OK, so what is causing this oddness? Running a bit funny.

[northwind]

Developed a bit of a running problem today, and I'm a little stumped. It's possible this problem's been there for a while but that I've just not ridden for long enough, this is the first time it's come up.

After about 20 minutes of typical mid-throttle riding on bendy a-roads, I develop a stutter. It sounds and feels like fuelling, in fact, it feels exactly like carb icing. I'm reasonably sure it's not a wet plug as mine is sealed up, and it wasn't that wet (plus, it didn't really feel like it, I think I was dropping some sparks but not all). So, what I'm getting is a mild surging and a very obvious loud misfire.

What's odd is that if I immediately roll off the throttle as soon as it starts, it fades away (though comes back a few minutes later) but if I carry on with a constant throttle, it gets drastically worse over 30 seconds or so.

Once it happens, the only thing that reliably fixes it is stopping- after a minute or so of idling or being switched off, it's happy. Oh, the idle is perfect by the way, even when it drops from heavy misfiring it goes straight to a stable idle.

But, it doesn't happen at all at higher revs- I managed 30 miles of high speed motorway without a flutter.

I've not changed the fuelling lately, and it all looked fine last time it was dynoed. It feels fine the rest of the time, nice quick pickup etc. Definately not a stuck choke. Oh, and I'd blame bad fuel but I refilled with only about 2 litres left from a different source and it still happened.

Any suggestions? I'm a little stumped... Going to check the plugs and air filter, and clean and balance the carbs tomorrow but that's just kind of random servicing, not really a targeted cure.

Perhaps along with cleaning the carbs check the fuel lines incase theres sh*t in there?

If you DID have bad/stale fuel, it may take several tanks to flush it out.. try using a few tanks of V-power before going much further. And yeah clean, flush and reclean the carbs..

Sounds a little like the issues I had with my curvy Northy.

Put that down to the Factory Pro needles wearing the emulsion tubes.

I'd check the signal generator, sounds a bit like my symptoms when it decided to start packing up.

[Bibio]

worn needle/emulsion, worn float needle, sticky float, water in the float bowl...

i'll go for sticky float or water in the float bowl...

im glad you mention this as this is what i have had the last few trips on the bike and was thinking i have a prob

watching with interest lol

[NedSVS]

Does it stop missing / stuttering if you wind it open from this midrange throttle position? It may have become too rich because of emulsion tube (needle jet) wear but the motor will handle a rich mixture better under load. I checked my emulsion tubes for wear by making pins from suitable size steel wire and filing / polishing the end to an elongated oval shape accurately measured with a micrometer. I found they were about 0.075mm (0.003") out of round at 32000 miles (almost half of which was done with Factory Pro stage 2 needles).

[embee]

....obviously check carb diaphragms too.....

[northwind]

It doesn't improve immediately under load, and the misfire's bad enough that it won't pull effectively up to higher revs so I'm not sure if that would actually cure it, since I can't actually make it happen Another observation- it's not heat/run time dependant, last night it happened after 2 minutes ride from cold. Until now it's always come on after more riding.

I think all the comments about the needle circuit are spot on, that's the obvious home for this problem. I would have thought that worn emulsion jets would be a constant issue? But maybe not. I've not had it opened up yet so I'll take a look today and see if there's anything obviously dubious, I've got some spare carb bodies so if worst comes to worst I can just swap bits.

Oh, the emulsion jets, are they replacable?