Having a carp swimming around in the carb can definitely cause problemsmy 2 cents ; carp in the carb', several instances where manufacturing swarf blocks jets.
If it idles well, yet misfires at speed, the misfire relates to either higher rpm or greater throttle opening.It seems to worsen as the engine warms up. It doesn't seem to be RPM-related. I haven't tried revving in neutral. It doesn't seem to matter if the engine is under load or not.
I've swapped out the BTH coil and HT lead. Non-resistor plugs and cap. I'm now planning to send the BTH back to the manufacturer in the UK to determine if there is an internal problem. It's happened before.From what I have read in this thread so far it seems to be heat related. I would be checking EVERY electrical connection associated with the ignition.
Change/clean/fix ONE THING AT A TIME.
Here in no particular order is what I would suggest next. I would start with the spark plug lead - actually I would replace it even if it looks OK, just in case. You want a copper cored lead and a non-resistor plug cap.
Check where the HT cable connects into the BTH coil for any sign of oxide or corrosion (or oil) and clean all that up. If you have a spare coil, put that on and see if matters improve.
Check all 12V wiries including the earth wires to be sure they do not have any trace of corrosion or oil in their connections and clean them up
The inlet manifold is hot but the carb body is not. There is a thick insulator between carb body and inlet manifold.Just wondering. Once the mis-fire starts happening have you reached down and felt if the carb itself is getting warm or even hot?
Brian, The magneto CoreyL is using is a modern BTH which is actually a self generation electronic ignition system so no condensor and no points used.classic condenser / short failure in the vid'.
in CoreyL's situation given the symptoms i'd look for something simple; is the misfire affected if the choke is applied judiciously at high revs, are the points in good condition ie spring, pads square nothing shorting at high revs.