I'll add my voice to the chorus telling you to look at the induction system. Loose carb mounting nuts, hardened intake tube packings, loose intake nuts, holes chafed in the intake tubes are all things I have seen cause just the situation you are describing.
So far as the vibrator/booster coils, they both are for essentially the same thing. The booster coil (at least on the 1340 on our BT) is a 12v coil that is wiring to the starter solenoid, and secondary side to the right mag distributor block. It's sole purpose is to add a sharp spark blast at VERRRY low engine RPMS to get the thing started. Important on ours since we still maintain the inertial electric starter (cause it sounds so dam cool!) The vibrators do essentially the same thing but operate differently. Usually 24V, as well.
Remember that the downfall and advantage of a magneto is that they produce a stronger spark the faster they turn. This is great at +1500 RPM, not so good at less than 150RPM at startup.
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Regards,
Jase
www.b26marauder.com
"I'm having a BLAST!!" 2007 CAF Wing Staff Conference
RIP Gary Austin..always in our hearts