Kubota diesel won't fire

billr

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Make
Bocat
Model
743
Year
1986
Miles
not many, top speed is 7 mph
Engine
Kubota diesel
I was going to put this in "Small Engines", but then decided it belonged here.

This is a 4-cyl Kubota of about 1785cc, in a Bobcat 743 skid-steer. Yes, it cranks.

It has sat outdoors unused for about 2 years, so I was worried about water in the fuel. However it fired up easily/normally and I used it for about 20 minutes before it started losing power under load and died. I cranked/fired it again and it ran for several seconds, then died. I repeated this several times until it would no longer fire. The battery is 12 years old, so cranking is poor, but no worse than when it first cranked and fired OK. I will provide plenty more details is anybody bites on this thread...

PS: Those of you that no nothing about diesels or Kubota, but do know about computers, can still help! I have files of the the manual for this beast, but they were sent "disjointed " because of size. There are 24 files in the format "74x.7z.001" through "74x7z.0024" If anybody knows how to "stitch" these together to make a PDF of the manual, sing out!
 
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billr

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Yeah, fuel starving is my guess too. I see four possibilities: crud in the tank, water in the fuel, the spin on fuel filter is clogged, or the pick-up tube in the tank is broken. That pick-up tube comes out the top. It broke once before, inside the tank where no leak is visible. Another hint: I have disconnected the fuel line at the injector pump and when I squeeze the "priming" bulb in the fuel line, nothing comes out the open end.

My immediate wonder though is this: I have connected fuel line to the injector pump, going up vertical higher than the tank and filled this line with (fresh) fuel; such that it is a small "tank" that is known to flow fuel OK. I still can't get it to fire. Even using starting fluid it fires only briefly, not as much as a gasoline engine would on the same blast of starting-fluid. I have cranked quite a few times now, both with and without starting fluid. The little bit of air in my fuel-line "tank" has been sucked up into the injection pump, and I would think fuel would be in there again by now too. Are diesel injector pumps that fussy about air?

What should I try next, pull off one of the injector lines and see if fuel is pulsing out there? Take compression? I know I will have to chase back toward the tank, find out why no fuel is coming out of the line; but I would feel a whole lot more confident of my diagnosis (and later success) if I could get it to run on my small known-good "tank". Also, if it was a problem of water in the fuel, I would like to get that flushed out with fresh fuel ASAP.
 

dabunk

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Diesels are very fussy with air. Go directly to your injectors and crack the line nuts loose then retighten. Then while cranking crack the injector lines one at a time to bleed any air out. Diesel injector pumps for the most part do not suck fuel they rely on a boost pump to get the fuel to them at low pressure. So your gravity feed system might not even work to get fuel to the inj pump maybe if you have an electric inline pump?
 

billr

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I finally got it to fire and run normally with a small tank connected to the "injector-pump". It ran fine and I promptly moved it out of the awkward area it was in so I could work on it easier. I used quotes for the injector-pump, because I think there is a boost-pump. It has Mikuni on it, and looks to be a rubber impellor or gerotor or crescent-gear type.

I was surprised how fussy it is about air, but I guess that makes sense since volume of each injector-pump stroke must be very, very small.

The root problem, as I suspected, is that the fuel pick-up tube coming in from the top of the tank (a "dip" tube) broke off. The boost-pump could suck only air.
 

billr

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Well, it is a slow night, so I will give you the "epilogue" to this saga.

After fixing the dip-tube in the tank, again had no joy getting it to start. Repeated cranking sessions using the air bleed valve open, per manual, was of no help. Finally, I discovered that the knob on the bleed valve had come loose from the stem. When I thought I was opening the bleed, I was just unscrewing the knob from the valve stem! The knob was attached to the stem with a "double nut", then soldered. The solder had failed, but provided resistance after a few turns of the knob; so it felt very much like the valve was opening normally. I only discovered this once the afternoon sunlight was shining brightly and directly on the valve. After I fixed the valve (re-solder knob), the engine started on first cranking.
 

JackC

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Good thing YOU found it because I doubt anybody online would have ever came up with that solution.

Have a wonderful Thanksgiving.
 

billr

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Actually, a guy in Australia that I got the manual from says it is a common failure with this series of Kubota engine. I got the files for the manual extracted with a utility called "7-zip". I had tried that back in 2015 when I first got the files, but no joy. It turned out that of the 24 files he sent, I only put 23 in the folder I was working with; the 7-zip gagged simply because it wasn't a contiguous set.
 
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