Help! Engine Rpm Increases But Speed Does Not + Rumbling Sound


Sungam

New member
I have Mercs 6.2 with Bravo III. When accelerating to go up on plane my engine rpm increases but the speed does not and I begin to hear a rumbling sound. Any ideas? It's only my second time out this season. Has the drives pulled this winter for servicing.
 
Are you sure your drives are down? I would verify visually in case gauges are wrong
 
I have the same engines and I believe it sounds like the same thing that happens to me occasionally. Usually around 3000 RPM one or both engines produce what sounds like aeration with a slight vibration or rumble and a slight bounce on the RPM gauge. I can silence it by adjusting the trim or increase the RPM's.

I hope this helps.
 
Thank you for the responses. I May not describing it correctly. When this is happening the boat does not pick up speed. It hits 12-15mph and the rpm is at or close to 2500 and stays there as you accelerate further. Then a rumbling sound begins.

I will however visually check that the drives are down and pray that is the issue.
 
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Yeah that sounds very much like cavitation..... you could try accelerating with the tabs completely down a bit slower and see if it still happens

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Thank you for the responses. I May not describing it correctly. When this is happening the boat does not pick up speed. It hits 12-15mph and the rpm is at or close to 2500 and stays there as you accelerate further. Then a rumbling sound begins.

I will however visually check that the drives are down and pray that is the issue.
Are you smelling anything like burnt rubber in the engine room?
 
Are you smelling anything like burnt rubber in the engine room?

I did not, but from what I could find online it feels like the engine coupler which emits a burnt rubber smell. But I did not smell burnt rubber.

Ran the boat all last season. This is something new that just started happening. It's just not getting traction as the rpm increases.

From what I read if the engine coupler goes it will begin to slip until it no longer turns the prop

Then have to take out the engine to replace it. Who designs this crap? Must be a millionaire? I know the guy who owns the marina is.
 
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Curious what the lesson learned was.. I had a similar issue on my former 270 SS.. after using the flushing attachment, I forgot to put the blue rubber cap back onto the water intake.. the engine would rev but not pick up speed.. was this your problem?
 
I learned that I was not accelerating as steady as I should have been and was hanging out too long at 2500 rpm. I also had empty tanks and then was fully fueled when this first occurred.
 
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