If the modified choke is actuating properly, the motor should be getting the fuel needed to start. Overchoking will quickly flood it and the spark plugs will be wet. What about the coils. Are they wired up to the correct set of points? The thumb pump on the tank doesn’t pressurise the tank, it fills the carb bowl full of fuel. Once the motor is running, crankcase pressure pressurises the tank via the second fuel line to the tank. Starting a motor with a near empty tank might require pumping the button a few more times until enough air pressure is built up to force fuel to the carb.
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