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Twin omc 5.7 Cobra drives slipping

D

David Crowder

New member
I have two 1989 cobra engines and drives. When driving above 1500-2000 RPM I can hear the engine rev louder the sync guage shoots up on the port side but the RPMs remain constant and the boat pull to port. I have changed the props no effect. Checked to ensure someone did not mix the drives Any ideas ?

Dave
 
The other site where you posted this i posted the same reply pasted here.

Bad prop, coupler going, or the dog slipping. So lets go thru the symptoms of each.

Bad prop you changed that to a new one or another old one ? Run a line thru the center of the prop if it doesn't line up after the slipping happens again then you know the prop spun.

Coupler is going. Look behind the engine for small pieces of rubber and the smell you will not mistake that for anything else.Also the temp will spike till you come down in speed see the water pump in the drive works off the shaft, no turning shaft no water being pumped. The engine has to come out for a coupler change.

The dog slipping. Common problem on cobras. If the lower cable is not adjusted correctly the clutch dog is slipping in and out. The lower has to be disassembled and the dog replaced. Actually on my friends i just turned it around how fast you go in reverse anyway ?
​
 
Master mechanic said:
The other site where you posted this i posted the same reply pasted here.

Bad prop, coupler going, or the dog slipping. So lets go thru the symptoms of each.

Bad prop you changed that to a new one or another old one ? Run a line thru the center of the prop if it doesn't line up after the slipping happens again then you know the prop spun.

Coupler is going. Look behind the engine for small pieces of rubber and the smell you will not mistake that for anything else.Also the temp will spike till you come down in speed see the water pump in the drive works off the shaft, no turning shaft no water being pumped. The engine has to come out for a coupler change.

The dog slipping. Common problem on cobras. If the lower cable is not adjusted correctly the clutch dog is slipping in and out. The lower has to be disassembled and the dog replaced. Actually on my friends i just turned it around how fast you go in reverse anyway ?
​
Click to expand...

Thanks MM for the reply,

The props are new Composite props with interchangable blades and no hub to spin.

The temp does spike to 200 and if I throttle back it does drop back to 150 but if I do not throttle back it will also drop and then slowly rise again. Although I just changed the thermostat 3 weeks ago it is acting like a sticky thermostat as far as the temp goes ?

I will look this weekend for for small pieces of rubber and the smell but I have been in the engine compartment quite a bit this summer and have not noticed anything . Will a coulple go bad slowly or does it just stop working ? In terms of reverse speed I have not really tested it other than using it to brake on docking which seems to work fine.

As for the dog clutch I did have work done on the drives this spring before launch with both lower cables replaced, bearing, both belows and water intake tube plus pivot pins ?

Thanks for all your assistance
 
in my experience, the coupler doesn't tend to survive more than 2 or 3 minutes of slipping and it is reduced to molten garbage.

however, you could have an intermittent issue I suppose. On my Mercruiser V6, I can actually get my head back into a position to see the coupler and driveshaft - amirror helps.
I can mark the coupler and driveshaft with paint much the same way a prop would be checked for relative movement.
You can see my blotch of red spray paint in this pic - I painted that in the hull years ago.
IMG_3502.jpg

Not sure if you can get in there on a Cobra at all.
I can also go for a run, shut down and then reach in and feel the coupler to see if it is hot (battery disconnected)

Do both of your drives have the same gear ratio in them?
 
Last edited:
hystat said:
in my experience, the coupler doesn't tend to survive more than 2 or 3 minutes of slipping and it is reduced to molten garbage.

however, you could have an intermittent issue I suppose. On my Mercruiser V6, I can actually get my head back into a position to see the coupler and driveshaft - amirror helps.
I can mark the coupler and driveshaft with paint much the same way a prop would be checked for relative movement.
You can see my blotch of red spray paint in this pic - I painted that in the hull years ago.
IMG_3502.jpg

Not sure if you can get in there on a Cobra at all.
I can also go for a run, shut down and then reach in and feel the coupler to see if it is hot (battery disconnected)

Do both of your drives have the same gear ratio in them?
Click to expand...

