During starting, the starter pinion is designed to disengage itself as soon as the flywheel starts rotating faster than the starter rotation. And normally this is due to the fact that the motor has already fired up and started running. However, there are instances when the flywheel will kick a second or two faster than the starter rotation because one or two cylinders actually fired but not the subsequent cylinders. Usually, this is refer to as a, "cough" or "sneeze" of the motor.
If this continued to happen there are usually two sources of problem, timing and also too lean or too low of idle settings. With too lean or too low of idle setting, the motor tries to start but unable to sustain such sneeze or cough due to low idle or too lean of a setting. Obviously, timing is an easy fix since you can actually see this with a timing light.
Now for a simple remedy, set the idle air mixture to two turns out (or even three) from slightly seated. Next set the idle screw setting so that it is pushing against the engine block and that the cam scribes are about 1/4" below the eccentric screw slot. Just to be sure, remove the air cover on the carbs and check to make sure the throttles on both carbs are actually slightly open.
This should get the motor started and if it does, then you can set you idle air screw settings accordingly and also your idle screw setting.
