Compression test on 1975 Johnson

Gradywhite3535 · Jul 8, 2013

I have 2 engines on my boat and i decided to take the 1975 Johnson 6RL75R. It's the 6 hp model. So I got it home today and did a spark check and compression test. When I did the spark check the spark was mostly orange. Does that mean a weak or bad spark. And when I did the compression test it was 30 psi on both spark plugs. Is this a good amount or too low for a 6hp? Thanks

orbanp · Jul 9, 2013

Re: Compression test on 1975 Johnson

30 psi looks low, but it depends on the gauge and on the way the measurement was done. You want to test it after the engine has run, at WOT with both plugs out, and at a good starting speed. You could also try another test gauge.
Spark should jump in free air about 7/16" - 1/2".
If the engine starts reliably then compression and spark is certainly fine. With lower compression you would have starting problems.
There is good info at this site: OMC outboard related articles

Regards, Peter

Gradywhite3535 · Jul 9, 2013

Re: Compression test on 1975 Johnson

Does anyone with experience with this engine or similar to it? What should the psi reading be for a 6hp? I bought the compression test and spark tester at harbor frieght. I know they are cheap but i figured I would only use it once or twice. I haven't tried to start it yet because i wanted to test as much as i could. If anyone knows what the compression should be i would appricate it thanks

Gradywhite3535 · Jul 9, 2013

Re: Compression test on 1975 Johnson

I would really hate to find out this engine has no use as i would prob have to buy a new one. I would like to try and get a year to 2 out of it.

boobie · Jul 9, 2013

Re: Compression test on 1975 Johnson

Try and start it once and see what happens.

raczekp1 · Jul 9, 2013

Re: Compression test on 1975 Johnson

i had 90 psi in 6 hp from 74'
and 100 psi in 6 hp from 72'

F_R · Jul 9, 2013

Re: Compression test on 1975 Johnson

30 psi is way too low. If that really is all you have, it isn't going to start. Better recheck those readings. As for spark, 7/16 to 1/2 is mighty optimistic. It isn't CDI ignition. 1/4" is normal for your magneto ignition. If it will do that, don't be worrying about the color.

boobie · Jul 9, 2013

Re: Compression test on 1975 Johnson

You don't know until you try.

Gradywhite3535 · Jul 9, 2013

Re: Compression test on 1975 Johnson

One more quick question. I don't have a portable tank but will i be able to just use a gas can just to see if she"ll start? Rather not go out and buy a tank and find out engine has seen it last day and have no use for it.thanks and i will keep you guys poated

boobie · Jul 9, 2013

Re: Compression test on 1975 Johnson

You need a primer bulb in the fuel line.

Gradywhite3535 · Jul 10, 2013

Re: Compression test on 1975 Johnson

Ok so today i got the motor started and ran quite well. I can barely see the marks for start stop etc... I think I put it on start in order to start the engine. If you want it in neutral what do you put it in and if you want it in gear to get the prop to spin what do you out it on? And also after running for a while i gave it power and it started to smoke a lil bit and got really hot. I take it that it was overheating? What could that be from?

gm280 · Jul 10, 2013

Re: Compression test on 1975 Johnson

The fact that you were able to actually crank it up and run it and you stated that is also ran well means your previous compression number were erroneous. Now you have to replace the water pump impellor before ever restarting it. That IS why it seem to run hot. Just hope it didn't get too hot. Never, ever, ever start it without any water available. That being either in a tank on muffs or on the boat in the water. Running for even a few seconds will trash the impellor because as the water impellor pumps water, that water also lubricates it as it turns as well. So running it without will provide enough friction to melt the blades very quickly...

Gradywhite3535 · Jul 10, 2013

Re: Compression test on 1975 Johnson

Who said i didn't have it in a tank? It was in a tank. I saw that the carb had a screw missing but read the leeroysrambleing and he stated that there isn't suppose to be a plug there. What could cause it to overheat? I never used this motor before and prob sitting around for 2-3 years. Is there anything I should do to the motor since its been sitting? And how do you put it in gear? I turned the throttle and it seemed to be at a fast idle and i couldn't get the prop to spin.

Gradywhite3535 · Jul 11, 2013

Re: Compression test on 1975 Johnson

Can anyone answer the question please?

Gradywhite3535 · Jul 11, 2013

Re: Compression test on 1975 Johnson

Please can soneone give me answers

raczekp1 · Jul 12, 2013

Re: Compression test on 1975 Johnson

gear handle is at left side of motor cover.
pushing to you=forward,
pushing backward =rewerse
pusing in midle=neutral

overheating because lack of water(not enough water in backet,or bad waterpamp impeler, termostat locked in lock posytion)

after siting while is good to chack spark, clean the carb, change waterpump impeler and termostat

Gradywhite3535 · Jul 12, 2013

Re: Compression test on 1975 Johnson

When I started engine today i let it warm it for 2-3 minutes and then put it in gear. As i did that it jerk pretty hard. Can any of these pics show a problem?

Gradywhite3535 · Jul 12, 2013

Re: Compression test on 1975 Johnson

Is there a fuel filter on these engines?

Gradywhite3535 · Jul 12, 2013

Re: Compression test on 1975 Johnson

A few more pics

Gradywhite3535 · Jul 12, 2013

Re: Compression test on 1975 Johnson

I noticed gas/oil comin from a few spots such as spark plugs, under carb, out of exhaust and i think from under flywheel. I would like to get the engine in good running condition. This engine will only be for emergencys.

Compression test on 1975 Johnson

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