Hi Martin
Well, so much for OPS carbs - all or nothing
I think you would have a hard job getting a good mid range, let alone tickover with that carb!
I can see where your 'name' for it came from - very accurate!
I have a VERY old Picco carb from a 40 which is also "single hole" and that was truly rubbish.
The bore looks surprisingly small for a 90, I'm guessing about 10mm (unless it's the size of your fingers
) so a change to a CMB carb would give advantages to top end speed as well. The bore on my 90 and 91 are both 12.0mm - even my Picco 67's is 12mm!
Your adaptor will (presumably) be able to mount the new carb the "right" way, ie across the engine, allowing you to connect to, and adjust both needles.
Danny
Edit - just had a thought - if you're REALLY clever (ie much cleverer than me) you could "mix" the channels on your radio so that, as you close the throttle, the mixture would also reduce
It would take some setting up and as you changed mixture during a run, it would alter the slow end as well, but it might just be do-able