Suddenly cutting out

[MalcW]

Bit of an odd one this. A dynamo/magneto 500 twin starts and runs fine, then after 5 miles or so suddenly cuts out on both cylinders with no advance warning. My first thought was the magneto failing when hot, a problem I'd had on my 650 twin. But whereas my 650 would take an hour or so to cool down before it would kick start again, this starts almost immediately. It will then run for another few miles, typically between 2 and 5 but occasionally more before it does it again. The fuel flow to the carb seems fine, but we've tried riding it with both fuel taps on, and with the fuel cap loosened in case of a blocked breather, but no difference. While it's running the throttle picks up well and it ticks over nicely. I'm sure it isn't a plug or HT issue as both cylinders are affected simultaneously.

All ideas gratefully received.

Cheers,

Malc

[dave16mct]

It might be the float needle sticking. The float bowl runs dry. You press the tickler and its away again.
Dave.

[1608]

Flow may be good to the carb, but have you checked the banjo filter. Short duration of running smacks of fuel starvation.

[MalcW]

Thanks Dave and John, both good points. I'll order a viton tipped float needle, as at the very least it means I've got a spare. The banjo filter looked OK from a quick roadside check, but I might swap it for another just in case. It does feel like a fuel issue (which probably means it is the mag!).

Cheers,

Malc

[Andy G]

Have you checked under the seat...a poorly routed ignition wire pinched under the seat can produce an intermittent dead short but only when sat on. Don't ask...

[MalcW]

Have you checked under the seat...a poorly routed ignition wire pinched under the seat can produce an intermittent dead short but only when sat on. Don't ask...

Hi Andy, a good thought but there's no ignition wiring under there as it's a magneto bike with the kill button on the end of the mag.

Malc

[raffles]

you could try removing the petrol pipes at the tank turn the tap on and check the flow
it could well be taps blocked in the tank

[MalcW]

you could try removing the petrol pipes at the tank turn the tap on and check the flow
it could well be taps blocked in the tank

I effectively did that by removing the banjo connector from the carb, then turned on each tap in turn to check the fuel at that end, which gave me a check of both taps and fuel pipes. The flow was good, and similar, from both sides.

Cheers,

Malcolm

[clive]

Have you checked under the seat...a poorly routed ignition wire pinched under the seat can produce an intermittent dead short but only when sat on. Don't ask...

Hi Andy, a good thought but there's no ignition wiring under there as it's a magneto bike with the kill button on the end of the mag.

Malc

I think that kill button is sprung loaded (although both mine have the comp end cap) no possibility of an intermittent fault on it is there?

[LynP]

I had a similar problem on a G3LS with monobloc carb. After a engine rebuild and with a cleaned and rebuilt carb, I took the bike for its first run. After about 2 miles it cut out. This happened on each of 3 further runs at about the same distance and each time it started easily after a couple of minutes. It turned out there was very small bit of something that was getting sucked up into the pilot jet and then it somehow cleared and the bike ran again, only for the bit to be sucked up again. Did another carb clean all fine after. Just a thought, LynP