Stuttering at 6k+

[ringer]

Went to Stratford-on-Avon last Friday after work - round trip of 120 miles. Very nice ride but did notice something for the first time. Up until now I haven't really pushed the bike too much. On Friday decided to do some overtaking on A roads (accelerating from 60mph to 75mph) but found the bike started to stutter (like an on-off hesitation) around the 6k rev mark (redline on mine is 7k5). Was very disconcerting and dangerous. Embarassing too as I was stuttering/crawling past cars and lorries. Now I'm sure I've occasionally redlined it (and a bit more) in the lower gears and not noticed this phenonemon. So what would cause this? So far I have come up with:

1. Engine seizing - don't think so, but have my hand over the clutch just in case.
2. Plugs / plug caps - was planning to replace soon anyway - will do so this week now
3. Ignition timing - tried altering it slightly to advance the timing - if anything it didn't run so well, so put it back to where I started
4. Ignition components starting to fail - could be very costly.
5. Carburation - I do know that the last owner had problems getting carbs set up and fiddled with the jets.

Any ideas?

[SteveMRC]

Manual fuel tap open ?

[ringer]

Manual fuel tap open ?

Yes - problems with electric tap mean I'm running permanently on the manual tap at the moment.

[3potjohn]

I have had similar on a different twin and the cause was water in the float bowls coming from water in the tank. Seemed like fuel starvation. My electric tap is OK (for now) and I got a new manual tap to replace my old one which was very stiff even after rebuilding.

[EVguru]

It could be a weak spark. The load on the engine is higher in higher gears so a problem shows up more.

My 250 2C developed an odd fault and would missfire in the mid-range if the ambient air temperature was high enough. This led to the almost unique situation of the missfire dissapearing when it started to rain!

I replaced the transducers with new ones I made up an the problem was cured.