24-07-2024, 08:16 AM
Hi
At risk of adding yet another voice. You describe good starting and running OK for several miles before getting a misfire under load. This suggests something changing when it gets thoroughly warm, and presumably coming good again when it cools. The condenser is Number 1 suspect for this sort of malarky, but you have already changed that. I think the coil is the next suspect, as others have suggested. The standard coil primary is about 1.5 ohms, and I suspect there are many other 6 volt coils that would work, at least for elimination purposes. Plug firing voltage requirement goes up with cylinder pressure, so if the ignition system is weak the voltage can be OK at idle but insufficient under load.
Also, I have sometimes found that dirty points work initially but become worse when warm - I have no idea what the mechanism is, this is just from experience. When I say dirty, I mean there is a thin invisible film of tarnish/corrosion on the contact faces. A strip of thin business card soaked in switch cleaner and drawn between the faces will help.
At risk of adding yet another voice. You describe good starting and running OK for several miles before getting a misfire under load. This suggests something changing when it gets thoroughly warm, and presumably coming good again when it cools. The condenser is Number 1 suspect for this sort of malarky, but you have already changed that. I think the coil is the next suspect, as others have suggested. The standard coil primary is about 1.5 ohms, and I suspect there are many other 6 volt coils that would work, at least for elimination purposes. Plug firing voltage requirement goes up with cylinder pressure, so if the ignition system is weak the voltage can be OK at idle but insufficient under load.
Also, I have sometimes found that dirty points work initially but become worse when warm - I have no idea what the mechanism is, this is just from experience. When I say dirty, I mean there is a thin invisible film of tarnish/corrosion on the contact faces. A strip of thin business card soaked in switch cleaner and drawn between the faces will help.