Not sure if you have the vacume operated fuel pump or just a strait petcock. If you have the fuel pump you will know since it will have two lines going to it at the bottom of the fuel tank. If so the top line goes to fuel and the bottom goes to vacume. Some carbs have on the right side just forward the throttle an air bleed valve. Vacume will go to it, then it will tee to the pump. Have you adjusted the valves yet, or checked compression? Compression should be no lower then 130psi and valves should be set accordenly, cold, Intake @ .003" and Exhaust @ .005". Either low compression or too tight valves will cause this hard starting. Ideal compression should be 150psi. If the compression is good and valves adjusted correctly then check for a vacume leak @ the intake. The intake could be cracked or the plastic isolator under neath, also you could have a cracked o-ring which seals the intake.
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