Engine dying

[MalcW]

Hi all. A friend's magneto '59 500 twin that he's s just got on the road has a tendency to die at any sort of speed, over a sustained 40mph roughly. Once it's stopped it will almost immediately restart and run for another mile or so. The fact that it restarts as it does suggests to me that it isn't the magneto failing when hot (been there, done that with my bike). We've tried a known good fuel cap so I don't believe it's that. That leaves fuel supply. Since it runs Ok at lower speed I suspect fuel isn't being delivered fast enough. Running with both taps open doesn't help. Low level in the float bowl maybe? Does the normal running at lower speed suggest the pilot jet is OK? Any other thoughts? Just occurred to me - would a dropped needle cause this.

Thanks,

Malc

[Mick D]

Hi

Sounds like fuel starvation, first thing I'd check is the strainer inside the fuel banjo on the carb, if there's debris in there then clean the strainers on the fuel taps aswell.

Another ride and if the fault persists, a full strip down of the carb, check all jets and blow through all the galleries.

If that doesn't cure it I'd start looking for an ignition fault.

Regards Mick

[Rob Harknett]

Sounds like fuel. Try removing the bottom nut on the carb, clean jets and let petrol flush through. I once found a bubble of water in that bottom nut.

[clive]

Debris in the bowl below the main jet will cause this symptom. As you ride along the flow of fuel lifts the debris to block the main jet. Stop and the debris drops down again. At tickover and starting the fuel flow is not through the main jet and the debris is not drawn up so it runs ok. Mind you making it actually stop is pretty extreme more usually it just makes it cough and splutter.

[les ward]

I would also check the fuel pipes. Ethanol can make old fuel lines swell internally, restricting the flow.

[robcurrie]

I have had a similar problem, turned out to be sticking needle and seat, fitting new parts cured it.

Rob C

[g80csp11]

what about the fuel filters (gauze) above the fuel taps . rust from tanks could have blocked these , or alternatively the fuel cap breather hole

[Ralph]

Has it bee reboard could it be nipping up?
seen it a few times when engineers used to modern engines bore them a bit tight.

[56G80S]

Rob and Clive's comment is an easy one to do. I remember this problem on a BSA B40SS when riding down to London a mere 20 odd years old. Some PO had used instant gasket, it disintegrated and kept blocking main jet completely.

The times I did it over the remaining 100 miles.

Johnny B

[MalcW]

Thanks all.

Weather permitting I'll meet the owner of the bike on Saturday and we'll check through these suggestions and try a couple of test runs.

Malc