The Gap on the Plugs is in the Neighbourhood of 0.035" I asked can the Spark jump a 3/8" Gap, much larger, cause a Spark at 0.035" at Atmospheric Pressure really means nothing as little to no Stress is placed on the Ignition System.
So the Motor will only Start if it is at Full Throttle? Are you using the Choke at all attempts to start, fast or slow?
What else have you Tinkered With/Adjusted? Did you fiddle with any part of the Throttle Linkage, as adjusting Screws?
While a Carb may look Clean, it has many Fuel Circuits that can be Dirty or Plugged that require disassembly to Clean .