1972 100 HP Johnson piston replacement

[57 7.5]

Ok I've put a lot of time and money into this motor and finally got it running great and then last time out it went from WOT at 37mph to 7mph and felt like I was towing a house with it. Took it home and did compression test and had 120 psi on left side and 120 psi top and 30 psi bottom right. Previously the bottom was 110 psi. Took the head off and the piston was bad (half of one side was burned up leaving a big gap. Hate to scrape the motor after all the work I've done and money I've spent. I know I can't afford a mechanic to tear it down and rebuild that side at like $80 hr. So I have the manual for the motor, how hard would it be to fix myself? I'm not mechanic by no means but can follow directions and you tube video's. Do you have to remove the power head to replace the piston? Thanks for all the great advice to those who have helped me get this thing running. Have had several good times on the water with her and would like to get some more.

 

[Intoodeeep]

I am in the process of finishing building my engine. a Johnson 235hp. Yes you do have to pull it down and take it a part. I am mechanically inclined and have been working on stuff my whole life. Stuff just comes naturally to me. I do think anyone for the most part can do it as long as they are patient enough to take the time to figure out how to do it and can follow the manual. You also need to figure out why you burned a piston up. That the big issue. You will also need to either hone or bore out that cylinder. I would even take that to a machine shop. That's my two cents. My last motor had a problem with both running a little lean I think and a internal water leak in the inner exhaust cover. your setup is a little different. But you will need to figure out what happened if you don't want to cook another piston.

 

[57 7.5]

Thanks, well I've been pretty patient with this motor taking the time to figure out all the issues it's had since I got it. Could it have been the bad pulse pack that caused it? I did find a piece of plastic in the cooling area around that cylinder. Yeah that cylinder wall needs to be honed or bored for sure.

 

[emdsapmgr]

You can replace just one bad piston. Make sure it's from the same manufacturer. Don't mix pistons from various mfr's in one engine. If the piston is a factory one, get a factory replacement. It will expand and weigh the same as the other 3. Good to know why the piston failed. Could be an overheat, and you've noted a potential problem in that area. Also, take the carb apart that feeds that cyl and make sure none of the jets that feed that cyl are restricted. (esp the high speed jet) If you are going to do the work yourself, you will need an original factory service manual. Check out this website: outboardbooks.com

 

[racerone]

Much newer motors are available for little money.------That 100 hp has a complicated / expensive parts ignition system.-----Not worth putting money into it.