OMC stringer thump sound from the rear.

[Serf27]

I have heard a thumping noise from the rear and I think I felt it too.
It’s only happened 3 times and it happens fast enough to where I can’t pinpoint what’s really going on.

Best I can describe it as is:
The thump/feel feels like when you’re lowering/tilting the drive into position until you feel a slight tap against the intermediate housing, knowing it’s all the way down.
That’s what I feel/hear.

When it has happened, 3 times.
When on plane, at about 3k rpm, I will ease off the throttle for whatever reason and then give it throttle and this is when I hear/feel it.

I have read it may be a worn dog clutch or the propeller?

The shift cable was installed as the manual says to do so, neutral and all other adjustments that where in the manual where done, fluid in upper/lower/intermediate are all new.

I haven’t drained the lower drive oil yet to check for metal shavings.

Could it be a prop issue since I THINK it is happening when slowing down for a second and then taking off?

The drive shaft between the upper and lower look, good, no missing teeth/splines.
No noise when cruising on plane or when shifting in/out of gear or in wake zones.

Thanks.

 

[Serf27]

I cannot edit the post anymore, but here is the lower drive drain plug removed after about 3-4 hours or usage time.
The drain plug went in clean the other day when I added new fluid.

 

[kenny nunez]

From what you have described it appears to be the tilt clutch, has anyone had it apart? If so and it is the early style where a thick snap ring holds the pack together the ring is not seated. The ring can only be properly seated while the pack is compressed either in a vise or a shop press.
If there is a socket head Allen screw under the plastic plug in the middle of the tilt clutch shaft then simply tighten the screw 1/2 turn.

 

[Serf27]

Thanks Kenny.
Yes, my original thought was that the tilt clutch is allowing some slip and letting the complete drive raise and then come back down.
I rebuilt the tilt clutch packs 2 years ago and everything has been working as far and up/down motion.

I also thought that maybe the Allen head screw just needs to be tightened down a bit.

But if this were the case, wouldn’t the drive just keep raising itself up?
Since rebuilding the clutch pack, the drive has always been up and it’s never sagged down over the past 2 years. But I just started using the boat, so maybe the clutches have worn down in the few times I’ve been on the water and the screw needs tightening?

On land, with the drive tilted half way down, I can tilt the drive by hand maybe like 1/4” or less. Should it not move at all by hand?

 

[kenny nunez]

You should not be able to raise or lower the drive by hand. First try a 1/4 turn first, if it still wants to thump give it another 1/4 turn.

 

[Serf27]

I cannot raise it or lower it, but the forcaes under water may be stronger than I am.
It is more like some play it has.

Here is a video of the play I’m talking about.

I will try tightening the screw.

 

[Serf27]

Kenny, I tightened the screw and the play is still there.
When the drive is all the way seated, there’s no play, but if I click the tilt up once very quickly, the play is there.

Here are photos of the tilt gears and in which position they are in when the play begins.

 

[southkogs]

Drive should be all the way down anyway, right? You don't trim the Stringer from the tilt.

 

[kenny nunez]

When the drive is in any position other than full down or full up it is normal to be able to feel the slack in the quadrant to lift gear . Also the worm drive to clutch pack gear clearance and there may be some clearance on each end of the worm gear.

 

[Serf27]

Yes Southkogs, the drive is all the way down when I’m on the water and the trim adjustment is made with the trim motor.

Kenny, so the play I feel when the drive is not fully seated is normal.
Can we rule out that the drive is not raising and causing the thump I feel?

What else can I look at? Propeller bushing?

 

[kenny nunez]

Try tightening the set screw in the tilt shaft.
Also check all the engine mounting lag screws in the stringers.
Is the propeller tight? It should not be able to slide on the shaft.

 

[Serf27]

I tightened it all the way. I need to loosen it and readjust it.
If this screw is too loose, it would cause the clutches to slip and the drive to rise up when driving the boat?

The mount screws are tight.
The propeller is “that feels tight” tight. I did not torque it down. It cannot slide back and forth and the nut has not came loose in 3 hour drive time.
The prop shaft has the thrust washer, prop, splined spacer, and the nut/cover/pin.

 

[southkogs]

Does the "thump" happen exclusively when you slow way down, particularly when coming off plane?

 

[Serf27]

No it does not.
It’s happened 3 times, maybe even only 2 times and it is quick so it makes it hard to pinpoint the thump.

Coming off plane and slowing for no wake zones, there is no thump.

Getting on plane, there is no thump.

Cruising on plane, there is no thump.

Slight Turning on plane, there is no thump.

Moving at idle speed in no wake zone and giving it throttle to speed up a bit, there is no thump.

One scenario where I think it happens is:
I am on plane, I ease off the throttle because of a big wave or I’m turning and then I push the throttle forward again. All this happens in about a second and the rpm drop from plane is about 2-300 rpm and when I give it throttle, it thumps and then everything is fine.

A second scenario is:
I am on plane, I ease off the throttle for the same reasons, a big wave or I’m going to turn but the rpm drop may be more than 300 rpm and then I give it throttle a little bit faster than the first scenario and it thumps then all is fine.

Could I be giving it throttle too fast and something is thumping because of the change of rpm/force?

That is all I can think of for now until I can recreate the thumping.
When I am getting onto plane, I slowly ease the throttle forward until I am 2500/3k rpm so there is no noise there.

 

[kenny nunez]

I used to have a OMC tool that after removing the quadrant gear it fit over the small gear, the other end had a 3/4” square hole that a torque wrench was inserted i at a right angle. This was how the “breakaway” torque was checked. So in your case just loosen the set screw until you can raise or lower the drive by putting pressure on the drive. After that turn the set screw until you start to feel resistance then 1/2 turn more. Increase 1/4 more if the drive still “thumps” on decelleration until it stops doing it.

 

[Serf27]

Thanks for the tip.
I initially did something similar when I set it up.

I tighten the set screw until I could force the drive up or down, I then tightened it until the tilt button would move the drive up or down.
Set screw too loose, motor just spins.
Screw too tight, drive doesn’t tilt.

I will try it your way and adjust accordingly to see if the thumping stops.

 

[southkogs]

Some (admitted) long shots: how bad/good are your ball gears? I'm wondering if you've got a little extra play on them and when you change RPMs quick, you feel a "hit" from them re-engaging.

Might also take a look at the drive shaft between the upper and lower gear case. If it's starting to wear, maybe you're skipping on it a little bit?

 

[Serf27]

The ball gears are not the best but I’ve seen them in worst condition and people mentioned they still had life. So I am assuming if mine look better than the ones I’ve seen, they are fine…?

I looked at the drive shaft when I replaced the lower case. All looked fine. I plan to remove it again tomorrow to double check.

Could side to side play in the drive cause a thumping sound?
My exhaust housing also has the TINIEST bit of play. Like paper thin movement.

 

[racerone]

Check clutch dog and forward gear.

 

[Serf27]

I removed the lower drive to look at the drive shafts. Shaft teeth on lower and upper look to be in excellent shape. No wear or missing teeth.

The drive shaft in the upper drive did have some up and down play.
I read it was normal, due to the woodruff key for the impeller. Is this normal and correct?

I also see which part causes minimal side to side play.

Part 81 seems to be what’s allowing the drive to move from side to side about a 1/4” or less. Is this normal?

I believe part 35 is the one with up/down movement. It’s the shaft that connects to the lower shaft when the 2 are joined together.