I have just finished a refurbish on a KF4 1200 and being bothered by major power loss and very rough running at random times.
A broken ignition wire was found so the pickups/ stator and modules were replaced.
Problem sorted almost ,some brief stumbles at part throttle and shortly after going to take off power.
I had taken the air box carb heat off,so ice was a suspect!
We manufactured some "hot blocks"to go on the carb bodies,and this cured the problem except on bad ice days when I don't think they could keep up due to the large amount of air blowing direct on the carbs(firewall the same as ambient temp)
An air deflector was made and installed(attached to the rear of the gearbox)
The engine bay temps are now 20-30c warmer than ambient,and I can't reproduce the stumble even after a long part throttle decent.
I hope this is of some help to any forum members who may encounter similar trials.