80 sportfire power loss

[Andyrobertson55]

Hey guys,
Disclaimer: definitely not advanced mechanically but I know more than average Lehman.
Newer member to this forum. Last summer I picked up a 80 sportfire that had some starting and idle problems. Essentially cleaned the carbs, changed the carbs and drained the old fuel. What do you know it fires after 3 pulls.

Fast forward a little. Went for a ride today everything was fine going great having a blast. Sled loaded up and backfired so I started heading home to get a look at it. I get to my neighbors house and the pitch of the engine dropped and sounded super deep. My assumption was a cylinder dropped. Along with this I had essentially no power. I limped it home and let it cool down a little. Tried to fire it up to get it fully in the garage and it was like nothing changed sounded like it was running right and had power again. It did back fire pulling it into the garage as well. Pulled the plugs and they look okay the electrodes are black one looks a little more rich (attached is a photo). Anyways I’m looking for guidance on what the issue could be.
Thank you in advance!