Had hoped to avoid this like the plague.

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macawman

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Checked compression in my 56 Johnson 10 horse today. Not good. Compression was 40 in the upper cylinder and 35 in the lower. Sounds like a complete power head overhaul is in store. Amazing it would even start, but explains why it would only run at medium to high speed. Guess I can kiss the beer money goodbye for a bit.
 
Usually a motor won’t run under 70. The fact that they are close would make me question my compression tester. Automotive testers sometimes don’t work well with small displacement engines. Before you start ripping it down, might want to confirm those numbers.


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I agree compression testing is not very reliable unless you’re using top shelf equipment and technique. When you pull the rope do you feel the chunk chunk chunk of compression?

Did you get the carb sorted out?
 
I can say with absolute certainly that it sure doesn't feel like low compression when you pull on the starter cord. I'm winded after 5-6 pulls. The is a thump-thump-thump as you pull on the starter cord and a definite difference when pulling the start with plugs in and plugs out. A friend of mine who has a manual start Johnson 20 horse tried it and said the compression felt good to him. Just purchased the compression gauge today, but that doesn't mean it's accurate.

The carb situation is improved, but still a work in progress.
 
Ok so if you feel that resistance you're probably ok. I think you just need to figure what's going on with the fuel system.

I'm only about an hour north of you on 65. If you want to bring it here I bet we could figure it out.
 
Pretty unscientific way to determine compression. :roll:
How about you take your compression gauge back and get another one and see what the number is.
Once installed, pull the engine through a few times until the gauge stabilizes at the highest number it will register.
The faster you pull the rope the better then number also. See what you can get out of it and let us know.
What fuel/oil ratio are you running in that engine by the way?
 
Compression is best taken on a warm engine which isn’t always possible. Most compression gauges are automotive and even the cheap ones are pretty accurate on most engines. My personal experience has been that once you get to 100cc or less per hole, they start to read under. Especially on piston port 2 strokes (I fiddle with chainsaws fairly regularly) . A little mix squirted into the cyl will help seal the rings and pull it over like a madman. The number isn’t as important as consistent numbers between the cyls.


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56 engines take 16:1. You can use 24:1 but for a motor that’s been sitting you should probably start out using 16. As long as you have minimal compression sufficient to get it started you really don’t need to worry about compression yet.

Get it going and let it run for a while then check the compression. The running will give the cylinders and rings time to reseat and break up any rust and carbon that might causing sticking rings etc.
 
Have you done a carbon clean? If you have not you won't get a good compression reading. I have had motors go up more than 40 points after a good over nite soak with seafoam. Also you will not get a full reading unless the carb throttle and choke lever are wide open.
 
timsmcm said:
Have you done a carbon clean? If you have not you won't get a good compression reading. I have had motors go up more than 40 points after a good over nite soak with seafoam. Also you will not get a full reading unless the carb throttle and choke lever are wide open.

There is absolutely NO NEED to open the throttle blades when checking compression. Not written anywhere in the books nor is it needed. "Wive's Tale"
 
Looks like the others have my thoughts pretty well covered. De carbon to make sure the rings aren't stuck and I would absolutely find another gauge to use and verify. I have used, on more than one occasion, a gauge that did not read correctly.
 
Got the seafoam today. Instructions on the can said to put it in through the spark plugs holes and to saturate everything so I put about 2 oz in each cylinder. I'm leaving it to soak overnight.
 
Sea Foam is fairly weak compared to one specifically made by Johnson/Evinrude (Engine Tuner) or Mercury or even Yamaha. Hopefully you will see some results with this. If not and you trust your compression gauge it would be well worth pulling the cylinder head, inspect the gasket, pistons, and walls.
If you still suspect that your compression gauge is faulty then replace it and run your numbers with a replacement gauge or borrow a known good one.
 
Thanks for the info, Pappy. Pulling the head is next if chemicals don't do the job.

I hooked my compression gauge up to my air compressor and compared gauge readings to compressor cut-off pressures. Assuming the compressor gauges are accurate, the compression tester reads about 10 psi low. However, adding 10 psi to the motor reading still isn't good.
 

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