I really like the PHVA's and have had a few real awesome Amal setups in the <18mm on a 50cc range.
If you wanna do it right right tho, make it USE that VM20. port the dicks out of it and get that pulling 5 digit RPMs, cuz on stock, to your original Q, yea it's kinda too big
I think the point most folks are making tho is the SHA is just an easier reliable setup if you're jusst building stock +
And swapping slides is pretty easy, if you're doing it in detail you're swapping needles in the Mik too
But I absolutely feel tuning what you've got. I'd say at least 4/5 builds just wind up with whatever carb I have, or have jets for. my comments are more considering/comparing buying new intake & carb - if I was gonna buy one
I've swapped equal size bings and SHAs and never felt like one was any bit better performing in the end. I like the click action of the choke on the SHA better for starting, and filters are cheaper and more available. SHA idle issues I've only ever had problems with the tiny little idle hole plugging (not the jet, the circuit under tha main bore), and that's easy. The bings maybe have less blowback? but yea just for having a needle vs slide i've got no hard prefs