Problem: Engine started fine, drove tractor out of garage to make room for another project (didn't let it warm up), shut it down and let it sit outside in 20 degree weather four hours, now it won't start. Carb pops and a flame shoots out.
Checked spark (weak but visible), compression (only 90 lbs but equal on both cyls), replaced plugs, replaced gas line and filter (clear tube had a crack anyway) with fuel rated hose, removed and tested fuel shut off plunger (and power to it, verifying ignition switch was OK), checked float level (plenty of gas in there, nice and bouncy floats). Did the crazy thing and ran a hair dryer on the carb to melt any ice, thinking that because I didn't let the engine warm up before letting it sit that somehow ice crystals were blocking the jets (getting desperate here!). Cranking doesn't flood engine/soak spark plugs due to the flames ;-). This is the extent of my diagnostic capabilities.
Robin engine mechanic found that the stator assembly was "shot" and hence the timing was virutually crazy. Not that the timing slipped out of adjustment, but that the coils were blown, barely firing the plugs and at the wrong time. Nothing I could have figured out! Parts were relatively cheap (about a hundred after a trade in value) but the labor to pull the engine, make the repair, and replace the engine was 4.5 hours. Actually amazing he did this so quickly, given he had never seen a PT before...and also took the hyd pumps out with the engine, so all those hoses were removed as well.
Related post:
http://www.tractorbynet.com/forums/power-trac/270560-have-you-considered-cutting-out.html