6748 runs, then stops

Hi all,

I have a 6748 that ran great until recently (when I needed it for the snow).

It began by not starting, then after using the hand primer it would start, then die. I changed the filters and hand primer as each time there appeared to be air in the pump so I assumed blocked filters or an air leak in the primer.

Now it starts, run for a short time (about 1-2 minutes) then stops. The cold start button is in for start, but drops out when it dies off.

It is as if the throttle is pulled back slowly until it stops. (the throttle position hasn't changed on the governor body)

I'm assuming something internal - any clues??

Ok,

I've now flushed out the governor oil - it was red (I think it was red fluid, not rust) but also dirty. I flushed it through with kerosene (JetA1 actually ) and slightly overfilled it with Castrol Edge 5-40 fully synthetic oil

It now runs for longer, but still drops out sometimes. I daren't take it out for a run to see how it is under load as I might get it stuck in my drive (track) AGAIN.

Should this fix it, or could there be an adjustment out?
The 24mm nut with the cold start screw spins of its own accord - could this be a problem?

More info.

I took the side cover off the pump and made sure the rack moves ok. It seems to, and I sprayed it with a light lubricating oil to make sure.

At max throttle, the rack seems to be at about halfway travel (around the pump pistons/springs), and with the cold start in, it moves across a bit further. With the engine running at 2500 revs, there is a little oscillation of the rack, but it is not fully open.

When the engine dies, the rack move across to the max fuel position, then stays as the revs drop off. Pushing in the cold start at this point will revive the engine revs. If not, it stops.

The throttle position is still at max revs.

There is some diesel leakage past one of the pistons, spraying up a little into the housing.

I tried adjusting the max fuelling (??) by slackening the locknut and moving the cold start nut, but I don't seem to be doing anything. The nut just spins without any apparent movement of the rack.

I took it for a run, and it went ok for a while, then stopped. Pushing the cold start started it again, then it stopped within a minute. Pushing the cold start started it again, then it ran for a while before stopping again.

It was very down on power - I had to drop it to 3rd to get up my hill.

Hmmmm

???

Sounds like a fuel supply issue, rather than internal,
Could be the seals in the pump, or a blockage between the tank and the pump causing it to suck in air through the hand primer.

Check the tank tap is fully on! ..... remove all pipe work and blow out with compressed air .......... see my comment on the 6718 from 5 mins ago

Could also be Diesel bug in the tank or filters, the small amount of water in the Diesel bug will freeze up solid in cold weather.
Bad diesel bug is a thick Jell that will block all the plumbing.

Do let us know what you find

Thanks Steve,

Ill have a look at those points later.
I have already changed the fuel filters (and checked they were fitted the coarse then fine) and fitted another hand pump, as I did suspect them. I haven't checked the tank outlet or blown the lines through yet, though.
It was the fact that pushing the cold start sorts the fault that threw me - I would have thought a blockage would not be 'cured' by this.
I did wonder about air - having had the filters and primer off, and then undone pipes for bleeding, I could see that being a good possibility. My next plan was to block the tank return and pressurise the lines with the hand primer to check properly for leaks.

I'll keep posting what I do and what I find for others, and obviously any ideas are very welcome!

Your comment about 'Diesel Bug' was interesting,

I asume you are talking about the fungus cladosporium resinae[/] which lives on water and hydrocarbon fuel (normally kerosene, jet fuel and diesel) in tanks.
I have some Biobor I 'found' that I can put through if I find it. I haven't seen any sign (brown slime) in the lines, bowl or filters though.

For others, if this is an issue, regular water draining will eradicate it. Beware of pouring a biocide in and running your engine - the fungus will die and release as a sludge into your fuel system and block it!
If you wish to kill it, biocide, then drain and flush is the way to go.

It sounds like a lift pump piston that seizes. The lift pump doesnt have to do much because its gravity fed, but if the piston seizes it does block fuel flow.

There is a big slotted screw, you wont have a screwdriver that big so i'd take a piece of 2mm metal and a vise grip. unscrew the bung and push with your finger if the piston moves (its spring return so easy to check, of the cam isnt pushing it out)

I checked the piston at the bottom of the lift pump, and it was fine. I removed it, gave it a clean, then refitted it.
I also blew back through the tank line, drained the fuel and refilled the tank.
It appears to have fixed it, although I'll need to try it under load first.

I'll post back here if there are any problems