small IC engine dying after few mins

[Jules]

So I've been screwing around with this petrol-powered hedge trimmer (single cylinder 2-stroke, small Walbro carb) which was given to me in non-running condition. Sadly owner had tinkered with the carb settings without realising that it was a split fuel line that was causing flat-out refusal to start, so I'm somewhat setting it up 'blind'...

I stripped the whole lot down (carb and cylinder) and cleaned it all out, after which I could get it to turn over a few times and then die. After some futzing with the low and high settings on the carb, it now seems to start well enough, and will then run happily for anything between 5 - 10 mins before abruptly stalling out.

Any ideas where to go from here? There seems to still be fuel in the carb's pump chamber when it dies - but I also see air bubbles in the fuel line when it stalls, travelling back toward the tank (but maybe that's normal for one of these carbs; I'm not sure if there's any valve to stop fuel draining back to the tank when it's not pumping). Whether it's pumping *enough* fuel is another matter. I presume it's not debris in the carb still, as then it wouldn't run for as long as it does?

It's perfectly possible that the carb's just utterly worn out, and I'm not sure if there's an easy way of testing short of just replacing components (pump's needle valve, diaphragm etc.) - but I wondered if there might also be other things which might cause a small IC engine to run perfectly well for a few minutes and then suddenly drop dead...

Hmm, I'm not actually running with all the baffles in the silencer right now (one was a really fine mesh and totally choked with crud that stubbornly refused to budge) - is the level of back-pressure from the exhaust critical in these small engines?

I do have another identical Walbro carb that I could raid for parts, but that one's none too healthy either, so it's not really a viable source of "known-good" bits. I've got an electric hedge trimmer too, but heck the petrol one does a *much* better job - when it runs! :-)

cheers

Jules

[Bob Eager]

Blocked vent in the fuel tank/fuel cap?

[Jules]

Good point - I'll check!

Thinking about it, I didn't clean the fuel filter either, because it's a goofy design with the filter inside the tank and no easy way of getting the thing out. I can probably rig up a temporary filterless tank and see if that improves things...

cheers

Jules

[The Natural Philosopher]

That was what my problem was/ Blocked klunk.also is a fuel filter.

Now I did get mine out by..hmm. Shaking the tank upside down until the thing fell out the filler or near enough?, and was then able to pull out with small pliers..use a clothes peg to hold the line out before detaching the klunk.

wash in fuel and blow through.

[Part timer]

I had that problem last time. I mixed a new 1 litre batch of fuel and the hedgecutter wasn't its normal self. Had to rev it hard, and once it's hot the engine normally restarts on the first pull of the starter (indeed on the first 10cm of cord). Then it just died with half a tank of fuel left, I unscrewed the tank cap and the air rushed in. Worked fine after that but I will dig out the manual to check about tank venting.

[Harry Bloomfield]

Is the tank cap vent clear?

That is normal.

It is, for best efficiency, but would not stop the engine after several minutes. A partially blocked exhaust could stop it after a few minutes though. A blow lamp would burn the crud off.

[Jules]

Just reporting back...

Filter seemed OK (it's some porous plasticy type thing, but wasn't obviously discoloured / dirty / gunked up.

Fuel tank vent was fine.

Next thing to try is the fuel hose itself, I think; the OD was slightly larger than the original stuff (ID the same) and maybe it's pinching at the point where it feeds through the tank wall.

If that doesn't work, I'll see what I can do with the 'spare' carb from the edger...

cheers

Jules