diffuser only controls the flow at low throttle settings, along with the needle, bigger bings use smaller diffusers cause they idle too rich with 2.00+. my drilled out 12 i had to go from a 2.22 to a 2.17, which is what the 15s come with, to get it to idle without leaning out the main jet too much for WOT. looke here for how the carbs come set up from bing:
Re: Bing 15-2.5 diffuser?
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