"""""- spark tested from end of ignition wire with tester while starting: seems good, regular"""""
So you have a good healthy blue spark at the end of a plug wire?
That says the ignition is working fine.
What's that leave?
Spark Timing - but you haven't changed that.?
Chain, but the compression is too good.
Fuel - not enough or too much. Pull a plug to see if it's wet - too much, flooding. An old tow truck driver's trick - starting a flooded vehicle - pull the coil wire out of the cap to give it a 1/4" or so gap. Start it up. After it warms up and clears the wetness out, put the wire back in. Or - remove the plugs, dry them out by letting them air dry, blow air in the chambers to dry them out.
Don't pump it when starting - use propane to enrichen it to get it started - you cannot flood it by using propane. Look at the sticky's - Propane Trick.