Perkins Revving Up

[mcdaida]

My old Jeanneau Sun Legende 41 has a Perkins 4108. Starting is excellent, runs fine at lower revs but recently when running at 2500 it suddenly jumped up to 3000 for a couple of seconds and then back down again. This is not the classic "hunting" as it only goes up and then comes back down again - revs don't go below where they should be.

I had the high pressure pump re-conditioned 3 years ago to try to remedy this same problem. At that time the over-revving was really bad, going high even when idling. Everything has been OK last season but after launching this season we have the return of the problem. Also had the diesel tank cleaned professionally 2 years ago as we were getting sludge in the fuel filters. I changed the oil recently and the fuel filters were changed back in September, just before hauling out. She was then out of the water for 6 months.

Any ideas would be appreciated.

 

[danielefua]

I had a similar problem with my 4.108 and it ended up being the "fine" fuel filter clogged. If the fuel filter (the one closer to the engine) gets partly clogged, the helmsman tends to crank up the throttle more than usual to reach the same rpm. As the clogging diminishes which may temporarily happen just for the gas flow, the engine revs up.

Try changing the paper filter again, it may fix the problem.

Daniel

 

[sailorman]

My old Jeanneau Sun Legende 41 has a Perkins 4108. Starting is excellent, runs fine at lower revs but recently when running at 2500 it suddenly jumped up to 3000 for a couple of seconds and then back down again. This is not the classic "hunting" as it only goes up and then comes back down again - revs don't go below where they should be.

I had the high pressure pump re-conditioned 3 years ago to try to remedy this same problem. At that time the over-revving was really bad, going high even when idling. Everything has been OK last season but after launching this season we have the return of the problem. Also had the diesel tank cleaned professionally 2 years ago as we were getting sludge in the fuel filters. I changed the oil recently and the fuel filters were changed back in September, just before hauling out. She was then out of the water for 6 months.

Any ideas would be appreciated.

we had the very same prob i.e run away revving

it was found by me to be the fuel system, fuel starvation was the problem.
check fuel lines & filters

 

[DownWest]

As you have had trouble before, I would suspect the governor in the inj. pump. Last fix might not have been complete. I had a problem with one years ago, not quite the same, but ended up changing quite a few bits in the pump. Corrosion being one culprit, and seals another.
A

Crossed posts. I hope it is fuel filters, the pump bits can be a bit eye watering. I found a spare and made one from the two.

 

[dovekie]

Same engine, same problem, a few years ago. To be precise, when warm, and at cruising revs, the engine would briefly rev faster - for a few seconds and then resume as before. Eventually revved and then died. In our case it turned out to be an air leak, into the hose between the final filter and the injector pump (corrosion). I think the otrher posters may be closer to the truth in your case, but it may be worth checking the fuel line into the injector pump.

 

[Cliveshep]

I'd go with air in the system. My Thorneycroft did much the same, in fact sometimes it went berserk, usually of course just when you wanted to be in control for some tricky manoeuvre. Going along nice and steady it would all of a sudden rev up like mad and we'd go careering off down the river. We lived with the problem for quite some time as at the time I had no idea what the problem was caused by nor how to fix it.

Finally it broke down coming up under Kingston Bridge, just died leaving me in the way of a river bus not about to slow down as expecting me to get clear, Fortunately for me another boat threw me a line and towed me to one side where a wonderful man who at first I thought was the local tramp (don't judge by appearances) offered to help, explaining that he was actually working on a commercial boat a few yards down. He changed the fuel filter automatically, and then spent quite some time bleeding the air out of my rotary injector pump, telling me they were a complete pig to get all the air bled out.

Once he did that it ran fine, never hunted again. The problem he said was air that had got in from somewhere, and he checked and tightened everything as well.

 

[JohnTH]

There are lube oil “fumes” in the crank case which are exhausted in to the inlet manifold through a separator. It sounds to me like engine lube oil "fumes" are being ingested and burned as fuel.

When your 4108 started revving out of control did you notice:

Black smoke?

Could you pull or press the “stop” button with any effect?

Did the accelerator have any effect?

Of course, when everything is going on all at once it is often difficult to rationalise anything however, noticing these phenomena is the key to understanding the difficulyt.

Or, just wash the separator out !

73s de

Johnth

 

[mcdaida]

Thanks Guys for all the replies.

I have 2 paper filters. I will change them both tonight and try tightening all pipe junctions and see how that goes. Will have to bleed fuel lines after that anyway.

When it does rev up it doesn't last for long so haven't tried cutting back throttle. I haven't noticed black smoke while this is happening but that doesn't mean it wasn't there. I think the instinctive reaction is to look in the general direction of the engine when it suddenly makes more noise, not the exhaust. I will try to check this tonight also.

I will post with results of tonight's experiment.

 

[Billjratt]

Hope you've found the problem.
FWIW, our 4108 did the random -revving bit, at first I thought it was a gearbox/slip problem. Had to bleed the system to restart the engine if it eventually stopped.
filters didn't fix, but found a graunched compression joint on the copper feed pipe - sucking air. Replaced the pipe, gone for ever!!
It's a fact that these old engines will have been attacked by non-tecs with molegrips and shifters at some time in their lives unfortunately.

 

[mcdaida]

Working again

OK, all is now working as desired.

I changed the fuel filters even though they both looked really clean, as I thought they would be. Filled filters with fuel using lift pump then primed through pump by disconnecting feed pipes at injectors and turning over with starter motor a few times. Then reconnected pipes and started up, let her warm up for a few minutes then left mooring and motored about 2 miles at 2500rpm. No indication of any variation in revs. Last week the revs were rocketing after a couple of hundred yards.

I suspect the filters are a red herring and there may be a small amount of air getting into the fuel lines somewhere which I cleared out with the priming of the pump. I had a good look for any fuel line connector problems around the engine and couldn't see anything, so I guess I'll be looking again in a while. At least it seems to be running well at the moment.

Thanks all for the replies,

Andy