Up to 3000 rpm you are using the primary carb and in part the idle jet. Hence we can say with reasonable certainty the issue isnt in the primary carb. Also this carb is cd-slide. i.e. forced operation
The secondary carb comes into operation at 3000rpm. However it is a venturii and will only come into full operation if the vacuum draw is sufficient. It was designed to assist mid range bog down of the primary. For this carb to come into full operation it really needs to have the throttle open wide consistently. i.e. flat out or when accelerating. Above 5000rpm matches this model hence my earlier supposition tht it is a secondary carb issue.
Have you ensured the emulsion tubes are clear?
Faulty coils that are broken have the reverse symptoms. As the engine gets hot they break down, and they operate better then above 4krpm than below it during these conditions.
However carbon tracking in HT leads will tend to arc out to earth before reaching the electrode (plug) too hence why I didnt discount it.
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