LEYLAND 4/98NV WILL NOT REV UNDER LOAD

[jeff mann]

Hi gentlemen,
New to this site ,looking for an expert ...I am a time served car mechanic, but that was a long time ago....
i have a boat with a leyland 4/98 NV engine (jcb i believe). it revs freely in neutral happily up to 2800 rpm. I put the boat in drive (borg warner velvet drive 1:1) ,and the revs will not go past 1400rpm...i have drained the fuel tanks,treated the new diesel with fuel bug killer,and put an injector cleaner additive in .i have renewed the parker racor filter element and put a new lift pump on the engine . there is a second fuel filter near the engine which has also been renewed ..only thing i can think of now is the CAV Dpa high pressure pump is shot ...do these pumps have some kind of advance and governing system ...i don't know ,but i imagine they are working ok cos it revs freely in neutral
Any boffins out there with any ideas ...pleeease
Thanks , Jeff

[philwheelton]

I would have thought 2800 rpm is very high for this engine as they only rev to about 2200 when in a JCB .
it could be a fault in the injection pump letting it rev too high when not under load.

[jeff mann]

thanks for that......well it wont go over 1400 under load , if i could get it past 2000 i would be pleased ....i think i need to get the pump recalibrated
thanks , jeff

[essexpete]

When you say under load do you mean running ahead with the throttle wide open? The reason I ask is that my cousin had a BMC Thornecroft in a boat that would rev freely to around 3000 but only pull 1800 under load. I did not grasp the reason to start with, the engine was not pulled down by over load but set so that in gear it would only rev to 1800. This set up, with the right prop on the 1:1 gear box, would give a max speed (sailing cruiser) of 8knots on flat water. The engine never got hot or smoked under that load. I do assume that the max BHP was not utilised but the engine was quiet and economical. I am wondering if your engine has been deliberately set to run at 1400 assuming you have single lever control? What type of hull have you got and how long is the boat?
Incidently my counsin updated to a Kubota engine that came with a different ratio box and opposite hand output so the prop had to be changed and the boat has never been as good. He now gets about 6.5-7knots running at a noising and less economical 2800-3000rpm.
In a nut shell have you tried removing the gear/throttle linkage and operating them separately?

ps apologies if you are aware of the all the above

[jeff mann]

hi essexpete
this is indeed a thorneycfoft leyland 4/98 . i am beginniing to wonder if this 1400rpm is as much as i will ever get out of this engine ...when i said underload, i meant in forward on the velvet drive 1:1 borg warner ...with throttle at halfway it does 8 knots as you said ...but it just wont increase rpm .by the way the boat is a colvic seaworker semi displacemnt man friday 22 foot length .... its no high speed hull , but i thought a few more knots would be there ,,,the guy i bought the boat off said he got cavitation at 2000 rpm ..i think he was fibbing
thanks for your thoughts though
jeff

[Mrsmackpaul]

welcome Jeff mate Im no expert at nothing much in life but it sounds like a weak spring or some such thing in the injector pump
if the exhaust isnt blowing black smoke and the motor isnt getting hot it would tell me the motor isnt getting over loaded and its a fuel problem ie the injector pump
Well those are the two tell tale signs of a motor being overloaded to me
good luck with it all

Paul

[jeff mann]

hi thanks paul


I think its going to be a recalibration of the dpa pump....ive done everything else ,all new ...if that don't do it,i guess it aint gonna speed up ever ....thanks for your input...by the way ,no black smoke or any smoke except a puff of blue on initial crankover ans start...probably valve guide seals ....she is 25 years old!!!!! but runs clean and healthy oil pressure ...I have taken my tachometer off an alternator phase tap so don't know how ultra accurate that is either !!!
jeff

[tctractors]

If you have hooked your Tacho into the W conection on your alternator, you will need to know a bit about the alternator (poles) the W connection is A.C. half main voltage as a more common standard 24v=12v 12v=6v it is easy to set the Tacho like a Durite incorrectly, I use an injector line RPM set up to check speed.
tctractors

[essexpete]

It might be something you have checked but is the throttle reaching the max stop when forward gear is selected? There could be a stop fitted in the hand lever. If it is a semi displacement then you might get a knot or 2 more perhaps but it would make the engine work hard and burn the fuel. I would guess you have a water line length of 20ft or less and the true hull speed would be not much over 5 knots anyway. A fiend had a Hardy 28 with a 4cyl Volvo Turbo allegedly capable of semi planing with 150hp but that would not go over about 8-9 knots. I am wondering if someone set up your boat for its best top speed for that engine/gearbox/prop combo.

[jeff mann]

thanks for that .i havent checked the throttle at the helm..i wll have a look
thanks again ..jeff