Compresson Test Results ???

[SAGE9]

Got compression test results back for 1999 Johson 150 (see related stinky oil/gas post) 2 stroke, oil-injected. Wondering if they seem O.K.

115 105
103 105
103 105

Thanks much.

[DaleH]

Sage:

You're fine ... and I'd almost discount that reading on the 115psi cylinder. The lowest cylinder reading should be > 80% of the highest, so in your case with a high of 115, the lowest would be 92, and you have 5 cylinders @ 104 +/- 1 . Your banks also average to each other, less the erroneous 115 reading.

It is not uncommon for the lowest cylinders on OMCs to read lower. FWIW I always run cylinder #1 first and last, specially if the guage hasn't been used in a while ... that might be why you had one high reading, plus technique is a variable here too, as is gauge variation ... that's why no manufacturer sets ranges for acceptable compression values.

Per the OMC Factory Service Manual:

1) Operate the OB until it reaches normal operating temperature, then shut her down

2) Remove and inspect all spark plugs, checking their condition

3) Ensure you have a good battery - fully charged

4) Install compression tester in 1st plug hole, remove "OB safety lanyard" (this kills the power to the coils)

5) With throttle in wide-open position, crank engine with starter thru at least 4 compression strokes, watching the needle bounce

6) The lowest cylinder reading should be at least 80% of the highest cylinder reading.

Example: Highest = 100 PSI ... lowest should be at least 80 PSI

7) On "V bank motors, the average pressure reading may vary between the port & starboard cylinder banks, but these averages should not vary more than 15% from one bank to the other

[SAGE9]

That's good to hear. Good information. Thanks!

[SAGE9]

Damn - forgot to ask. What about comparing compression to "like new" compression. I heard that this engine comes new with ~120 compression. Does it matter that the cylinders now average 104?

[DaleH]

What about comparing compression to "like new" compression. I heard that this engine comes new with ~120 compression.
I don't know where you got that info and I can't verify it. I know that all V6 loopers from the late 80's to the early 90's were between 90 and 100. You might have 60-degree block motor and I don't know the ranges offhand for all of OMC/Bomb models.

However, no OEM service manual, factory or otherwise, prints any values for the compression ranges - as its the ratios that are key. We could test your motor with 3 different gauges and get 3 diffeent ranges of values. I still think you're A-OK.

FWIW you might want to do a decarb with Seafoam if that puppy hasn't been decarb'd before. For that 2-stroke, I'd do so every 50 hours, see here. (http://outdoorsbest.zeroforum.com/zerothread?id=2589) If you can't use the pony tank method, buy Seafoam's "Deep Creep" aerosol and use that, mving from carb to carb (remove the air silencer, which is simple on the OB).

[SAGE9]

Sounds good and thanks again for the useful information. I'll give the SeaFoam a try.