1979 Evinrude 15 hp thump... Help

[Elliomcmh]

Hi,
I have a 1979 Evinrude 15 hp short shaft, tiller steer motor mounted on my duck boat. Haven't had any problems with this engine since I bought it several years ago. Two days ago, I was backing up while trailering the boat and I hit a curb, with the engine lowered, which scraped the keel of the motor and also jarred it pretty good. There wasn't much noticeable damage at all.

When I put the boat in the water, 45 minutes afterwards, everything worked fine at idle speed. As soon as I hit full throttle, I heard and felt a violent "thud" like I hit a log. It continued doing this every 5-15 seconds until I backed down to 1/4 throttle. The motor doesn't sound like its slipping, and the engine doesn't rev up at all when it happens. Also, the motor stays in forward gear the whole time this is happening. Hitting a log is the best thing I can compare this to.

I drained my lower unit oil, and although it was blackish in color, there were no metal flakes. I pulled the lower unit and the vertical drive shaft splines looked fine. The prop shaft didn't appear to be bent either. I pulled the piece that holds the prop shaft in place which let me access the gears. I'm no expert on outboard motor gears, so i didnt remove them, but they didn't look bad, no missing teeth or anything like that.

I did notice that If I put pressure on the shifter with my hand, while I'm driving the boat (past the normal position that the shifter rests while in the forward position), the motor runs great and does not "thud" at all.

Sorry for the long post, but I just wanted to make sure that I included all of the info I had. Thanks in advance!!!

 

[Bosunsmate]

Re: 1979 Evinrude 15 hp thump... Help

Hows the clutch dog look?
You could try adjusting it so its more in forward when in gear

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

At the time, I didn't really know what I was looking at, I was mainly just looking for broken gears or something extremely obvious. I wish I would have posted this thread and then taken the lower unit apart, so I would have known what to look for.

 

[steelespike]

Re: 1979 Evinrude 15 hp thump... Help

I'm wondering if you broke a motor mount.I believe that can throw off the shift linkage.
Its odd for a motor to go from normal operation to severe clutch dog failure over night so to speak.
Its also possible the shift linkage components were damaged.

 

[metriccrescentwrench]

Re: 1979 Evinrude 15 hp thump... Help

I'm wondering if you broke a motor mount.I believe that can throw off the shift linkage.
Its odd for a motor to go from normal operation to severe clutch dog failure over night so to speak.
Its also possible the shift linkage components were damaged.

That makes more sense than anything else. If nothing else something in the shift linkage was bent.

 

[Joe Reeves]

Re: 1979 Evinrude 15 hp thump... Help

(Jumping Out Of Gear - Manual Type)
(J. Reeves)

This pertains to lower units on all OMC manual shift outboard engines, or any OMC engine with lower units defined as a Shift Assist or a Hydro Electric Shift unit which incorporates a "Shifter Clutch Dog".

Within the lower unit, splined to the prop shaft is what is most often referred to as a clutch dog, hereafter simply called dog. The dog has at least two lobes protruding from it on both ends, facing both forward and reverse gear. The forward and reverse gears also have lobes built into them near their center area. When the engine is running, in neutral, the gears are spinning constantly via the driveshaft being connected directly to the powerhead crankshaft, but the propeller does not turn due to the fact that the dog is centered between the two gears, and the dog lobes are not touching either of the gear lobes.

When the unit is put into either gear, shift linkages force the dog (and its lobes of course) to engage the lobes of the gear. The lobes of the spinning gear grab the lobes of the dog, and since the dog is splined to the prop shaft, the propeller turns.

The lobes of the dog and gears are precisely machined, most with right angled edges that could be installed in either direction, and some with angles slightly varied that must be installed in one direction only (one end only must face the propeller). Dogs that can be installed in one direction only, if reversed, even if the dog and both gears were new.... would jump out of gear almost immediately. Keep in mind that the lobes are precisely machined with sharp angles!

Due to improper adjustment or worn shift linkages, but usually due to improper slow shifting, those precisely machined sharp edges of the lobes become slightly rounded. Now, with those lobes rounded, as the rpms increase, the pressure of the gear lobes upon the dog lobes increases to a point whereas they are forced apart (jumping out of gear), and due (usually) to the shift cable keeping tension on the engines shift linkages..... the unit is forced back into gear giving one the sensation that the engine has hit something, and the cycle continues.

Some boaters with manual shift engines have the mistaken belief that shifting slowly is taking it easy on all of the shifting components..... Wrong! Shifting slowly allows those precisely machined sharp edges of the dog and gears to click, clank, bang, slam against each other many times before they are finally forced into alignment with each other..... and this is what rounds those edges off! The proper way to shift is to snap the unit into gear as quickly as possible.

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

Thank you so much for the quick replies!! I will attempt to check the condition of both the clutch dogs and the motor mounts today.

