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3.0 cobra possible coupler failure

P

pjlucky

New member
Hello all Its been 4 years since this boat was in the water. It was taken out because it was giving a sever bump and some rev up. It was thought at the time by the user to be out drive problem. User = son
Dad now has the boat and after careful examination and cleanup determined it should not be the lower unit all the gears run smooth and the shift dog engages fully. I would expect a ratcheting effect if this was skipping.
last evening on a shake down I had the experience my son told me of intermittent bump mild to sever
like hitting bottom or rocks with the prop.
As we approached the landing it got worse and began to rev up. turning made this worse. throttle up still delivered thrust to the prop in-spite of the slippage.
The motor is pulled ant the coupler as well with no real evidence it is the coupler .
I spun a coupler years back on a mer-cruiser. I saw more evidence on that one. Has anyone experienced this ? the splines are OK and the rubber does seem soft and lateral play is possible while I have a coupler from a merc. it is far stiffer.
Regards
 
A note about my post I realize that internal failure of the couplers rubber could be ragged and cause the bump effect while still giving the appearance that nothing is wrong.
I am looking for some affirmation if any.
A second issue while removing the engine I see there was some kind of damper in the down port of the exhaust below the riser. this is melted into an unrecognizable lump hanging from the small shaft. What is this items purpose.?
 
The purpose is to prevent water from coming up the drive and into the motor.

Water cannot come up the riser from the transom if the motor is running.

If you beach your boat it is thought to be a problem of waves coming up the riser into the motor.

Or if you come off plane to quickly and a large wav hits you from behind.

Remove the shutter.

You tried of course a different knowen good prop before pulling the motor ?
 
I have to say no the the prop Q
This was replaced because of a spun prop. There is no evidence it is the cause here . A turn applied to the gimble would have little effect on a prop .My reason for ruling it out is just that it is tight and shows no evidence while the coupler exhibits a soft flexible lateral play. and a turn to the gimble would aggravate the condition.
Also I can see nor feel gimble bearing failure it rotates smoothly and has plenty of grease applied. I thought possibly a hang up from roller cage failure there . It is this mild and severe bump /bang that has be baffled. the rev up is only occasional and was only experience at the end of the nights use. that was mostly stopped and fishing while a run up wind and drift back .very little actual run time was applied. 30 minutes total maybe
 
you said it's not the coupler.

You say it's not the dog.

You say it's not the prop.

All thats left is the lower cable and adjustment.

After you read the link it's not so easy to do.

Oh look at the dog again.

Good luck !
 
On the contrary I believe it is the coupler I was just looking for affirmation that it can do all this and still appear
perfectly Innocent. I have no choice to continue with my plan of elimination. I can and will use the merc. coupler which I know is good. in-spite of it being 1 inch shorter in the overall length flywheel to end the spline is the same. The engines are the same HP and why a OMC is so large a dia. using more rubber to encompass means more flex and more susceptibility to failure. the overall length is a bell housing issue so I will have a bit more spline exposed. it will prove the failure regardless to be the coupler or not and I can choose to take a proper resolve. Its only more work. at no cost at this point.
 
About my above plan to use the merc coupler.Does anyone know if the elevation to the gimble mount is the same for the two bell housings I have a few merc bells and the gimbles are probably very different elevation does any one know for sure?
I could use a merc housing if it were compatible and reduce the shaft exposure.
Of course this would mean a complete realignment if I did.
You can tell I am a renegade I do not believe in toeing the line everything can be modified.
 
20.jpgMerc.parts would mean a big change of parts.
The bell housing then the transom assy in and out then the drive.
From the sounds of it the dog is the problem.The shift mech.has a part that's called a bellcrank.It gets corrossion around it and stops it from shifting in and out of gear.#21
Profile? You might be close to someone who can help
 
If you can match the couplers.Then yes it will work.They are the same just different part #s from different companies.
Try Sierra's catalog.Sometimes they cross Merc and OMC.
One more thing Serial# and Model# help find the right parts.J
 
Hystat
I was sure it was as you say,at the beginning. When I first began work on the drive and removed it I found no evidence,and reassembled the drive.Also there however is no bump when it reengages at rev up it just smoothly goes on. and why would turning aggravate this, even at slow speeds and with some trim up approaching the boat ramp.
I will give you the answer tomorrow since its back together and will launch in the morning.
 
Last edited:
Hystat
I have to eat crow on that. You are probably right on. The test run provided no resolve it still bumps.I didn't have any rev ,just bump.
As for using a merc coupler as a substitute most certainly can get away with that.I am not recommending that as resolve for someone it is just possible.

Hystat I mislead you I never took the lower unit apart its is fused together by calcification and need some tender loving abuse to get it apart.I was hoping it was not a broken gear and the smooth motion told me it probably wasn't a gear. The rev gave me hope that the problem was elsewhere ,maybe there is two problems.

So lets assume its disengagement of the clutch dog.Can it be caused by other than wear down? linkage Etc. I did apply heavy forward adjustment enough that reverse wasn't engaging and it still did the bump going forward.

For now I will have to use it as is. I needed the boat only as a tow and safety platform for an entirely different test program . So when I have time to take the lower unit apart I will drop back in and post.
 
post 6 in this thread did i post to look at the dog again ?

So you never looked at the dog or the gears ?

You got the link.

I'm done.

Good luck !
 
failure of the clutch dog is caused by a faulty lower shift cable
3 or 4 clunks under load and its done, the lower unit needs to be rebuilt.
then replace cables and adjust meticulously. Visit the Stuart Cobra site Chief pointed you to, and get the latest and greatest procedures for your exact model of drive...
Or, consider the SEI conversion to a Mercruiser drive if the boat is worth fixing right.
 
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