Splits in the Stromberg diaphragm can be small and hard to spot. Also means that when they are very small their effect on the piston movement can be inconsistent. The diaphragm can also become distorted which also affects piston movement. This distortion is probably more likely now due to ethanol fumes.
Ron's comment "direct it to the correct spark plug" also means the correct plug type for your engine.
I agree with checking the advance and retard mechanism. The rotor should spring back when turned against the mechanism and released. Check that the vacuum diaphragm moves the breaker plate. Sucking by mouth on the pipe is usually enough to see if it's working.
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