Volvo Penta 740 Big Block - noise and a knock

[MarkMaunder]

I have a 1990 Santego 2767. The motor is a Volvo Penta 740 Big Block (pic attached).
Anyway, we were having fuel issues for a month or so (stalling, etc.) and so after changing the fuel filter a couple of times, we decided to drain the tanks of dirty gas with black bits floating around, and fill them with fresh fuel.
With our new, clean gas, the boat was running better than ever. A lot less smoke, quiet (smoother), etc. Then, last week we went out and it was again going great, when suddenly it got loud (like a muffler blowing off on a car). It might have lost a little power (hard to tell) and I think I can hear a bit of a knock.
I thought a hole might have developed in the exhaust somewhere, but I can’t see it.
Any suggestions would be appreciated!

volvo penta 740.jpeg

[km1125]

Wonder if you blew out a baffle in the muffler? Is that a straight shaft boat?

Are you still pumping plenty of water through the exhaust?

[bud37]

Thats an outdrive ......if there is a hole in the exhaust in the wet side there will be water in the bilge I believe as well as lots of fumes, be careful.....Sorry not too familiar as to the exhaust path in the volvo duo prop.

Sometimes if the raw water pump impeller has a catastrophic failure the lack of water in the exhaust will make it quite loud and the rubber will burn from the inside out, so if that is what is going on here, be sure to check the exhaust tubing on the inside and replace to complete your repair.

[MarkMaunder]

QR_BBPOST Wonder if you blew out a baffle in the muffler? Is that a straight shaft boat?

Are you still pumping plenty of water through the exhaust?

It's an outdrive duo prop. Not pumping as much water as I recall, but the boat is fairly new to me. The tapping sound is rhythmic. I was thinking a baffle too.
Thanks!

[MarkMaunder]

QR_BBPOST Thats an outdrive ......if there is a hole in the exhaust in the wet side there will be water in the bilge I believe as well as lots of fumes, be careful.....Sorry not too familiar as to the exhaust path in the volvo duo prop.

Sometimes if the raw water pump impeller has a catastrophic failure the lack of water in the exhaust will make it quite loud and the rubber will burn from the inside out, so if that is what is going on here, be sure to check the exhaust tubing on the inside and replace to complete your repair.

The raw water pump is new last year. I hope it's not that, although there doesn't seem to be as much water coming out the exhaust as before. Not sure what you mean by "exhaust tubing on the inside".
Thanks for the help!

[bud37]

QR_BBPOST Not sure what you mean by "exhaust tubing on the inside".

The rubber exhaust hose that acts as joints between the exhaust manifold and that cross section etc. Those hoses can get burned on the inside if the engine exhaust is run thru there with little to no water flow. It will not be obvious from the outside, those sections would have to be taken apart and checked on the inside to be sure there will be no failures while out boating. Now if the hose is old already, chances are it would be a good idea to change it out if in any doubt.

[plittle2005]

I have seen dockmate's VP exhaust Y-pipe, aluminum, corrode through where it joins the outdrive housing under the rear of the engine. A leak here can sink the boat. Bilge water can corrode from the outside, cooling water from the inside.

Engine must be pulled to inspect and replace this aluminum casting.

Phil Little

[Viper]

Interesting setup for a sterndrive application. Usually there's no muffler as the exhaust usually goes through a metal exhaust horn/Y pipe, through the transom assembly, through the drive via a bellows, and exits under water so no need for a muffler. Is your exhaust directed through an exhaust port in the hull instead?