How fast should a Perkins go?

[Hunter34]

I have just fitted a new fuel injection pump and injectors to my Perkins 4 - 108 which seems to run fine but only revs to 2900rpm max.
Under load when motoring in flat calm it revs to 2800rpm.
I have checked the throttle lever on the pump and it is open all the way to the pre set stop.
Obviously the max speed setting on the pump cant be adjusted as it has a seal on it.
How much should a 4 - 108 rev to?
It is a hydraulically governed pump so I wonder if something else could be limiting the rpm.

Any info much appreciated.

Andrew

 

[LeytonC]

There are 2 types of 4108, a low rpm 1 and a high rpm 1, which should rev to 3800. I cannot remember what the low one is, i think (of the top of head its about 2800-ish) if you cannot rev higher than this, its either a low 1 set right (presuming i am right /forums/images/graemlins/wink.gif) - or it could be having fuel starvation. I had the same problem on my 4108(s) and it was caused by muck in the fuel lift pump not allowing the non-return values to shut correctly. I would check the fuel lift pump and filters first...
(i am presuming you mean 2900 out of gear and 2800 in gear)

(I am assuming you have tried simple things - like stop control full home etc etc)

 

[jimboaw]

Sounds OK to me assuming you are getting satisfactory boat speed?. Never had to run my old boat's 4 108 above 2200 and most of the time a gentle 1800 was plenty to push a 33 foot MS along at 5.5knts

 

[Roberto]

Andrew,

I had the same problem on my 4108, this is what I did

there is a plate on the injection pump where the correct number of max rpm *at idle* is indicated: if you give full throttle for one second with the engine in neutral it should run up to the max rpm, otherwise the pump + command lever should be checked;

my pump has a 4100 rpm indicated on it, and the engine in neutral at full throttle immediately reaches them

once you know the pump is correctly calibrated, then you can investigate why the engine does not reach the stated max rpm under load
there are several possible factors, in my case I found
1.a fouled primary filter (decanter)
2.some brownish deposits in the internal membrane of the lift pump, which was failing to give full fuel when at high speeds

you may also try to check what happens when you open up one engine locker hatche (air starving), or if the engine runs better if you open the fuel tank deck inlet (fouled breather), or also try to check if the fuel return line is not clogged

I went through all these steps and the engine recovered a few rpm every time, the biggest problem was with the filter and fuel lift pump anyway


hope this helps

 

[rich]

Andrew, I have copied this from my workshop Manuel.

Perkins 4108,
49 shp @ 4,000 rpm, for high speed planing craft only,
46 shp @3,600 rpm max intermittent rating,
37 shp@ 3,000 rpm max continuous rating,
engines should not be operated at max intermittent speed for periods in excess of 1 hour after which, the engine speed should be reduced to the continuous rating speed for at least 15 minutes before returning to max speed,

I run mine at 2,600, Hope this helps. Rich.

 

[ChrisE]

You don't mention whether these speeds are under no load. If they are under load and post implies they are then the thing that will determine your max revs will be the size of the prop. We had Thorneycroft 90 which is not so different from a Perkins and with a big fixed prop we never got more than 3000 revs out of the engine, changed to a smaller feathering prop and max revs went to over 4000.

 

[Hunter34]

Thanks for the help

Thanks for the replies.
I did mean that the engine will only rev to 2900rpm when not in gear and 2800rpm when in gear.
The filters, lines, injectors, lift pump are new and the tank vent is clear.
The bottom has just been painted and I have a 14" autoprop which is working just as it should.
I received an email from the company I bought the pump from who tell me its either the tachometer or the pre set stop has been set wrongly as my model of Perkins should rev to 3600rpm when not under load (out of gear).
As I have tried another tachometer which read exactly the same as mine I know this is OK plus the engine just doesnt sound like its reving 3000+.
I suspected the screw was possibly set wrongly as there is very little throttle travel (on the pump lever).
I guess I will have to try and adjust the screw without breaking the seal which is attached to it an assume in future that not everything factory set is going to be right.

Andrew