My 65 electraglide FL misfires and breaks down under acceleration with the headlight on. Its fine when the headlight isn't on and also just cruising around with the headlight on. I dunno how long it been like this cause I don't ride at night these days. But late home the other day and start getting dark. Where to start looking.

 

1 check the charging system by the shop manual

2 the 65 does not from the factory have any fuses or even a circuit braker so - if your flip switch - headlight wiring - ignition switch has a short

it will do what you have when you turn the HL on

 

If it was shorting, which I agree with you by way, why doesn't it do it all the time the headlight is on. Only seems to miss and splutter under load

 

see check charging system = you have a draw pulling the battery down while you turn the throttle - what it is - is in your checking the things that are using or not using electric ---------- this is the first step it could be something else