[embee]

Just another comment. I had one occasion with somewhat similar symptoms, it would run under some load but went onto one cylinder at idle (and easily stalled as a result). I thought it would be fuel related because of the symptoms, it happened suddenly during a ride.
After draining carbs, checking idle mixture passages etc it made no difference. Eventually purely as an effort to eliminate the easy stuff I fitted an old pair of plugs and it ran perfectly. After fitting a new pair it continued to run perfectly so it was definitely one of the plugs which had gone down, though I've never experienced a plug failing in quite that way before it clearly can happen.
Try a new pair of plugs first, that's a cheap and easy one.