that was my first thougt as well when reading this post.
Especially when it pops into the exhaust. A fixed timing advancer can cause these issues. Needs disassambly, some cleaning and greasing (only inside!) to work again.
It is always a good idea to check the timing advancer !
Prior to disassambly and just to check, remove the ignition cover, keep fingers, children and pets away and start the engine. From approx 3.000 rpm you should see the weights of the centrifugal governour moving to the outside / a larger radius. If they don’t do that, you have found the culprit.
Not sure with points ignition of the Bs, but the same mechanical timing advancer is used with the pointless ignition of the LTD. I use WD40 or similar to lubricate that stuff.
Will that work with the Bs as well, if used carefully ?