Tractor Repair: igntion failure under load, spark plug wires, timing light

Question
IH 354 (4700 hrs) has intermittent misfire at slow and fast idle, runs in shop for 30 minutes with no change, but when working (snowblowing, etc) within about 10 minutes the misfires become more frequent, leading to rapid ignition failure one cylinder at a time, with finally just firing on one, then total failure, all taking place withing a few minutes. Have been able to restart after a cooldown period. Points, cap, condensor replaced, properly gapped. Plugs good. Dealer suggests possible fuel supply problems but that doesn't seem to fit the sequenced shutdown. They have also suggested checking all low voltage circuits (ignition key, etc). I'm thinking coil. Wife won't let me buy a new tractor so forced to repair this one....Help!

Answer
 If it only does it under load that does sound like a fuel supply problem, but you will need to verify if there is spark to the spark plugs when the engine dies to know for sure.  You could hook up a timing light to the spark plug wires and as the engine dies you would see if the timing light still flashes or not.  Since the flashing of the timing light is triggered by inductive magnetic fields in the spark plug wires caused by the high voltage spark, it will tell you if there is spark or not.  If the timing light does not flash, there is no spark.  Or you could pull off the spark plug wires with an insulated pliers and hold the wires close to the engine to see if they spark.  If there is no spark, replace the coil.  If there is still spark, then it is not an ignition problem and there is a restriction in the fuel supply.  Verify that the fuel line from the tank will run full stream.  If there is only a small dribble, that's not enough to run it under load and the tank outlet or fuel line is restricted and will need to be cleaned out.  The cool down period is either letting the coil cool off enough to start working again, or it is letting the fuel system fill the carburetor up again if it is starving for fuel.  By checking for spark as the engine dies, you will find out if it is an ignition or fuel problem.