Wiring priming fuel pump
From a post of mine, - yrs ago.
If the vehicle has been sitting a while, the carb and lines may empty, but it is not nec to continue cranking till it fires ..
After 2 crankshaft revolutions the cams one revolution has cycled the pump's lever once, compressing a spring/diaphram inside the pump.
Stop cranking, wait a sec. and the spring/diaphram maintains suction for a few seconds moving fuel. Give it another short crank,wait a bit. - the lever once again cycles the pump, fuel moves after key released.
I usually gently pump throttle on 3 rd try, and it usually fires pretty quick.
An example, how often have you pulled/opened a fuel line to the carb after the engine hasn't run for a while, and got a spurt of fuel ?
That's the spring in the pump maintaining pressure to the carb, while it maintains vaccum to the tank . .
Really helps when you have a weak battery to understand pump function.
True on all manual diaphram pump vehicles to my knowledge.
Cheers .