92 johnson 40 hp low compression

Took my 40 hp in to get a tune since I just bought it last year. Problem was it didn't troll because it loads up at idle in gear. Shop told me I have 125 psi in # one cylinder and 75 # 2. Said possible rings stuck but guessing scored piston causing the problem. Anyway to check either? can I take off the side plate next to cylinders to inspect, don't want to take off plate with spark plugs until needed. Any suggestions??

Re: 92 johnson 40 hp low compression

I would first do a decarbonization. Then repeat the compression test.

I would expect cylinder damage to drop the compression lower than 75#, but carboned up rings can do that easily.

Re: 92 johnson 40 hp low compression

decarb, if not improvement, pull the head, could be as simple as a head or exhaust gasket.

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.

afterwards compression.recheck

Re: 92 johnson 40 hp low compression

Should I disconnect the oil injection for the decarb or just let it run at the extra ratio? Or just disconnect my fuel tank and hook up the pre mixed tank.

Re: 92 johnson 40 hp low compression

if you have a small tank with gas and seafoam use it, the OMS will still mix your oil.

i have a 3 gallon tank for my 3 hp, that works great for this.

Re: 92 johnson 40 hp low compression

Thanks! I do have a small tank I can mix up in. Hopefully this works, I was pretty discouraged when they told me about motor. They made it sound like I had a worst case scenario, and I just bought the boat last year. At least you guys gave me some hope. And some things to try.

Re: 92 johnson 40 hp low compression

If you get a can of Engine Tune from a Johnson / Evinrude dealer it'll have a hose on it. That hose connects to the schrader valve on your primer. Full instructions on the can. Real simple, real easy.

Re: 92 johnson 40 hp low compression

Finally had a chance to try a decarb between rains (that we really need out here in California). Took both cylinders to around 145psi so what a blessing. Found a 4 stroke motor in the mean times so now at least I can feel good about selling the johnson and give someone a great deal since that's what I got on the yamaha. Thanks so much for your help!

Re: 92 johnson 40 hp low compression

decarb does wonders again, congrats on the fix, and the new motor.