4.3 - Alpha 1 loud clunk

[anthonylind]

Have a 1995 Larson with the 4.3 Mercruiser/Alpha 1. Bought last summer and we've used it a handful of times and loved it. Last week when out pulling a tube I punched it to start pulling a flat 2 person tube with 2 teenage boys on the tube. Almost immediately it sounded like I hit a rock. Killed it and checked everything out and can't see any visible signs. The same sound happens consistently inconsistent if that makes sense. It's not a grinding noise. It's a loud clunk and I can feel the vibrations in the floor of the boat. Doesn't happen as much at low speeds/rpms. I was able to get the boat loaded back up just fine. I cannot replicate in my drive way using muffs so it only happens when under load. I was loaded when the sound happened. I think I had 7 people in the boat plus the 2 on the tube. Any help would be greatly appreciated. I can't believe the knowledge on this forum and all the help you've been in the past. The only thing that crossed my inexperienced mind was it sounds like its momentarily coming in and out of gear. I don't seem to lose power as its just a short (less than a second) clunk that seems likes its coming from the lower unit.

 

[AShipShow]

Hmmm nothing jumps out to me... Maybe the experts will chime in but ill give my 2 cents...

Sounds to me like it could be a few different things... ever checked/ changed u joints and gimbal bearing? Also, you may want to watch achris' video on shift cable adjustment.. Maybe the dogs are slipping out of gear under load?

 

[anthonylind]

I did watch a video on shift cable adjustment and did that yesterday. Hope to take it out this morning and see if that fixed it. From what I have read I don't think it is the gimbal bearing because it isn't a constant noise that gets worse when I accelerate. And it doesn't get worse or happen when I turn the wheel so it shouldn't be the u-joints. Hoping someone else has an idea. I also emptied the oil in the lower unit and I haven't found any shavings or debris.

 

[alldodge]

It might be the dog clutch slipping and then locking in. The cable adjustment was a good idea. Suggest draining the lube, let it sit for a few hours in the pan, then slowly pour it out to see if any metal is in the bottom. Could also use a magnet and run it thru the lube to see if metal is id found.

 

[anthonylind]

No luck on the shift cable adjustment. Just took it for a short spin this morning and the same thing. It seems to happen around 2000 rpm and up. When it tarts to happen it does seem to be a consistent/rhythmic thing. Maybe every second or two. Could a tooth be out on one of the gears in the lower unit and its just a matter of time before something major blows up? Or if something is wrong internally in the lower unit I would here it when I run it with muffs in my drive way. Shaking my head on this one and need some expertise. Worn shift cable?

 

[alldodge]

Sounds are hard to determine over the net. With it happening every second or two do you think it would be slop in the steering arm, any kind of left or right turn?

 

[anthonylind]

It was happening when I was going straight....no turns. Did not try making a turn once the noise started. To scared something major was going to blow up and I would be stranded in the middle of the lake! I understand the bit about diagnosing noise over the internet. Should have done a bit more testing this morning I see now. Should have trimmed up to see if the noise was still there. We are going camping today and I don't think it's going to be ready!

 

[anthonylind]

Took back out again and it doesn't matter whether we are turning or going straight. It does not happen in reverse. Happens when full trim down all the way to trimmed up and almost out of the water. I don't think my prop is that bad but could something be wrong with that? I put my hand on the top of the drive when we were out in the water and when it jolts it definitely bounces the whole lower unit up and down slightly. Like I just grazed a rock. I'm about to just pull the whole lower unit and see if I can see any problems. The whole engine jolts slightly when it happens. I've replaced an engine coupler on a 165 and I hope thats not the problem.

 

[alldodge]

Doesn't sound like the coupler, but does sound like the dog clutch is slipping in the drive. Drain the lube and check for metal

 

[anthonylind]

I've already drained the lube in the lower unit and I'm still in the process of trying to filter it through a paper towel. Still no metal shavings and I ran a magnet through and nothing.

 

[anthonylind]

Can I remove the lower portion of the lower unit without removing the whole lower unit from the boat? That's where that dog clutch system is correct?

 

[harleyman1975]

Sounds like it is coming out of gear to me as well.

 

[alldodge]

Can I remove the lower portion of the lower unit without removing the whole lower unit from the boat? That's where that dog clutch system is correct?

Yes, its easy to do, but you cannot see the dog clutch without disassembly of the lower unit, and reinstalling the drive is easier if the whole drive is removed.

Hey achris and Bt Doctur any thoughts?

 

[anthonylind]

So is the dog clutch system in the upper half or lower half? Trying to find a good repair manual or video on this system and I'm not having any luck.

 

[alldodge]

Lower and you have a Gen 2

See shift spool item 40
http://www.mercruiserparts.com/bam/subassembly/30858/1017/90

The rest of the lower unit
http://www.mercruiserparts.com/bam/subassembly/30858/1017/80

Manual is at Boat info No 14
http://boatinfo.no/lib/mercruiser/manuals/mercruiser14.html#/0

 

[alldodge]

Many folks with Alpha drives put the drive in gear slowly, when it should be quick. As its starts to go in gear, you hear the gears clipping (grinding) until it is in gear. Doing this will round or chip the corners on the clutch. In the pic below it shows how to inspect

 

[anthonylind]

So I messed up on the year. Mine is a 90 not a 95! Does that mean it's a gen 1 instead of a gen 2? Thank you for all your help. Had to leave it at home for the weekend and get to our campsite.

 

[alldodge]

Being a 90 it appears to be a Gen 1, that said the issue would be the same.

 

[QBhoy]

Probably mentioned above somewhere, but is the rpm drop/interrupt micro switch working as it should ?

 

[achris]

Drain the oil. Pull the top cover off and look for broken teeth on either gear. If the problem is not there pull the drive off and split it. Check the splines in the bottom of the upper drive shaft. Still no problem, you're into pulling the lower housing apart.

Chris