1978 25 hp Johnson misses and shears woodruff key

[78evinrude]

I just bought a 1978 25 hp Johnson that has a problem I hope someone could kindly identify for me. At idle the motor runs fine. Half speed it misses every 30 seconds, at fast speed it misses every 15 seconds. Even with the lower unit locked in, it hits the transom hard, and has sheared the Woodruff key between the flywheel and shaft. It runs fine otherwise, I put new spark plugs in and cleaned the contacts on the coils. I'm thinking it must be the coils, cdi box, or propeller hub. I can't see how it is the timing because it runs fine otherwise, I can't find out how to time it either. It is unlikely the propeller hub because it seems that once that fails it wouldn't recover. Thanks for any help, since it is an old motor I'm thinking someone has had the exact problem. Here is a link to a video of the problem http://courtneychapinceramics.com/cc.../evinrude.html

 

[Chinewalker]

Slamming the transom sounds like a clutch dog issue, not an ignition issue. The cog that drives the forward or reverse gear wears out over time to the point where it slips off the gear. It is somewhat spring loaded into gear, so when it slips out, it slams right back in again. Stop running it until you get it repaired, as it may take the forward gear with it, if it hasn't already.

 

[jimmbo]

Sounds like a clutch dog. If original, it lasted almost 40 yrs. That's a long time for that design

 

[78evinrude]

Thank you both for your responses, I appreciate it, I'm going to go start turning wrenches thanks to your inputs, it will take a couple of days but I will post the results, thanks again.

 

[78evinrude]

I ran the motor without the shift cable attached, and the motor would slip out of gear. The clutch dog showed very little wear, but I guess it is enough? I ground down the gear so I could fit a cotter pin in the shaft to keep it in gear, will test it next summer (this might not be one of my best ideas). Thanks again for the posts.

 

[Silvertip]

Shearing a flywheel key can also be caused by the nut not being torqued to the required spec.

 

[F_R]

Most people look at those clutch dogs and gears and see nothing wrong. But it takes surprisingly little wear to make them jump. By your original posting, it is clear that is the problem though. ALSO--it should not be shearing flywheel keys. That is another problem.

I am amused by your "fix" for the dog. Can't wait for summer to hear the results:watermelon::laugh:

 

[oldboat1]

Don't think you want to run it with that fix.

I was a little curious about the spray coming off -- was thinking the motor might be too high on the transom, but guess it could be something on the hull -- transducer, maybe.

 

[boobie]

Could you post a pic of that "fix" ??