A cut and past from my profile info.
Tillotson racing carbs
49cc up to 80c V1L, Honda PA, pug 103 & AV10 moby with stock cases.
Venturi 18mm throttle Bore 22mm good carb for a 49cc street riper.
Venturi 19.8mm throttle Bore 25.40mm
49cc spinning12K+ RPM or over 60cc, big reeds
Venturi Diameter 20mm throttle Bore Diameter 26mm
63cc and up or track 49cc setup.
Venturi 22.50mm throttle Bore 25.40mm
Venturi 22.9mm throttle Bore 25.4mm
The newer tillys venturi shape and bleed ports work so much better in the mid-range down to 1/3 throttle.
Thy get funky when your not quit on the pipe at lower rpms but that how it is with all carbs.
The spring rate popoff is the key to the mid-rang jetting = to the slide and needle setting on a standard carbs tuning.
What throws off tuners is the hi low mix adjustments.
You have to think of its fuel feed as tho it's running down a long pipe.
The first part of the feed pipe has a 4 way tap with the left tap feeding the low speed and the right feeding the high speed.
After the 4 way tap you have a flow diverting presser regulating bypass tap.
The flow diverting presser regulating bypass tap controls all flow / presser and mid rang mix adjustments, AKA the popoff spring.
But if your hi low 4 way tap up stream is using more or less fuel you will need to change your popoff accordingly for the mid rang jetting mix.
So the hi low jetting mix titer-totters with the mid-rang jetting mix.
And the low RPM and hi RPM mix adjustments have a offset titer-totter with each other.
The upper mid-rang tends to adjust a touch with the hi RPM mix adjustment.
So a rich low can give you a lean hi, but a rich hi will make your low just a pinch lean.
Most fuck up by having a rich low and go so rich on the hi speed adjustment that it makes the lower mid-rang lean.