Engine tone change for a few seconds

[38mess]

My Perkins M30 runs fine, but every now and then it changes tone for a few seconds, the revs don't drop but it just sounds like they do then it sounds normal again. I have changed the filters and made sure there is no air getting in, also it's clean fuel
It might not happen all day.
I am at a loss

 

[Pye_End]

Exhaust outlet briefly covered by a wave?

 

[Stingo]

SWMBO's jaw stopped flapping for a nanosecond?

 

[RivalRedwing]

piece of rag in the tank (happened to me)... good luck in sifting through the many and varied answers

 

[Sandy]

Orcas swimming under the boat.

Plastic bag, sewage temporary blocking the cooling water input....

 

[38mess]

Orcas swimming under the boat.

Plastic bag, sewage temporary blocking the cooling water input....

Good call

 

[[2574]]

I've had this, it had me worried. Then by chance I found it was high load on the alternator. There was belt slip, as it slipped/loaded it changed the load on the engine causing a tone change. 100A alternator. Fitted a serpentine belt, problem solved.

 

[oldgit]

"but every now and then it changes tone for a few seconds, the revs don't drop but it just sounds like they do then it sounds normal again. "


this normally only ever happens when the weather on the horizon between you and your destination lots like.... ?

 

[38mess]

"but every now and then it changes tone for a few seconds, the revs don't drop but it just sounds like they do then it sounds normal again. "


this normally only ever happens when the weather on the horizon between you and your destination lots like.... ?

Definitely

 

[Adios]

I had a 4108 that had occasional tone changes for a few seconds, which made me feel I should investigate it at first. In the end I put it down to engine character. Try not to worry about it.

 

[Spirit (of Glenans)]

Propellor chewing on a plastic bag/ clump of Saragasso Weed?

 

[Chiara’s slave]

Propellor chewing on a plastic bag/ clump of Saragasso Weed?

Theres a lot of weed where we are. We get the noise thing with our outboard, but I can see what’s going on cos it’s only 3 feet away.

 

[38mess]

Propellor chewing on a plastic bag/ clump of Saragasso Weed?

I don't think it's weed, I checked my gearbox oil and it was almost down to the bottom mark. Perhaps it was little seizures. Just clutching at straws I guess. It's been fine all day today.

 

[wombat88]

If you have one, aged diaphragm in lift pump?

 

[BornAgainIdiot]

Could be a bit of fuel starvation, as mentioned lift pump or crud (but you said you'd done filters..)
Have you tried hitting it with a hammer?

 

[Pye_End]

Would have thought a problem with the fuel system would result in an rpm drop?

 

[Chiara’s slave]

Could be a bit of fuel starvation, as mentioned lift pump or crud (but you said you'd done filters..)
Have you tried hitting it with a hammer?

And if that doesn’t work, try a bigger one.

 

[Daydream believer]

My son & I had a situation in the bay of Biscay when the engine changed tone for a few seconds for no apparent reason.
It went on doing this for 15 minutes & we could see nothing on the instruments to indicate why. My son looked over the stern to be shocked to see about 4 ft of the tail of a fish poking out from the stern.
It seems that dolphins were swimming up to the saildrive & somehow blowing air, or water, into the prop. This caused the prop to cavitate momentarily. This altered the engine sound.
This happened twice more in the area & one dolphin actually followed us right into Le Pouligen marina & then started swimming among the boats. I was actually able to kneel on the pontoon & stroke it.

 

[Buck Turgidson]

It's a change of load on the prop or pressure in the exhaust. If it was fuel I would expect an RPM change momentarily.

 

[penfold]

Change in tone but not revs suggests, aside from the alternator load already suggested, the waterlock cycling and regurgitating a slug of water. Is it a regular thing or just once in a while?