Haven't seen a carby engine with a fuel return line,only a vent to the charcoal canister or the tank breather,which usually has a small filter to keep dust out of the tank.If the tank breather was blocked,you will hear it suck then the fuel cap is released after running for a while.This would lead to fuel starvation corresponding to how hard you pedal it,not at any particular time,temp etc.You could check this by taking it for a run then releasing the cap,listening for the vacuum release.Okay fellas, update it maybe completely useless but going back to the possibility of a fuel issue I changed out the back fuel filter again with a clear one, my guess return line? And had the car running for 5-10 minutes and it stayed bone dry is that normal?
From your symptom description,I suspect it is electrical and agree with Peter Connan,run a bypass wire clipped direct to the coil,
Cheers,G.