[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.