compression test on 8 hp O/B

[jw998]

I did a compression test on my 1998 8 hp ob. Both cylinders were at 90 psi, which the book says is good because they are the same. But the book does not give any numbers for how much compression there should be. What would the compression on a new motor be, and what is the low number for needing to consider head work?

 

[carholme]

Re: compression test on 8 hp O/B

If you have the manual, see Compression Checks in Section 1C, page 9, General information.

Normal Compression (All Models) 115 -- 125psi (787 -- 856kPa) for models 6
8
9.9
10
15

Check and record compression of each cylinder. The difference in compression readings
between each cylinder should not vary more than 15%. If compression varies by
more than 15%or compression for both cylinders is less than 100psi (685kPa), the powerhead
should be disassembled and inspected.

This is from the Mercury manual P/N 90-827242

Gerry

 

[jw998]

Re: compression test on 8 hp O/B

Ok. Thanks. I'm using the Seloc and Clymer manuals and couldn't find it there.

 

[jw998]

Re: compression test on 8 hp O/B

Also, I figured out the oil leak issue in my gear case. It wasn't gear case oil, it was just LOTS of unburnt fuel oil. Fortunately it's still onshore and not at sea. Thanks carholme

 

[JB]

Re: compression test on 8 hp O/B

Because of rope start compression numbers can vary all over the place. I would pronounce 90psi and even a healthy outboard and leave it alone.

 

[jw998]

Re: compression test on 8 hp O/B

I believe that I have a bad stator which is giving me lots of unburnt fuel (and smoke).

 

[jw998]

Re: compression test on 8 hp O/B

I like 90 better (as it saves me a rebuild). And yes, we are rope starting.

 

[jw998]

Re: compression test on 8 hp O/B

I did a Black stator resistance test and got a low speed reading of 30 (spec is 32-38) and a high speed reading of 29 (spec 31-37). I know it's under spec, but is it bad enough to give me the smoking on throttle, and unburnt oil that I'm getting?

 

[carholme]

Re: compression test on 8 hp O/B

Well if you like the numbers, you go right ahead and don't pay attention to it, it is your engine after all.

Gerry

 

[Mi duckdown]

Re: compression test on 8 hp O/B

RUN It.

 

[racerone]

Re: compression test on 8 hp O/B

I would say the stator is very good and the meter being used is very bad ----Run this motor.------The sludge you see is very normal.----On a 2 stroke motor oil and fuel go right through the motor and are wasted.----That is the nature of your 2 stroke motor !-------Oil and gas go right thruogh the motor even a brand new up to spec motor right out of the box.