15 hp Johnson Compression

[Khenders]

I have a 15hp Jonson (J15RCOB) that tests ~65 & 70 PSI.
After just rebuilding the top end on me jet ski, I am not sure I want to tackle replacing the rings on this engine.
How involved is it?

 

[wbeaton]

Re: 15 hp Johnson Compression

I haven't done one of those, but it shouldn't be too hard. Its a pretty simple machine.

Does it run at all? Maybe your tester is out. If it runs I'd at least try a decarb before commiting to a rebuild.

 

[Khenders]

Re: 15 hp Johnson Compression

I used the same compression tester on my jet ski and it came up to spec compression. So I don't doubt its accuracy.

The engine does run. It just doesn't have the power it had before.

 

[wbeaton]

Re: 15 hp Johnson Compression

Then I'd try a decarb before a rebuild. It might help bring up compression. Either way, for $10 its worth a try.

 

[Khenders]

Re: 15 hp Johnson Compression

What exactly is a 'decarb'?

 

[tashasdaddy]

Re: 15 hp Johnson Compression

i helps if you put the year so we don't have to reference it. here's the correct manual if you decide to tear it down. http://www.outboardbooks.com/index.cfm?fuseaction=models.main&year=1985&ID=121798

the motor is worth repairing if you plan on keeping it.

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 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.

afterwards recheck compression.

 

[Khenders]

Re: 15 hp Johnson Compression

Thanks for the link to the manual.
My understanding of low compression is due to worn rings.
What exactly does seafoam do to the engine to restore compression?

 

[reload]

Re: 15 hp Johnson Compression

Decarbing removes all the carb left from combustion and most any other crud that has accummilated during use. It allows the rings to free up in the grooves so they more closely follow the cylinder walls.

 

[tashasdaddy]

Re: 15 hp Johnson Compression

low compression is also cause from stuck rings, in there seats on the piston, debarbing removes the crap sticking the rings. it's worth $10.00 to try it before tearing into the motor.

 

[Khenders]

Re: 15 hp Johnson Compression

When I add premixed gas back to the tank, do I add it to the seafoam blend or replace the seafoam blend with regular premix? Its a 6 gallon tank and will take quite awhile to burn up...especially if it smokes the whole time.

 

[reload]

Re: 15 hp Johnson Compression

I think if you do it right, you can add the fresh premix on top of the seafoam mix. You are only mixing a small amount of seafoam, then running it for a while. Then you let it sit awile and then run it awhile. Let it sit a gain and change plugs add premix to tank and go.

Initially you add 12 oz to a gallon, run it twice. Lets say you still have a gallon. Now you add 5 more gallons of premix (assuming a 6 gallon tank), now the ratio is 12 oz to six gallons. 2 oz per gallon. This is the maintenance ratio, should be fine.