I just purchased a 1977 evinrude 6 hp, model # 6705M. It starts and runs. I noticed alot of carbon inside the combustion chamber when I pulled the spark plugs to inspect them. I went ahead and pulled the head off and cleaned all the carbon from the head and top of the piston. I then put some ChemTool on top of each piston to let soak down through piston rings to see if I could maybe free up some carbon there too ( all day soaking). Reassembled and checked comression. Readings are #65 on both cylinders, same as before I started cleaning. Does anyone know what a bare minimum compression reading is for this motor and if this motor is worth going any further with? Also any ideas on how I can get more compression?
Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.
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Check your compression with another guage. Compression guages aren't always accurate. In any case, if the engine starts easily and runs well, then its compression is good enough. It's more important that the two cylinders be close to each other in compression.
A good decarb, as described above, might raise your compression a little, but if your 6hp is running well...it's fine.
It sounds more like a concern of engine performance then compression. I agree with the other guys when they said if it runs well leave it alone.
But if you are experiencing less power/performance then what you have experienced with that same motor then as TD said a good decarb would be the way to go. Not only will it smoke but it can get kind of messy so if you store it in a building or whatever then it will possibly drip black carbon on the floor after a good decarb.
After decarbing my motor and running it in a barrel my engine's foot looked like it was dunked in burnt motor oil.
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1977 Challenger 15' Bass Boat with a 1978 85HP Evinrude (finished project but keep finding stuff to add to it)
Projects include: To many boats, to many outboards, and 1 I/O that I hope to be rid of soon.
Compression of those small engines depends on the speed of rotation one can acquire. Cranking via the manual starter vs installing a rope around the flywheel and giving it a hefty pull might result in a higher reading.
I've been buying and reselling smaller outboards for the past year and it seems all these used small outboards between 2-6 hp have a compression reading between 60-80 lbs. Your motor is fine.
When you get around 10 hp the compression should be around 100+lbs.
Thanks Whoopbass thats the answer I was looking for. Everyones replys are really helpful and worth trying. Its a relief to know this motor is still worth continuing on and fixin up. This forum is awesome and thanks for all the help. Take care and good boating.
Compression tests are a diagnostic tool to find out if there is ONE cylinder that has failed. That is why the spec is the differences between cylinders.
The likelihood that both (or all) cylinders would fail the same amount at the same time is just about zero.
The fascination with psi comes from, and is valid only for, 4 stroke engines because valve sealing is critical. 2 strokes don't have valves.
Compression??? 1977 Evinrude 6 hp
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