compression test on an older johnson 225hp outboard

[gmulligan]

Can someone help me estimate the ranges (I know it varies OB to OB based on hours) to look for when compression testing an outboard motor during survey? I am having a survey done on a 1989 hydra sports 22ft walkaround and the motor compression is said to be btw 80 & 85 on five of the cylinders, and the sixth is at 78. does anyone know if this is too low (should they be reading closer to 100 or 110 across the board vs. 80 - 85)? Any advice/guidance would be greatly appreciated. thanks.

[seabird23]

I just bought an older ('97) Evinrude where the compression was in the 90-100 range on all 6 cyls. Sounds like that motor might be a little tired. From what I understand, as long as they are not more than about 15 psi from one another, you shouldn't have a problem. Not much help, but I'm sure someone else will chime in with a more technical answer. Good Luck!

[gmulligan]

thanks,.... I really appreciate the info (I am kind of going into it assuming a need to repower in the next year or so).

[danielb]

Beside a compression test do mechanics do a blow down test on the each cylinder to determine the condition of each cylinder? It can be done on 4 strokes, don't know if it can be done on 2 strokes, but it is a very valuable test of the cylinders tightness.
They would presurize a each cylinder with a 100 psi of air and read a gauge to see how much of the air leaks out and how long which gives you an idea of how tight the rings are etc.

[ericjp]

Actually the difference in pressure should be 10% or less not 15 lbs.

[Mulldogg]

do the heads need to be removed for the blow down test?

[abeal2]

Heads don't need to be removed for a "blow down test" It's a very similar process as in a regular compression test except you fill the cylinder up with compressed air to say 80 or 100 psi and hook a gauge up to it to see how much leaks out over a specific period of time. That's the way piston airplanes are compression tested.

[Mulldogg]

is there a number (say 70) which the reading should not fall below? ie: if the 6 cyclinders all come out with a reading of 65 lbs plus or minus 2 lbs, is that worse than if the come out at 80 lbs across the board plus or minus 2 lbs? I know it is an older motor with some hours on it so the compression should be lower than younger engines, but at what point is the compression reading too low (less than 50 lbs)?
To all who read and post, I appreciate your patience and insight, I am new to this, but determined to learn. Thanks in advance for your postings.

[Mist-Rest]

Blow (leak) down tests will work on 4 strokes, not on 2 strokes.

[Harpoon]

This post brings up another question.

Is it worthwhile to do a ring-job on a good running outboard when it gets over 6-700 hours ?

Any mechanics want to comment ?

[RussH]

I used to have a 1983 235HP Johnson outboard that I estimated to have 2,000+ hours. The guy that bought the boat did a compression test and all the cylinders measured in the upper 90's except for the bottom cylinders were in the low 80's.

I never noticed any change in the output power of the engine the whole time I owned the boat. The new owner run's the boat almost every day and the only thing he has had to do is change the fuel jets to adjust for the altitude in Phoenix, AZ.

I think your fine as long as you don't get some strange PSI ratings like a 50 in a cylinder.

Make sure you run the engine to operating temp, remove all of the plugs, and push the throttle to wide open before you measure the compression.

Russ

[Reel-Rascals]

OMC V6s of that vintage were usually 90-95 from day one. so you're not that far off. Use 2 different gauges and you get 2 different readings. I do it like Russ stated and use a $20 remote starter to the starter solenoid to bypass the keyswitch and leave that turned "off", that way no e-juice goes to the coils.

Decarb her good with Seafoam (1 pint to almost 1 gallon of fuel), run @ 1500 RPMs for 15 minutes, shut off for 15 minutes, then repeat until the mix is gone.

Prop her to run 5500 RPMs, decarb every 50 hours and you're golden !

"Life's too short to own an ugly boat ..."

[Mulldogg]

Thanks a lot guys, much appreciated.

[f4phantomii]

Regarding the blow down test, could you just do the normal compression test and keep the pressure on the gauge and see how long it takes to bleed down?

Seems like that would be easier than pressurizing the cylinder with air.

Would that work?

-Michael

[seaclusion]

No. For the most part, compression is measured on each upward stroke of the piston. Usually three or four revolutions will show true compression on the gauge. Most gauges will hold the highest achieved reading until it is reset.