I think you guys may be right with something electrical, I just took it over some speed bumps somewhat faster than one is meant to go over speed bumps and with each bump I got (air) a small amount of the stutter.... it felt like multiple misfires. This bike's electrics are 99% gaffer tape, 0.5% prayers and 0.5% prior owners strange ideas of DIY.
Luckily I've got a mostly empty day tomorrow so it'll be tank, fairings, airbox off and have a dig about.......was hoping to not need to do anything to this bike till I gut it in the summer for project parts.
edit to add - Mine is a zx400k (zzr400) jap import so no carb heaters. I think the UK model 600's had them though.
