Our neighbor has a Riding lawn tractor with a B/S engine... 28N707 type: 0150-01 code: 9409124A. The engine starts pretty good however after only 30 seconds - 1 min. the exhaust pipe becomes "red hot" WHY? In addition to that when he is ready to shut down the mower (idling it down) it backfires with a blue/yellow flame. Why??

Hi,
Air cooled engines get very hot. It could be that it has a little trash in the carb making it a bit lean or that it has a partially sheared flywheel key that has changed the timing a bit.
Peace,
Clarke