Misfire

If it's only happening when lights on, suspect lighting wiring touching/frayed to earth somewhere. Headlamps are good place to start looking on old Brits. If happening without lights, suspect rectifier. Check you are getting 6V (12v if it has been converted) or more with engine running using voltmeter. Ammeters themselves can be a problem, short it out, ie remove wires from back and join together. Sorry difficult without looking. Is the battery itself ok?