JCB 3C111 Loss of Engine revs/Power

Can anyone help me here.....I have a 1979 JCB 3C111, and over the last few times I've been using it, the engine revs have been dying away resulting in obvious power loss. I suspected fuel starving, so I have cleaned the sediment bowl out, cleaned the lift pump gauze and also cleaned the fuel filter (all filled with loads of rubbish), re-bled the system and its exactly the same. The odd thing is that if I turn the engine off for just a couple of seconds, then start back up, she will rev up, but will only hold this for a second or two before it just dies back to a tickover. Its as if the fuel pressure is there, but its starving in some way and when the demand isn't there, the pressure puts enough fuel through to allow a brief rev up but then goes back to its 'govened' state.
Any ideas?
Steve

hi had a problem a bit like this before on a david brown 1490 after an injector pump overhaul kept airlocking turned out to be a small crack in the pipe from fuel filters to the rear of injector pump cav rotary pump.i'm assuming your's has the same inj pump hope this helps.

HI THOMAS, I had a problem kinda like yours, ended up being scaled rust covering the fuel line inside the fuel tank . I got the junk out of it , been good since. good luck!! D.G.

Look on the top of the fuel pump and see if there's a fitting about an inch long with a return line coming off of it. If so unscrew the line and remove the fitting from the pump and look at the male end of it. You should see part of a really small ball that, along with a spring, makes up a check inside the fitting. Some injection pumps have this check valve coming off the pump and if it gets clogged it causes excess pressure in the pump casing which will cause the problem you describe. It's pretty much impossible to tell if it's clogged and about the same to try to clean out if it is. I've never seen one with the problem you describe that had that valve that wasn't clogged if that means anything... Your best bet is to take a small diameter punch and knock the ball out through the bottom (punch in through the female/top end and knock the ball out through the bottom/male end). This leaves an open passage through the fitting but you'll still need to clean it out to remove any loose pieces of crud. Put it back together like this and if the machine runs fine afterwards that was the problem, for now. Unfortunately it's also an indication of further problems. There's a coupling inside the pump that will slowly disintegrate over time and the pieces will clog that valve. The machine may run for another day or more likely several more years depending on use, before it becomes a major problem. Eventually though it will need to be addressed with a rebuild. If you want to put in a new valve before rebuilding the pump (around $30 last time I checked) I can tell you it'll most likely stop up just like the old one did so it's really not worth it if the machine will run without it. I've seen some that would and some that were better but still not exactly right without it. Good luck and I hope this helps.

1979 JCB would have a Leyland 3.8 engine usually fitted with a CAV DPA pump,any dirt you found in the filter or feed pump came from the tank so you need to check the flow coming from the tank,the tank setup is a right SOB to get dirt out of as its part of the frame,either take the line off at the nipple on the bottom of the tank or the feed pump,you should have a good flow of fuel,if not and you have compressed air look in front of the windscreen,there should be a breather on either side,undo the one on the left,with the cap on the tank put the airline on where the breather goes and pressurise the tank the idea been to blow whatever is blocking the outlet to get pushed out rather than working from the bottom and pushing it back in,once you have a good flow coming from the tank clean the feed pump gauze and pump body,if the filter/s are clean bleed the system,there is a sequence for bleeding, some had two filters,and others one taller one, open all the bleed screws,prime the feed pump till clear air free fuel comes from the back filter,close the bleeder,then do the front one,then the bottom one on the pump finishing with the top one,keep priming till the feed pump lever goes limp,open each injector pipe at the side of each injector a couple of turns, on full throttle spin the engine over till fuel spurts from the pipe unions,tighten the pipes and start the engine.
AJ