71 Johnson powerhead options?

[Catwagon]

Hi everyone,<br /><br /> New member here and I really appreciate your forum and all the great folks that have been helping others get their problems sorted out.<br /><br /> I have a problem of my own to ask about. I was just given a '71 25 hp Johnson (25R71S)that fried itself due to a failed impeller. The motor is pretty nice and fine shape cosmetically and I hate to see it junked. I'd really prefer to hang it on the back of my Grumman 4.4 at some point if at all possible.<br /><br /> Here's question one, The powerhead isn't locked up and still has 50 and 60 psi compression on the cylinders. It appears to have cooked a seal(s) somewhere since there's a light coating of oil all over the engine and it was thick enough in places to have actually run down the sides. When an engine is in this shape is it possible to save it?<br /><br /> Question two. If it's not saveable or just too big of a job what other powerheads other than another 25 will fit this chassis? I've noticed that with Johnnyrude there seems to be quite a range of horsepowers availible from certain families of shortblocks.<br /> I'm not really particular and I wouldn't care if I had to go down to an 18 hp or up to a forty. I just don't know what engines out there will bolt up so I can get this girl up and running so she doesn't go to waste.<br /><br />BTW, I do have a service amnual on order but I haven't recieved it yet.<br /><br /> Any thoughts or suggestions for this newbie?<br /><br /> Steve

 

[R.Johnson]

Re: 71 Johnson powerhead options?

I would start by removing the cylinder head, and see what the cylinder walls' look like. This is really a very basic engine, and would be a good one to start with, for someone who has never rebuilt an outboard. Doing the work your self, with the aid of the manual, and the help you can get here, you could end up with a very good fishing engine without spending a whole lot of money. It would be easier to tell you what parts will inter-change when you let us know what you need in the way of replacements.

 

[Catwagon]

Re: 71 Johnson powerhead options?

I removed the head earlier today and was surprised to see the cylinder walls in the shape they were.<br /><br /> Each cylinder had one very small area each where they were scored ever so lightly and nothing else. If you closed your eyes and ran your finger accros them even then they were pretty hard to detect. In addition, there were no ring grooves, scuffed areas, nothing. You could even see the factory cross hatching most of the way around the circumfrence of each cylinder.<br /><br /> I know for a fact that the impeller failed because it got so hot that it badly discolored the paint on the outside of the mid section and also the paint on the head is badly discolored.<br /><br /> Growing up and seeing some of my buddies' 2 cycle motocross engines when they were in bad need of rebuilds made me expect something far worse than what I saw on this Johnson. Is what I saw still bad news or have I accidentally lucked out on this motor?<br /><br /> Is it possible that something else actually caused it to stop first and prevent more damage like the diaphram in the fuel pump and the crankcase seal melting due to the heat? I'm pretty clueless on 2 cycle outboards so far but I'm learning.<br /><br />Steve

 

[Solittle]

Re: 71 Johnson powerhead options?

Steve - I would be tempted to put a new water pump and thermostat in it and give it a try. You will of course need a new head gasket.<br /><br />If it runs OK you might try another compression check with a different gage. I would suspect that the compression gage is not reading accurately. What is interesting is the closeness of your readings. I would suspect that if you did some serious damage that the compression readings would differ widely.

 

[itstippy]

Re: 71 Johnson powerhead options?

The world is full of OMC 18-20-25HP, 22 cubic inch motors that have overheated and blown head gaskets. There is no temp guage or warning horn/light so when the impeller goes blooey and things get real hot in a hurry there's nothing to let the operator know unless they're "in tune" with the motor. It is not good, of course, but not uncommon. If you got 50 and 60 PSI compression without replacing the head gasket, and the cylinders look like you say they do, I second SoLittle's advice. Those things are tough. Clean it up, put it together, put it in a barrel of water, fire it up, and watch real close. <br />By the way, the world is also full of 18-20-25 OMC powerheads.

 

[Catwagon]

Re: 71 Johnson powerhead options?

I'll be happy to do like you both suggest and order the parts to do it, but aren't the 50 and 60 psi compression readings awfully low? I've been doing searches on this site using "compression" as the search word and from the posts I've read it seems that anything under 90 psi is sort of low for an outboard. When they're healthy and running normally what compression range should these 25 horse motors be operating at?<br /><br /> (I'm not trying to be argumentative or anything at all, just trying to learn my way around these little 2 cycle motors better.)<br /><br />Steve

 

[itstippy]

Re: 71 Johnson powerhead options?

50 and 60 PSI is way low. The motor overheated and now the compression's shot. We all hope it's the head gasket that blew. An easy and cheap fix. When you do a compression check on a motor with a blown head gasket you get low readings. The pressure leaks through the bad gasket. Replace the head gasket and the readings go back up. That's what we hope for; cross your finger and go for it.

 

[itstippy]

Re: 71 Johnson powerhead options?

They've got head gaskets and impellers and t-stats here in the iBoats MegaMall. Sierra parts are fine. iBoats is reasonable cost for parts and shipping. It is worth the gamble, in my opinion. <br />Really study the throw pattern of the oil slick on the outside of the engine. Can you tell where it was throwing from? What did the gasket look like when you pulled the head - any obvious signs of damage?

 

[CATransplant]

Re: 71 Johnson powerhead options?

Do the head gasket replacement and check the compression again, as folks have suggested. If it's still low, then you can rebuild the motor...or...check with an outboard junkyard like tcoutboard.com and ask them what they want for a powerhead with good compression. <br /><br />As someone pointed out, there are literally thousands of these 25hp outboards out there. Some have blown powerheads; some have blown lower units. Some are just hanging in someone's garage needing a minor repair to go back into service.<br /><br />If yours is nice, cosmetically, it's worth fixing, in my opinion. They're good outboards.

 

[Catwagon]

Re: 71 Johnson powerhead options?

Itstippy,<br /><br /> In response to your question it appears that oil was slung evenly all around the engine compartment except that there was none at all on the upper or lower flywheel. For that reason I'm assuming that maybe the seal on the upper crankshaft failed. It looks like the interior of the engine compartment was lightly "fogged" with oil everywhere and there's also a small amount of clear, thin oil sitting in the bottom of the lower engine cover. I haven't pulled the flywheel to visually confirm whether the upper seal is melted though.<br /> <br /> The head gasket seems to be in nice condition overall except for one little spot. In this spot the gasket material in the narrow area between the cyinders looked pulled out of one of the metal gasket rings towards one cylinder. The sealing rings themselves looked fine but I know that can be deceptive sometimes.<br /> <br /> I forgot to mention this earlier, but the ceramic insulators on the plugs didn't appear to be burned or discolored like I would expect to see on an auto engine that had overheated. It may not matter but I just thought I'd mention it anyway.<br /><br /> I have a head gasket, upper seal, impeller and a thermostat on the way. When I get it all put back together I'll fire her up (In plenty of water!) and then report back to you kind folks what I find. <br /><br /> Steve