Rome/KG - I checked my valves and everything seems to be in order. I called my friend who use to be a Cat engine mechanic and is into old equipment and gas engines. He told me to check my condenser on the mag. I think that will be the next thing I look at. Thanks.

check all valve clearances. inlet too tight, or exhaust too sloppy mabe can cause it.

Change the plugs. I had a similar problem, plugs looked good but were not good any more. New plugs solved the problem.
Chuck C

-you mentioned your plugs look different? thats odd? can you switch them around and see what happens...? but the best would be to replace with new

-plug wires...good idea Neil....easy to miss and overlook...most wires can be checked with an ohmmeter...get a number and see if they're reasonable resistance..

-i don't like to wildly replace stuff, esp if it can be checked out but you never know

Some odd firing twin cylinder engines are known to have problems with the one cylinder running leaner due to the uneven flow through the carburettor. Could explain your spark plug differences. Maybe the mag gets time to generate a better spark as well during the longer gap. Only a guess on that, someone will know better and tell us all.
cheers WF

Cracks carbon tracking in the Magneto Cap?
May as well check this while replacing Condenser.

I had a problem with a small skid steer. Sounded like carb problems. Rebuilt the carb, no difference. I then remembered the "Old Saying" 90% of carb problems are electrical. Replaced the condenser and problem solved.

I lean to cracked cap or condenser. I had a racing engine that would backfire under heavy load on a long run.. ADDED fuel over and over > bigger carb, more fuel pressure. End cause was that several plugs were to hot and glowing and pre igniting the fuel. Right plugs ? try a cooler range plug also.

I finally got back to my project and got the pony running smoothly. I ended up removing the pony head and noticed that #1 intake valve was a bit sticky. I cleaned and oiled that valve and it runs like a champ again. Hopefully others can learn from my frustrations with this pony. I actually think that the pony on my 46A might have a slight issue with one of the valves too. These little flatheads don’t get run much and do a lot of sitting. Whatever supplemental lubrication we can give the valve train can only help I believe.