I've been doing a bit of a rolling upgrade on the bike, fairly recently I've replaced the diode board, coil, ignition module, HT Leads, spark plugs. The bike has developed a stutter/misfire around 2000 to 2500 RPM in the last few days, then runs cleanly up. The carbs have been cleaned and balanced and fresh fuel put in, but I can't fix it. It starts OK and ticks over OK Any ideas?