7.4L over-revs when trying to plane

C

Candylicious

New Member
Dec 14, 2015
7
Fort Worth TX
Boat Info
1993 290 Sundancer
Engines
7.4 Mercruiser w/Bravo I
Hi folks,

Looking for any input on my issue - 1992 Sea Ray 290 Sundancer, 7.4L 454 w/Bravo II od. Boat is struggling to plane - full throttle, pushing 4K RPM it just wont get up. Trim is down along with trim tabs in bow down position. Tried raising the tabs to get a little more speed and that's when the engine RPM "over-revs" (5K+).....kinda like an automatic transmission slipping.

Earlier this summer the old girl would get up on a plane fairly easily (~5-7 sec) and run on top at about 4800 rpm @ 32 mph.

My question is this: could the prop be slipping on the bonded drive (hub)? I cannot think of anything else mechanically that would cause the engine to over-rev like that when pushing hard.

Thanks in advance for any input!
 
If your prop has a rubber hub (which a B2 prop could have), yes, that could be slipping. The other mechanical piece that could slip is the coupler. The engine has to be pulled to change that - take a real good look at your prop and/or try another prop, first. With 24 years on it, either could be the issue. Also, check the running gear for obstructions or damage.

There are plenty of threads about your issue on this site - if you wanted to search for yourself.
 
I ASSUME THIS IS A I/O OUT DRIVE.

Is your lower unit all the way down?
If not this could cause the problem of getting on plane. If it is up to far and you lowered the trim tabs it may also be out of the water when the nose goes down. Not likely but, worth a look.
 
Thanks for the tips guys. I will check the prop by marking it against the hub and seeing if it slips. Hope the coupler isnt bad...thanks for the heads up on that one Mark!
 
Hello again folks. Been some time since I posted on this subject so I thought I would provide an update. Turns out the problem was the "whale tail" or hydrofoil that had been mounted to the anti-cavitation plate had come loose and was causing heavy drag. The anti cavitation plate had worn to the point that the front hydrofoil mounting holes were completely compromised thus allowing the hydrofoil to flip up and act as a anchor. Once removed I could get it back up on a plane however, I believe the anti-cavitation plate is worn to such severity it may be contributing to the same problem as it will over-rev when trying to get up with a full load (water/fuel) on board. Thought I would inquire if anyone has seen this and can comment.
Thanks for all your previous reply's.
 
Well... it's actually called an anti-VENTILATION plate and it helps to prevent... wait for it... ventilation! :) So, yes, it could be causing the issue. Put some tape over the holes and see what happens. FYI, a foil on a boat that size will do next to nothing as far as helping to get it to plane faster. I wouldn't worry about EVER putting it back on.

Other thoughts... Make sure the tabs are trimming all the way down... think about putting bigger tabs on there. Make sure the drive is trimming all the way down... See if your drive has a "celery stick" -- remove rear hydraulic ram mount and look in the hole for a movable spacer -- it can be repositioned.

It could still be a hub or coupler, in addition. Did you mark the prop?
 
Sounds like a hub slipped. Been there, done that. Check easy things first, I always say. Check tabs to see if they really are going down. I don't think AV plate would cause that much drag so as to cause over revs.
 
Lazy Daze....yes.....trim is all the way down......tabs are too......had it out of the water and checked both......also has a SS prop that I took in to have the hub checked.....shop put 1200 psi on it and it didn't budge.....not the hub (marks are aligned).....I will check for the "celery stick" as you mention...appreciate the advice.
Is there any other coupler between the engine and outdrive (or in the outdrive) similar to the prop hub that could be contributing? What is odd is that I have to pull the throttle back to idle to get the prop to "bite" again.....seems like if it was cavitating it would start to bite after it simply slowed down a bit......almost feels like a clutch that is slipping but as far as I know everything between the engine the prop is mechanical.
 
"Any other coupler"... Yes, as mentioned above, the "coupler". It's essentially where an automatic transmission would mate to an engine in a car. And, it works pretty much the same idea as the prop's hub.

If the prop is in good shape, it's highly unlikely that you are cavitating. But, if the holes in the AV plate are large enough and in the right spot, it could be causing the prop to ventilate.
 
Lazy Daze said:
It's essentially where an automatic transmission would mate to an engine in a car. And, it works pretty much the same idea as the prop's hub.
Click to expand...

Have you ever known a coupler to slip intermittently? My experience has been that it slips for a nanosecond before it strips for keeps.

Being a boat, it could be the Johnson rod or a million other things for God sakes, but my guess is the prop hub.

The statement about lowering RPMs to idle to get “bite” points in that direction.
 
The Bravo's have a cone clutch in the upper section of the outdrive for shifting from fwd to rev. They can fail. There has been a few updates to the earlier units as in your case. Earlier allum shift linkages were also prone to wear out and have been replaced with steel. It can limit the travel of the collar or allow the end of the cable to disengage leaving you stuck in fwd.
 
Should also mention, the b2's have room for a 19" prop. If you are running a 17, it should give you a better bite for taking off.
 
Soulshine said:
Have you ever known a coupler to slip intermittently? My experience has been that it slips for a nanosecond before it strips for keeps.

Being a boat, it could be the Johnson rod or a million other things for God sakes, but my guess is the prop hub.

The statement about lowering RPMs to idle to get “bite” points in that direction.
Click to expand...

I have, yes. There's no "hard and fast" rule. Too many variables... age, typical load placed on it over the years, etc, etc.
 
You must log in or register to reply here.

Forum statistics

Threads
113,171
Messages
1,427,822
Members
61,082
Latest member
wavespestcontrol
Back
Top