perkins 4.018 40 hp engine randomly reving up 100rpm

[c.K.]

Hi
My perkins engine is great
but recently the engine reves up randomly 100 rpm for second or 2 it has never done that before any ideas?
Air starvation?, I close of the cabin and have a exhaust fan running
Thank you

[tkeithlu]

Hmmm. The problem is that it speeds up, not lags, unless "usual" is actually lagging and occasionally it is freed from a restriction.

All it's got is fuel, air, compression, and load. Check the intake for baby birds. That handles the air. If it were restricted cabin air you'd have bleeding ear drums. Check the transmission for slipping and the shaft/propeller for anything in the way, such as the shaft being difficult to turn. Then load is not a likely problem. Compression can pretty well be eliminated because the problem is intermittant. Fuel will be more of a challenge. Pump problems, clogged filter, and a bit of trash in an injector come to mind.

Getting more specific is going to be difficult (for me and my limited set of ideas) without more details. What other changes occur with the surge? Under what conditions?

[Lepke]

Alternators can draw as much as 25 hp from the engine. It could be your alternator is charging and when the charge is complete, the alternator stops charging and is no longer an engine load, and the governor takes a second to make the adjustment. Another possibility in governor inside the injector pump has some wear issues. Or there is play in the cable or speed control linkage.

[pdxsailordiver]

When my 4-107 did that it was because, I needed to adjust the Paragon transmission.
I’ve had to adjust it 2-3 times over 22 yrs.

[wingssail]

Originally Posted by c.K.

Hi
My perkins engine is great
but recently the engine reves up randomly 100 rpm for second or 2 it has never done that before any ideas?
Air starvation?, I close of the cabin and have a exhaust fan running
Thank you

My guess would be that the alternator is backing off. My Balmar regulator sometimes does that (reduces the field current) momentarily and I can hear the engine speed up when it happens.

[skipperpete]

Next time it does it, check the cold start device ( assuming your perkins has one) and see if its hot. Possibly you have an electrical problem thats causing it to momentarily activate and change the Rpm
Definitely check the linkages and the stop lever for looseness
The governor in the picture is often fitted to your engine and the fuel control marked in red can get sticky.... could cause low idle fluctuations

[c.K.]

thank you all
Right now I'm on the "hard" hope to go back in today
I will check out all advice
CK

[KadeyKrogen38]

I must have spent $1500. Chasing this same issue. Turned out to be a very minor problem. The fuel input copper nut on lift pump had a minor crack was letting a small amoun of air into system and causing the slight rev ups.

[Scubableu]

I second PDXSAILORDIVE .. If the RPMs are changing while underway mine required transmission adjustment.

[bioandy]

I have a similar Westerbeke 4-108. In the past 10 years, the 3 times my revs were erratic was caused by a clogged fuel filter. I'd be curious as to your ultimate fix. Keep us posted.
Andy

[Gary Matthesen]

I suddenly became aware of fluctuations in the rpms. Also had the throttle wide open to maintain speed, not normal. Primary fuel filter, did both while I was at it. Only 3 years without a change, maybe it was due.

[zeehag]

when mine did that, i learnd after the fact my fuel pumps had failed--yes 2, a liftpump, mechanical style, plus my electric assist pump..both failed. i made it to mazatlan from barra de navidad , 300 ish miles, with this issue, running on only the high pressure pump, which thankfully had been freshly serviced. yes i know--damned near impossible, but done.
one other reason for accelerating rpms is extra oil in sump--is diesel and indicates a fuel pump (mechanical lift pump, usually, altho can also be injector pump) failure
the other reasons i found for surging rpms is air in line and dirt in line and faulty pickup.. those reduced rpms not accelerated them. fuel pump issue the rpms accelerated by around 100 rpms, not much, but enough to create concern as the misadventure was only beginning.
best of luck in your repairs and smooth sailing

[Captndave1]

I agree with kadeykrogen 38, a small air leak on the suction side of the fuel system will cause the engine to run lean for a few seconds then run normal until enough air causes it again. When an engine runs lean like that, the RPM will increase momentarily.

[c.K.]

thank you all
Great replies , think I have the answer since for the first time my engine did not want to start, a good Samaritan came by , had it running in 2 min with the help of some primer and he noticed that the little primer pump did not work again causing fuel problems
I motored home without any problem 3.5 hrs
Again thank you all
CK

[nolakennedys]

Originally Posted by KadeyKrogen38

I must have spent $1500. Chasing this same issue. Turned out to be a very minor problem. The fuel input copper nut on lift pump had a minor crack was letting a small amoun of air into system and causing the slight rev ups.

Yes, this or a loose fuel line fitting somewhere. Rev up a little and look for shine or pooling on all the fuel lines and fittings.