This set up is very common on drag bikes in the UK, yes I do have a pinch screw to hold the needle but as I said no float bowl because SU carbs can't put enough Methanol through the needle valve, I was working on a Holley float bowl mod but for now I'm just plumbing into the bottom of the carb and have modified the inside to take Holly Methanol jets. The setup works well with one jet size on most drag strips around sea level, I'll just wing it and make 3 or needles and jets but it's a pain making a needle on a 16" swing lathe.
Yeah, that's not what would be my first choice either!!
In an attempt to be helpful here:
What size jet do you use?
Will a .100" or .125" jet flow enough methanol with a "standard" needle that might be available?
Without an idle circuit, you could use say .090" needles in .100" jets which would be
waaaay richer than a gasoline setup, and might be close to the flow you need for methanol . . . . . . ?
Or maybe .125" needles with a custom diameter jet . . . . . ?
Can you raise the fuel pressure and use a return line with a bypass pill to alter the flow rate through the needle and jet?
Sorry, just tilting my head here and letting the thoughts flow out.
I'm unsure if any of this is useful. My main line of thinking here is to try to find a way for a "standard", available, needle group or sub-group, to fill your need so that you don't have to fabricate custom needles.
Fordboy