an ex-rental. I bought a jetski once.....once!
Like they say above....anything over 100 and even is good. Reason being I could test that engine with my tester and get 130, 125, 132. That is a great test. You could then use YOUR tester and get 110, 102, 112.
posted 06-05-2002 04:21 PM ET (US)
Test each cylinder a few times and take the average, or throw out any really high or really low numbers. If you are using the type of guage with a tapered rubber inlet, it can sometimes be difficult to get a good seal. The idea is to make sure you have enough data to make a good evaluation.
posted 06-06-2002 03:42 PM ET (US)
Good point...if you do not have a tester that screws into the port, junk it. Those rubber ones are as useless as......on a bull.
posted 06-07-2002 01:59 AM ET (US)
Thanks for all the helpful feedback! I'll get a screw-on to replace the rubber tipped one.
posted 06-10-2002 12:05 AM ET (US)
Anyone have an online source for a compression tester that they recommend? What size adapters are needed for marine engines? 14mm? 18mm?
posted 06-10-2002 09:54 AM ET (US)
Any autoparts store like Pepboys, etc. They have a decent kit with an adapter that fits most plug holes, like $22-25.
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