I'm having a very similar problem with my 1985 1900 dg at the moment. Fine from cold but a bugger to start (intermittent) when hot. Also the occasional 'pop' backfire as the spark is lost. Going to go the route of replacing individual ignition items (coil first,HT leads,dizzy cap and rotor arm,then finally leccy ignition unit)
Hope you clear yours up,let me know what cures it and i'll do likewise.
distributor problem
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