When I disassembled the lower unit, I was only able to get as far as removing the reverse gear, so I could only see the clutch dogs that connected with the reverse gear. I wasn't sure how to remove the driveshaft pinion gear which I guess allows me to access everything else including the clutch dogs and the forward gear. Can you give me any advice, links, or tips on how to remove all of this?

As for the motor mounts, I believe there is a lower and upper set??? I have included a picture of them, with what I believe to be the upper mount circled in red, and the lower mount circled in green. There is paint on the upper mount which is why it appears to be cracked, but no chunks appear to be missing from the rubber. And as far as what I believe to be the lower mount, there is a large amount of "play" in it, when I move the motor side to side and front to back. I really don't know much at all about motor mounts, and are these even the motor mounts in question? Any advice?

******I also forgot to mention, everything seems to work fine when I put the "ear muffs" on the engine and run it in my drive way with the water hose connected. It does not slip out of gear at all..... It only seems to slip out of gear while the boat in water*******

Pease don't hesitate to ask me any questions or give any other ideas, thanks for all of the help!!!

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

Also, if the problem is the clutch dog, can you reverse the dog on this model motor, to see if that is the problem?

 

[boobie]

Re: 1979 Evinrude 15 hp thump... Help

Some dogs you can and some you can't. Some dogs had a different cut for the reverse gear.

 

[Joe Reeves]

Re: 1979 Evinrude 15 hp thump... Help

Even the shifter dogs which appear that they could be reversed, they cannot be reversed without affecting shifting performance.

These would have the message "Prop End" imprinted on one end of the dog, the message of which is obvious... OR... there will be machined score lines cut into the forward end that must face forward gear.

 

[Bosunsmate]

Re: 1979 Evinrude 15 hp thump... Help

To remove the pinion I use a plumbing wrench to hold the driveshaft (with a bit of cloth to stop the shaft being damaged) and i undo the pinion nut with either a socket or if the motors smaller i use a ring spanner. Dont use the claw end as the nut is on tight so the nut will end up rounded if you dont get the right fit.
Im not sure about motor mounts, you may want to wait for someone to answer that first before you try removing the gears

 

[the machinist]

Re: 1979 Evinrude 15 hp thump... Help

As for those motor mounts, the uppers would be the weakest. They are buried below your red circle in the photo, & one of the two attachment screws per side is the Phillips that is showing just below your red circle. To test it, with the motor on the boat, try to push/pull the powerhead forward or rearward. If you get any movement, that may be your problem.

Also you might want to check for a bent prop shaft.

 

[Joe Reeves]

Re: 1979 Evinrude 15 hp thump... Help

The fact that the engine "thuds" every 15 seconds or so when power is applied indicates a shifting problem, not a m/mount failure, HOWEVER.... if the engine DID NOT have this problem before backing into that curb, it is more likely that the problem IS NOT a damaged dog and/or gear but rather a shift linkage problem (not engaging properly).

Did this problem exist any at all, just not as often before hitting that curb?

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

Awesome, thanks for the replies. I am finally off work today so, I'm about to start the disassembly and hopefully find the problem. Does anyone have any advice as far as where to purchase or view a Downloadable service manual? There are no marine stores near me and I would like to start working on this today.

Joe Reeves- this problem Didn't exist at all before I backed into the curb.

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

There was no play whatsoever when I wiggled the top end of the motor around. But there was A LOT of play at the lower thrust mounts when I wiggled the lower end of the motor. There has always been some play there, but I just can't remember how much

 

[Bosunsmate]

Re: 1979 Evinrude 15 hp thump... Help

Awesome, thanks for the replies. I am finally off work today so, I'm about to start the disassembly and hopefully find the problem. Does anyone have any advice as far as where to purchase or view a Downloadable service manual? There are no marine stores near me and I would like to start working on this today. .

Boatinfo - 1971-1989 Evinrude Service manual

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

Boatinfo - 1971-1989 Evinrude Service manual

Thanks for the link to the service manual, it definitely helped with the disassembly.

 

[Elliomcmh]

Re: 1979 Evinrude 15 hp thump... Help

I disassembled the lower unit today and found a little chunk missing out of the clutch dog that contacts the forward gear. Do you think that this could be the cause of my problem? Here are the pictures of the forward side of the clutch dog and the forward gear. The reverse dogs and gear looked fine.









Also, I wasn't sure how to access the top motor mounts, but here are some puctures of the lower mount. How do they look? And should I worry about inspecting the top mounts?

 

[Joe Reeves]

Re: 1979 Evinrude 15 hp thump... Help

In all of my 30+ years experience, I have never seen a shifter dog damaged such as that one regardless of how the lower unit was hit.

Yes, that would definitely be the cause of the jump thump scenario.

 

[boobie]

Re: 1979 Evinrude 15 hp thump... Help

Agree, that dog is really bad!!