Hi NormK,
It's likely to be the starter o-ring which soften and allow the fuel to excape and air to enter during cycle which you lost suction of fuel into the cumbustion chamber. If you're lucky to have purchased this particular starter o-ring that will just soften after couple of used. This o-ring will soften even a day of use and without warning. Of course couple of things to check - diaphragm cover possibly comes off, faulty decompress but unlikely, blocked jet nozzle. As you said, there is a spark, changed float needle and have fuel in the reservoir. Is the spark plug wet or dry during crank? If wet likely to have starter o-ring, if dry likely to be the jet, carby body vacuum issue, diaphragm or fuel supply. Hope it helps.
Cheers,
TheCarbyMaster
