The curious bump bump bump....650 Merc question

[sparticus]

No I'm not trying to move a post to the top of the list. Its the symptom of my first trip out with my boat this year. I have a 16' Hydroswift with a mid 60's 650 Mercury. After I had launched and started the engine (it starts with a single turn of the key) and engaged the lever to go forward it would make a bumping action (the motor not the lever) the faster we went the more it bumped. After clearing the bouy for the marina we gave it WOT. This worked ok for a time. After sitting and fishing for a time we decided to move to a different spot, the bumping contniued. In wot occasionally it would even rev up like it wasnt even in gear causing me to rapidly retard the throttle, really got my attention. To me (novice outboard motor owner) it seems like it isn't fully in gear. The lever is harder to move than I remember in the past. Is there any checks that I can do to rule out problems. I am a highly experienced trouble shooter in the aircraft maintenance world and I would like to think that some of that can transfer over to this. Just need to be pointed inthe right direction. I have looked at the prop hub thinking the rubber might have broken loose but it seems to be intact. I have lubed up the linkages inside the motor cover but the throttle lever seems to just as tight as before. Thoughts? Thanks!

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

Would a broken prop hub act like this? Wouldnt it be obvious if you looked in the open area around the inner hub? I see 3 "vanes" that attach the inner hub with the out hub and they appear to be attached although "flexed", somewhat reminiscent of a water impeller. Thanks in advance for those that could give some input!

 

[moorecomp]

Re: The curious bump bump bump....650 Merc question

As an A&P myself, and someone who recently overhauled a 1966 Merc 650, I would recommend disconnecting the shift cable at both ends (you have to open up the control lever box) and see how hard it is to move. It could be not moving fully (backlash in a push/pull cable). One the engine side, just loosen the nut holding the small locking lever and rotate it a little. You can then lift the shifter cable off of a small stud. One of our cables would move when using the control lever, but when we removed it and tried it by hand, it was almost frozen. Found 2 new cables on Ebay for $25/ea. New ones are smooooth!

Craig Moore
Mancelona, MI

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

Thanks for the heads up on the cables. Backlash in the cable has crossed my mind. Not to mention that I have no idea how old the cables are. At this point everything is suspect. That will be my next step.

 

[coastalrichard]

Re: The curious bump bump bump....650 Merc question

You should also check the fluid in the lower gearcase. The fluid should not be milky (possible water intrustion) nor should it contain any shavings (they will stick to the drain plug) which would show excessive gear wear. I recently had a similar issue (bump, bump) - it wasn't pretty!

And, if you do not have a manual for that motor, you should get one! Believe me, you will save more than it's cost!

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

Thanks coastalrichard! I will also take a look at that. When you say...."it wasn't pretty" are you saying that your lower unit was bad?

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

Ok after tonights TS session....throttle cables were disconected and it was nice and smooth in both directions. Reconnected and the stiffness returned. Checked the oil in the lower unit and it looks as clean as the day I put it in there, not a bit milky. So I can deduce that the stiffness I am encountering and hopefully the bump bump bump is in the motor somewhere. Is there a way that when the lower unit was dropped last year for a water impeller change that something happened then? Its the only thing I can come up with at this point that might have changed. Any thoughts from anyone else?

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

Thoughts?

 

[BoobieBouncer]

Re: The curious bump bump bump....650 Merc question

This is a complete guess, but comes from your thoughts about the lower unit water pump chage.

If the water pump wasn't "shimmed" properly could that be the problem?

 

[sparticus]

Re: The curious bump bump bump....650 Merc question

I think tomorrow evening I will drop the lower unit and check the shift shaft. Looking at the area from where the cables operate the linkage is a small bellcrank for lack of better words that goes straight down into the front of the motor. It doesnt look right, it was (and still is) partially out of its housing. I have tapped it back over the area that it appears should be its proper place but it won't drop. Its obvious that its displaced by the markings on the bearing surface showing twisting movement. I suspect the impeller change was what caused this. As much as I really don't want to drop it (really awkward to handle) I feel thats the next step. If I drop it and the throttle is still hard to move then it will explain it all. I can never have anything thats normal.