Glowing pipe vs spark, to overheating of the legs than normal is to read the spark plug for a direction:
Rich: Black coating on the porcelain. Ignition too retarded. Sparks too late, has fuel residue that is unspent. Moving thru the stork cycle's, hot unspent fills the pipe on one end, fresh air on the other; the flame front continues inside the pipe. Heats up and glows. This is riding to cool the pipes, not sitting still for over 10 mins.
Lean: Kinetic heat + too lean a mixture: parts per million kind of heat exchange of gases; lack of fuel for cooling; it's something like a oxy/acetylene torch; you turn off the gas; run pure oxy; and acts the cutting torch kind of meltdown.
How to read a plug:
Follow the porcelain all the way down inside till it ends deep into the plug. That is the proper reading of a plug. Not the nose. But you need a special type plug reading tool. So being stuck looking at the nose of the porcelain, black can show a shinny liquid type residual for a retard condition...if... the tune always had the correct jetting, where nothing has changed fuel wise, now look for an ignition problem.
White or clean porcelain finish. Where that porcelain finish is going to melt like sand to glass and show a gloss, if not crack the porcelain at or near the center electrode.