25 hp Johnson 25R71S, Exhaust in midsection?

[Catwagon]

I posted a few days back about a 25 horse Johnson (25R71S) I got that supposedly had a failed impeller and just quit due to overheating and low compression (50/60 psi). The cylinders were just as pretty as they could be, and the motor wasn't locked up.<br /><br />Original post <br /><br /> Well, I started taking it down today and found something that I suspect isn't supposed to happen. I started the dissassembly with the bottom end since I was told the impeller was ruined and I suspected it myself since the paint on the midsection was discolored like it had been very hot. I dropped it to get access to the impeller and immediately saw that everything inside the midesection had a deep layer of carbon caked on it like it had been subjected to pure, hot exhaust. In the deep cavity behind the impeller housing was a deposit about an inch deep in loose carbon particles and the shift rod and driveshaft were caked as well. The inside of the access cover where you separate the shift rod sections was black as coal as were the two retaining brackets on the shift rod. On top of the impeller housing is what I think used to be a rubber sleeve where the water tube joins and goes up into the engine with the cooling water. Well, that rubber sleeve is burnt to carbon and very brittle and hard. I'm assuming that's not a good thing, huh?<br /><br /> Now on to the impeller. I took that apart of course and when I opened it I saw what looked like a brand new rubber impeller with only a couple of hours on it. There was no melting, scarring, missing areas or anything else on it to suggest that it had failed. I was also careful to check to see if it had been properly mounted on the keyway after reading several posts here and there were marks suggesting that it had been rotating normally.<br /><br /> I'm just guessing here, but did a gasket let go on the exhaust system somewhere and then dump hot exhaust into the midsection where it wasn't supposed to be and cause the engine to overheat and quit? If that is what happened, did the exhaust pressurize the midsection forcing the lakewater out and the impeller to starve for water to cool the powerhead?<br /><br /> Teach me folks, I'm trying.<br /><br />Steve

 

[R.Johnson]

Re: 25 hp Johnson 25R71S, Exhaust in midsection?

On your engine there is an inner exhaust tube, or what may be called an exhaust mega-phone. This may be broken at the top of the exhaust housing where it is held in place by four screws. I have found this several times on this size engine from that period. You have to lift the power head to check this. That is not as diificult as it sounds. Disconnect the throttle, and shift link, along with the fuel hose. Take out the hold down bolts from the underside, and the power head lifts off. Many times, this size engine is run way over propped, and is sevearly lugged. That is where all the carbon comes from. If you find the above conditions, and make the repairs, prop the engine to run at about 6000 RPM. Using OMC carbon guard mixed as to the directions will clean a lot of that carbon out of the engine. My son, and I run one of these engines on our 14 Ft. flat bottom fishing boat. I use a 9 x 9 prop to get it up where it belongs. This has made it a very reliable, strong running engine for it's size. We use carbon guard with every fuel mix, as in all of the engines we run.