270 Sundeck . Bravo III

Old Man In The Sea Ray

Old Man In The Sea Ray

Member
Dec 15, 2021
46
Sarasota
Boat Info
2003 Sea Ray Sundeck w/496 HO Bravo III
Engines
496 HO Mercruiser w/B3 drive
Greeting all.
Forgive my rambling but I'm frustrated and at wit's end. And, I consider myself a damn good mechanic. I'll add that I've owned boats for over 50 years and this is my first (and last) I/O.
My 2003, 270 Sundeck, has a 496 HO shoehorned into it. Recently it's developed a "throbbing vibration" between idle and roughly 2500 rpm... only in gear. While slow cruising I would swear a prop or prop shaft problem. Between 2500 and 4500, it's smooth and strong. I discounted an engine vibration because while I feel it in the seat of my pants at low rpm, I believe the engine is running smoothly. I confirm this by opening the Captains Call... nice steardy purr at all rpm.
The marina I took it to says it's not a lower unit problem. They "suspect" it's coming from the engine. They may be right but I'm not willing to pay $125/hr (plus parts) while they scratch their heads. I'll bring the boat back to my lift and scratch my own head.
So where do I start? Plugs? Wires? Injectors? Fuel delivery? Is it possible the cone cluctch is not fully gripping at low rpm? God I don't need this.
Your wisdom please.
Russ
Edit (while ideas pop into my head): I have what's commonly called the "sport exhaust bellows". This is an abreviated bellows that doesn't seal directly to the outdrive. It relys on contact when the trim is full down, otherwise the exhaust just escapes to atmosphere... just below the waterline. I'm wondering if this burble could create a vibration sensation.
Did I mention that I'm at wit's end? :eek:
 
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Forgive me not being familiar with the 496, but could it be the cap and rotor? When was the last time that was changed? I know on my 5.0 it was running rough one time at low rpm’s until I changed it. All the metal inside was green.

I doubt it’s the bellows. I had those installed one time and all it did was make the boat louder
 
You're saying the vibration is only when it's under a load and feels like its coming from the transom? Have you considered the gimbal bearing might be going bad?
 
Sounds like it could be a gimbal bearing, U-joint, engine coupler, or prop hub.
I just changed and Alpha gimbal bearing that howled at lower rpm’s.
How many hours you got on the engine?
How long since last bellows/gimbal job?

Why are you saying it will be your last I/O? Had other problems?
 
DWABoat said:
Sounds like it could be a gimbal bearing, U-joint, engine coupler, or prop hub.
I just changed and Alpha gimbal bearing that howled at lower rpm’s.
How many hours you got on the engine?
How long since last bellows/gimbal job?
Why are you saying it will be your last I/O? Had other problems?
Click to expand...

Less than 200 hours on 2003 boat
I changed all bellows recently.. gimble no.
I/O's are persona non grata down here in saltwater. Almost no mechanics will touch them and I can understand why. I'd rather service a freestanding OB than work on my knees in tight quarters wrestling with an I/O.
 
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Old Man In The Sea Ray said:
Less than 200 hours on 2003 boat
I changed all bellows recently.. gimble no.
I/O's are persona non grata down here in saltwater. Almost no mechanics will touch them and I can understand why. I'd rather service a freestanding OB than work on my knees in tight quarters wrestling with an I/O.
Click to expand...
Wow! Low hours on an ‘03.
Definitely understand on the saltwater/I/O thing. Those duo-prop units have a lot of SS-aluminum battle going on.
It sounds like you lift yours though.
Does your unit have duo-props or a single prop?
 
To correct a few things that have been suggested…..

Bravo 3 has 2 props
Neither has a hub
8.1’s don’t have distributor caps

They do, however, like to wear out spark plugs rather quickly. You’ll probably get 300 hours or more on a small block before the plugs need to be replaced. Probably not on the 8.1

Does the boat reach the rated WOT RPM? A misfire can be tough to feel on a marine engine but it will show itself as low WOT RPM.

When you replaced the bellows, how was the gamble bearing? With the drive removed, You did check it to make sure it was “smooth spinning”, correct?
 
Just saw this post and was going to ask about the drive / engine alignment and greasing the coupler and the u-joints. Also, when was last time drive oil was changed?

You can try removing that exhaust tube and see if it makes any difference.
( I had one on mine, took it off, and wife hates the noise (I like it) when on plane. I tell her to turn the stereo up)

The B3 is an odd beast with the twin props (I know, I have one) and has it's own quirks. One suggestion I would make, also, is pull props and have them checked / tuned.

A harmonic drone at specific RPMS / loads can be a bear to chase down. In all my vehicles, it usually winds up being exhaust or gear train related.
 
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All good ideas, thanks. I'm just trying to minimize the trial and error approach.
[QUOTE="
They do, however, like to wear out spark plugs rather quickly. You’ll probably get 300 hours or more on a small block before the plugs need to be replaced. Probably not on the 8.1
Honestly, the marina wants to replace the plugs. My first reaction was that's a waste of time. I'll do it.

Does the boat reach the rated WOT RPM? A misfire can be tough to feel on a marine engine but it will show itself as low WOT RPM.
Good question. She does run like a banshee at 4500. That's enough for me.

When you replaced the bellows, how was the gamble bearing? With the drive removed, You did check it to make sure it was “smooth spinning”, correct?[/QUOTE]
No. Not sure how I would have done that on bunks.
 
Attention please, clean up on my mind.
Alrighty, here goes. I'm one who prides myself on doing all my own work, but to be honest, I'm out of my "wheelhouse" on this I/O. I met with the marina today and they were more than gracious enough to let me speak with the mechanic. He answered all my questions and I believe his assessments... engine problem. One plug is black. Bad plug? Bad injector? I don't know and he can't either. Unrelated, he showed me an oil leak in the lower unit. Strange, since my OD oil reservoir shows full, but true.
I hate it, but to save my sanity, I've decided to open my checkbook and let them have their way with it (and me).
Words I seldom speak: "what the hell it's only money".
So that's where I stand. Thanks for all your inputs. I'm going fishing.
Russ
 
Forgive me, really. He said the OD is leaking, you said reservoir is full.
Are you sure you looking at the outdrive pump reservoir and not the trim tab reservoir? Just an idea.
 
So plugs and wires on a 496 are maintenance items that need done far more than most think. Plugs every few seasons regardless of hours. Wires, maybe 4-5 years at most. That motor just chews them up. The 496 is a beast and easily runs on 7 cylinders, just not as smooth. So let them at the plugs and wires first. FYI - Merc OEM wires are crazy expensive. Volvo or crusader or aftermarket for the 496 will all be $200 cheaper.
 
paulswagelock said:
So plugs and wires on a 496 are maintenance items that need done far more than most think. Plugs every few seasons regardless of hours. Wires, maybe 4-5 years at most. That motor just chews them up. The 496 is a beast and easily runs on 7 cylinders, just not as smooth. So let them at the plugs and wires first. FYI - Merc OEM wires are crazy expensive. Volvo or crusader or aftermarket for the 496 will all be $200 cheaper.
Click to expand...
As I'm discovering...:eek:
 
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