Honda outboard mechanical bump

[markot]

I have a 1982 Honda BF75 that my father bought new in 1982. It is a great engine and has only had minor issues over the past 30 years. This past season, (we use it on a 10' rubber Zodiac boat), it developed a mechanical jerk that happens several times a minute at the exact same time when the boat is running for the most part wide open under load. I am very mechanically knowledgeable and I am 99% sure this is a mechanical jerk and not an ignition or fuel cut out. I read a post in this forum from some years ago and someone had this exact problem with another Honda engine but there wasn't really an answer as to the cure. They described a bump sensation every 15 seconds under load only.

I pulled the lower housing apart and the prop, sheer pin, bevel gear set and engagement dogs look fine. I had a problem with a 1974 Honda CR250 dirtbike popping out of gear years ago due to worn gear dogs and they were visibly worn- these look fine. Likewise the prop hub is tight.

Any ideas? Maybe something I am not seeing? Of course it's also only in forward and like I said under full load. I may try locking the prop and applying torque to the input shaft in various positions. I hate things like this....

Thanks for any help!

 

[pvanv]

Re: Honda outboard mechanical bump

Confirm that the shifter is properly set. If you are not 100% in gear, that could happen.

 

[markot]

Re: Honda outboard mechanical bump

Good point- since I had to pull shifter linkage apart anyway- I usually don't expect anything to be simple so I sometimes neglect to check the obvious possibilities- thanks

 

[markott]

Re: Honda outboard mechanical bump

Good point- since I had to pull shifter linkage apart anyway- I usually don't expect anything to be simple so I sometimes neglect to check the obvious possibilities- thanks

I wanted to post about my own repair. I cleaned and closely examined the drive gears in my Honda BF75 outboard that was experiencing a mechanical bump noise. The lower unit of course has forward and reverse bevel gears and an in-between drive cog that is locked to the drive shaft. With the shifter in forward, the drive cog is released and is spring loaded and meshes into the forward drive bevel gear. The shifter is not holding drive cog in place- just releasing it. For neutral, the shifter pulls the cog to a center position where it spins freely. In reverse, the shifter pulls the cog, and holds it in a meshed position with the reverse bevel gear. There are two dogs, or fingers that the gears lock themselves together with when meshed. The forward locking dogs where very slightly rounded on the drive edges and what was happening was in forward under load, the gear dogs would work their way apart at the same interval each time. As soon as they did, there was a 1/2 or 1 revolution slip until the spring force would cause them to re-engage and start driving again. This was happening constantly and at the exact amount of time in-between.

Instead of just replacing the involved gears, I used a Dremel tool and small grinding wheel to re-square the drive edges of the dogs. The engine now works as perfectly as when new.

 

[MattFL]

Re: Honda outboard mechanical bump

Great repair! Did you check how the dogs contact the gear to be sure they contact at the same time after filing them down?


Also I am really surprised you were able to open the case on a motor that old, was it used exclusively in fresh water?

 

[markott]

Re: Honda outboard mechanical bumpt

Re: Honda outboard mechanical bumpt

Great repair! Did you check how the dogs contact the gear to be sure they contact at the same time after filing them down?


Also I am really surprised you were able to open the case on a motor that old, was it used exclusively in fresh water?

My Honda out board gets put in water a few hours at a time and stored dry. Where we use it is the upper Chesapeake so it's low salt at most. The drive comes right apart as I service it and recently replaced impeller and gaskets- it's really in great shape. When I ground the gear dogs I was very careful about checking the contact surfaces of the drive edges- not to mention that it was a very small amount of grinding to square the edges. I have ridden and raced dirt bikes since I was 11, (now 53), and I used to obsess with examining engines and wanting to understand how things worked to the point of my mother asking me why can't you be like a normal boy- then I went through the same thing with cars later in life. Now I endlessly explain things to my 16 year old son. The thing I really admire are the people that design and create engines and transmissions or outboards from an idea!