I would personally suspect the carbs first, I always ran the plugs Dave Silvers sent out ( being dead lazy I just ring up, give bike reg no, have the parts they send , never do I do part numbers) in my k3 when it was running to no ill effect. I have had a few dud ngk plugs over the past few years where the spark seemed ok outside the cylinder but once run a few minutes the bike refused to run - new plug ( again) sorted it, or severe abraiding of the electodes - I never figured out the answer to why this was the case, but had it on several machines and with ngk plugs from several suppliers. Every time the plug seemed ok but refused to fire in the engine, re gapping - no efffect, swap plugs for new or known good, ran fine.
