Yanmar 1gm 10 won't rev

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danbridger

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Well it will but not for long.
Recently my 1gm10 has started misbehaving.It starts promptly on the Tickover position and runs smoothly with minimal smoke.However,if I accelerate to over about 2/3rds (sorry no tachometer) it revs up for a while but then the revs drop until I throttle back and it is happy again.In my fuel line I have an outboard motor bulb to help with purging the system of air after filter changes.If I give that a squeeze or two the engine will rev up. to date I have tried:-leaving the fuel cap loose,disassembled all the pipes,ensured they flow freely and trimmed the ends so the jubilee clips are on fresh rubber,changed the filter and the one in the engine and bled the low and high pressure systems. the diesel is clean with no sign of water or bug.
It doesn't feel like an exhaust side problem to me.From previous threads I suspect the answer will be that there is an air leak I haven't found yet, but does anyone have any other ideas?
 
Could be the lift-pump misbehaving. Does it have to lift fuel to the engine filter, rather than be gravity fed/
If the former, could you arrange a small temporary tank at higher level to try the engine?
 
earlybird said:
Could be the lift-pump misbehaving. Does it have to lift fuel to the engine filter, rather than be gravity fed/
If the former, could you arrange a small temporary tank at higher level to try the engine?
Click to expand...

The fact that it will rev when the primer bulb is squeezed supports this theory.
 
Is there some restriction in the fuel line? Got called to a brand new Elan with similar symtoms, traced to a small piece of cloth in the fuel tank being drawn into the diptube as fuel demand increased, dropped free as flow rate decreased. Suggest you rig up a temporary fuel tank with a positive pressure supply such as a pop bottle of diesel held higher than the engine as VicS suggests.
 
Thanks for the advice so far, the tank is below the filter,so I guess the lift pump does have to do some work.I did try using a jerry can of diesel as a temporary tank, there might have been a slight improvement but I had the temporary tank sitting on the old none ,so it wasn't much higher.
I have just looked up lift pumps.They don't seem to be a repairable item,just fork out £110 and hope it fixes it.
 
The most common source of air getting in is through a poorly seated sealing ring in the top of the fine filter on the engine. Difficult to get right as the locking ring is in an awkward place and easy for the bowl to not sit straight on the seal.
 
Tranona said:
The most common source of air getting in is through a poorly seated sealing ring in the top of the fine filter on the engine. Difficult to get right as the locking ring is in an awkward place and easy for the bowl to not sit straight on the seal.
Click to expand...
The first 1gm that I fitted had this problem, and I'm pretty sure it was a manufacturing fault as I just could not get a seal. Solved by buying fatter O rings. Really annoying as it took so long to trace the leak and in that time I completely lost faith in the engine. Basically a good little engine but spoilt by some stupid design faults (who decided to put the water pump out of sight behind the crank pulley) and poor assembly (why do they use bolts with heads made of cheese, and then torque them up with 6 foot scaffold poles).
 
in the end,fortunately,it proved to be an air leak. I think it was the extra1/4 turn on the primary fuel filter cartridge that did the trick .Many thanks for all your suggestions, thankfully I didn't have to explore most of them!
 
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