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Ignition fault would be my first thought, so check the condition and security of the points, the condensors and all the wiring leading from them. Some Honda fours, 550's I think were prone to the yellow and blue wires working loose where the snap connectors join in the rubber shield near the back brake pedal.
If its a 78K with PD carb and starts without choke then the carbs just aint set right
check the resistor plug caps, they normally have 5k resistance,but over time,when hot ,they fail,I had one go 10 days ago,new cap,smooth as silk again ( I hope,well it run sweetly now)does anyone understand electricity and resistance? with electronic ignition,I get a FAT BIG spark,so I run 33thou plug gap,how does 5K resistor plug caps affect them,would the bigger gapcause the cap to fail sooner ??
I does sound like an ignition problem. There's two separate, independent ignition systems, one firing 1 and 4 and the other firing 2 and 3.Try to determine which by feeling the temperatures of the header pipes.What ignition set up have you got; is it the original points or have you gone electronic?
Ignition related I am sure from the pair of cylinders going like that - assuming the filters have had a similar life ( possible for outside pair to get more weather , maybe washing out any oil etc, depends on the pod filter type. I have had issues in the past with condensors malfunctioning when hot ( belive it was that, I described the symptoms to a collage mechanics tutor who recommended chainging the condensors and this seemed to resolve it.
I agree. I rode home 70 miles on my 400four going like a 200two. I was just a dodgy capacitor.