Although I don't own or run a Phantom I'm with you Red. The smaller the carb more bottom end and mid-range, and the more tractable and controllable the delivery and drive.
A 34mm Miki sounds about right to me. You could try a 36 or even 38 but for most mere mortals at MX I would expect a 34 to give the best results in the most situations, and there would only be a poofteenth at the top end anyway.
On hard packed MX circuits and Dirt Track the 36 or 38 might start having advantages.
I think carby size is associated with Penis Envy
. Carby size was definitely part of masturbation marketing back in the day
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