Thanks great information I will see if I can get back there, its prety tight for a 6'1 240 pound old guy ;) As for the gear ratio as far as I know they are exactly the same.
 
Master mechanic said:
So the drive is slipping and the temp rises hmmmmmm re-read my first reply to this thread.
Click to expand...

Thanks I got that point in your first post. I have been having overheating issues on both drives so I was / am not sure if they symptoms are related. If the coupler is going will the temp drop on its own and does the coupler go slowly over time or all at once ?
 
You have a rubber coupler mine has no rubber just splines when mine goes i'm dead in the water.

You didn't mention overheating issues with both engines.

So.... lets start over. What has been done to the boat last? What kind of water do you boat in ? For example elbows changed last ? Impellers changed last ?
 
David Crowder said:
Checked to ensure someone did not mix the drives
Click to expand...
checked how?
If this boat is new to you, you might want to count gear teeth in the upper drives.
Or if it is floating, get someone to turn the engine 5 revolutions (in gear, strap wrench on the crankshaft dampener + wrench on alt pulley to turn engine, batt disconnected) and accurately count prop revolutions, compare to the other drive
 
Master mechanic said:
You have a rubber coupler mine has no rubber just splines when mine goes i'm dead in the water.

You didn't mention overheating issues with both engines.

So.... lets start over. What has been done to the boat last? What kind of water do you boat in ? For example elbows changed last ? Impellers changed last ?
Click to expand...

Thanks everyone for all your help
Ok so lets start over

Last year was the first year I had this boat.
Splashed the boat in the spring. No leaks a good start :D
Drove to dock both engines heating up to 220 degrees
Replaced Thermostats - No Change
Installed Impeller Kits - Over heating resolved
Took boat out for first drive past breakwall noticed slipping as heard by increased revs and increase on port side of sync guage.
Asked for advice from multiple sources spun prop identified as likely cause
Changed both props - No Change
Noted Exhaust bellow on Port engine broken
End of season - unable to resolve issue.

This year I had marine mechanic ;
install two new lower shift cables
install two new gimbal bearings
install two new exhaust bellows
install two new drive bellows
install two new water supply hoses
Change drive lube

Took the boat to dock Port engine overheating
Replaced thermostat - No change
Installed impeller kit - resolved over heating

Left my dock to drive to fuel dock;
Port engine fine
Starboard engine overheating
Changed thermostat - no change
Changed Engine circulation pump - no change
Changed impeller - no change
Removed impeller again checked housing seal and plate re-installed - overheating resolved.

Last weekend First time this summer I have been able to get past the breakwall

Engines seem like they are still slipping like last year and as described
starboard engine overheating above 2000 RPM
reduce RPM temp drops
keep RPM up temp goes to 220 on guage drops back to 150
engine still slips at higher RPM

hope this provides a better picture.
 
Last edited:
sure sounds like the port side coupler is slipping. how it hasn't melted down I dunno.
I'd verify drive ratios too.
Port side drive may be carrying most of the load due to mismatched gearing and therefore slipping.
 
hystat said:
sure sounds like the port side coupler is slipping. how it hasn't melted down I dunno.
I'd verify drive ratios too.
Port side drive may be carrying most of the load due to mismatched gearing and therefore slipping.
Click to expand...

I was afraid of that ! No matter what the cause it does not sound like it will be an inexpensive or this summer fix :mad: Maybe next year I can actually get to cruise on my cruiser
 
Yep back to the same conclusion. What i didn't see is when the elbows were changed last. And the flappers checked and replaced or discarded. Me i would discard them.

Before you bought the boat the sea trail didn't reveal any of these faults ?
 
Master mechanic said:
Yep back to the same conclusion. What i didn't see is when the elbows were changed last. And the flappers checked and replaced or discarded. Me i would discard them.

Before you bought the boat the sea trail didn't reveal any of these faults ?
Click to expand...

I have not nor do I know if the elbows have ever been changed. As for the flappers when the drive work was done this spring the one flapper was found as a ball of melted plastic and not replaced.
As for the Sea Trial the boat was bought during winter here and no sea trial was done
 
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