Suspect fuelling problem

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Oct 25, 2007
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Hi chaps - I'm tearing my hair out with this problem:-

I have a 920 Commando which was fitted with a single 36mm Amal Mk2 carb - it ran OK but starting was always a bit of a problem and it also tended to run 'rich' thus I recently fitted a similar 34mm version. I was delighted with the result and on its first run it was brilliant. On the next run it started from cold fine and I set off - got about 100 yards and it started to misfire and then died. It would only restart with the choke applied and ran like a dog (if I shut the choke off it died immediately). I removed the carb and cleaned it out (although it looked fine in any case). Removed the petrol taps and checked them - good flow and clean filters). Put fresh fuel in the tank and checked and cleaned the fuel tank breather hole in the fuel cap. The plugs look fine (Denso Irridium items which I recently installed)with a good fat spark - Ignition is Tri-Spark Classic Twin). It started fine with choke from cold and I let in run for about a minute and then switched the choke off - the engine started to 'miss' a little and I adjusted the pilot a screw for 'best running' which was about 2 turns out (which seems OK to me) - It then idled and 'picked-up' perfectly. I then set off for a ride and again she 'conked-out' after about 100 yards i.e. the same symptoms a before. I'm convinced that it's a fuelling issue and I'm loathed to change jet settings etc. as the bike ran so well on it's first run but I'm at a total loss as to what the problem may be.
Any ideas please ?

I'd be looking at the float and the float needle plus the seat, could be the float is not allowing the needle far enough off the seat to allow enough fuel in.
Mark — after you follow Kommando's advice. if it's a Mk2 Amal, you should have two float chamber vents with little hoses attached. I assume these are not blocked or kinked, because it does sound from your account like it's running on the juice in the float chamber after initial start up and then not getting much after that. Have you run it with the cap off the petrol tank? Other people have been fooled by petrol taps that appear to give good flow but there was an air lock somewhere. If it conks out and you wait a few minutes, will it start again? If so, how long does it take for it to start and does it always need the choke to start? Do you have one of those inline fuel filters? I chucked mine away because occasionally they would cause temporary air locks. I assume you really cleaned out the air and fuel circuitry with brake clean or carb cleaner (and/or compressed air). Are you certain you don't have an air leak from the inlet manifold and rubber hose?

Finally, although it does sound like a fuel problem, check that all your earth connections are tight and in good order, because bad earths are devious little buggers that can masquerade as fuel starvation.

90% of fuel problems are electrical. 90% of electrical problems are electrical. :mrgreen:
Hi Chaps - thanks for the ideas - the idea ref coils could be an issue - I've just fitted brand new Tri-Spark coils - I wonder if one of them is duff ? - I've kept the old Lucas items - I'll refit them and see what happens. The carb to manifold link is good (brand new 'hose' and manifold gasket).

90% of fuel problems are electrical. 90% of electrical problems are electrical.
That is truly often the case. Mine went down recently, wouldn't rev but ticked over returning from a run. Eventually it stopped completely after popping and banging. Turned out to be the modern blade fuse I had on the battery, it had fractured internally but not blown. However since yours is presumably not popping and banging before it stops ? (signs of intermittent elec fault) does sound like fuel. With a duff coil it would run on one very weakly.
Hi all - an update - I've re-checked the carb for leaks, flooding, blocked jets,filters and tubes etc. - I've re-fitted the old coils - the bike started from cold easily enough ran for about 10 seconds and then just 'died' - fitted new plugs (NGK) and restarted - this time it was coughing,backfiring (flames from the offside silencer) and mainly running on the left cylinder. I'm beginning to supect ignition. To re-iterate the ignition is Tri-Spark Classic-Twin (coils are in series - thus 'wasted-spark') - timed correctly with a strobe.I know that Boyer can suffer with intermittent electrics and produce 'weird' effects. The strange thing is that I'm sure I can hear a faint 'ticking' noise coming from the ignition switch when 'off'. All other electrics (lights etc.) seem OK. The battery is at 12.6v. I'll have to re-check all of the ignition wiring, switch contacts etc.

I'll keep you posted - this is so frustrating as I've had to postpone two MOT tests as I've broken down with this fault each time I've tried to get to the test centre. The test centre guys are very understanding !

mwoo said:
The test centre guys are very understanding !


They've been dealing with British vehicles for a hundred years, of course they are. :mrgreen:
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