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Likely Cause of Low Compression on an Older Johnson 4hp

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bigtime_mcalpine

Regular Contributor
Hi,
Looking at an old Johnson outboard on Kijiji. Owner says motor is in great shape except for one small thing: low compression on cylinders. Can anyone guess on most likely cause of low compression for a motor of this vintage? Owner suspects that rings need to be replaced but suggests that he can’t find new rings anywhere. Wondering if it is much more likely to be something else. Comments/thoughts welcome.
Here is a link to the ad fyi.

https://www.kijiji.ca/v-powerboat-m...p_ios&utm_medium=social&utm_source=ios_social

Thank you!
 
Don't jump to conclusions. Could be a blown head gasket. Or stuck rings. Or overheat damage. Or something flew apart. Hey, you are buying it knowing there is some sort of a problem. If you are willing to take a chance and the price is right, go for it. Otherwise, run.
 
Parts can be found ( new and used ) if you know where to look.-------evinrudeparts.ca-----Might be able to help.----I agree it could be as simple as a blown head gasket.-----If you get this motor you need to do a complete evaluation before spending beer tokens on parts.-----Inspection of splines in crankshaft is a must.
 
Hypothetically speaking if it WAS rings and assuming rings could not be sourced new Could one switch out some used pistons and rings or is that crazy talk?
 
I think it might be possible to put used pistons to the engine. At least i did so, while i made rebuild to my 3hp Lightwin. And engine works really good now. It is good idea to hone cylinders at the same time like i did.
 
Not crazy talk if the used parts are in good condition.-----There are an amazing number of these motors that have few hours on them.-----Northern Ontario season is short.
 
Update: couldn't resist at $150CAD. Motor is now in my living room. It's too cold to drag the motor stand out of the shed but did manage to take a few pulls to check compression on top cylinder. 100psi. I'm thinking that's just fine. Too lazy to check bottom one yet.

Buuuuuut, I may have found the issue right off the bat (or maybe a contributing issue): first sparkplug was quite literally hanging by a few threads. A previous owner stripped most of the threads so that may explain the lower compression when plug is in. Anyway, I'm starting there with a re-thread and will proceed with proper compression check of both cylinders next. Given that I need to rethread, I'm also thinking that I'm now committed to at least taking the sparkplug cover off to clean all metal debris out of cylinder from rethreading.
 
update: top cylinder: 115psi, bottom cylinder 105psi. Thinking this is not excellent but probably acceptable do you guys agree?

bigtime_mcalpine said:
Update: couldn't resist at $150CAD. Motor is now in my living room. It's too cold to drag the motor stand out of the shed but did manage to take a few pulls to check compression on top cylinder. 100psi. I'm thinking that's just fine. Too lazy to check bottom one yet.

Buuuuuut, I may have found the issue right off the bat (or maybe a contributing issue): first sparkplug was quite literally hanging by a few threads. A previous owner stripped most of the threads so that may explain the lower compression when plug is in. Anyway, I'm starting there with a re-thread and will proceed with proper compression check of both cylinders next. Given that I need to rethread, I'm also thinking that I'm now committed to at least taking the sparkplug cover off to clean all metal debris out of cylinder from rethreading.
Click to expand...
 
update #2 (more for laughs than anything else)
Took the carb off last night and gave it a good cleaning. Residue in there no longer smelled of gas...more like paint thinner. All went well. Oh but then I slipped on stairs to basement and carb went flying across room onto tile. Bad news, bent one of the fuel mix brass needle thingy's. Good news, the brass thingy took the brunt of the hit. Great.
 
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