Why would the nozzle gasket be useless?-----Just don't leave them on your work bench.----Install them.-----Once carburetors are full the pressure can drop off on the bulb.-----Once motor is running the bulb does not need to be hard !
A sheared flywheel key will still show good spark.----But at the wrong time.----And no you can not find a sheared key with a timing light.----Pull flywheel or check timing pointer with #1 piston at TDC
Usually when I suggest checking the flywheel key the answer is.----" there is nothing wrong with it ".----Hopefully you assembled it clean and dry and torqued the nut to the correct value.
Some trouble shooting is required.-----Sounds like it is running on 2 of 3 cylinders.-----compression., post the numbers.-----Spark.-----Carburetor linkages.----Timer base moving.-----And does the tell tale hose come off the side of the block or was it moved per a service bulletin.----Should...