Ran my bike along a dual carriageway en-route to trials centre near me for the first time since sorting carb leaks last year.

Meant I had the throttle quite open in fifth gear - I'd say ~35-40mph-ish, and after about 1/2 mile the bike cuts out.

Pull over, stare at it, have no idea what to do, take the tank cap off although I know that's full. Decide to kick it (!) then change my mind and decide to try starting it! Thing starts first kick and burbles away, staring back at me with, like, 'what's the problem!' Grrrrrrr

Decide discretion is the better part of valour and turn around and head back down the dual carriage to home - 3/4 mile back down the road (through some green lights) and it cuts again - coast to a halt, gather myself, starts first kick again. This time just a r/b and then slower roads and make it home with no further incident.

Last year I replaced the float needle and jet when I had carb leaks and bought a 200 size, as per manual, and found when I stripped the carb that it had a 250. I kind of assumed that maybe the oversize float jet might have been a contributory factor in the leaks, but now I'm wondering if it actually needs a 250...? Just to confirm, the main jet is a standard 122 size, making the 200 float jet > 1.5 x as big, the recommended ratio, as I understand it.

Note also that 3-4 times over the last couple of months I've been a mile or so up the road, no long open throttle runs, to a closed car park where I've been practicing slow stuff and had the engine running for 30 minutes or more, all on low throttle/1st gear, with no problems.

Fuel tank breather / one way valve blocked / partially blocked and causing a vaccum?

Or maybe an issue with the fuel pump / breathers if the bike has the tank at the rear? cant remember which year it changed.

 

It does indeed Need a 250 to allow enough fuel into the bowl. For road running, that is.

Edited April 15, 20197 yr by copemech
E

Doh!

I'll check the breather but I think that's fine unless it's got some recent blockage. It is the rear tank version, with the Dellorto vacuum pump. My understanding is that this will still flow fuel even when not working, i.e. fails 'open'...?

Looks like a carb-off, larger float jet jobbie! ☹️

these were reknowned for trouble with the vacuum taps, also as mentioned the tank breather

 

Did another run last night, and this time pulled over to the side of the dual carriage and paused for a moment with the engine idling before pulling away again, just before where it cut the last time. Got all the way to the trials centre, and then ditto on the way back.

I think I can manage this as a medium term workaround.