No I have not flow tested the OEM Mikuni BS40 carb.

I have included below some information I have gathered over a considerable amount of time on the internet doing research in this area.

Keep in mind that Mikuni does NOT provide airflow numbers for their carbs (Ever wonder why ?)

Calculate Carb CFM needed for a given engine size

Required Intake CFM for an given engine size

Volumetric Efficiancy (VE) is affected by intake type, camshaft grind and type, boost, and many other factors.
Here are some guidelines.
Factory engine: 65%-75%
Mild street performance, flat-tappet cam: 80% - 83%
Mild street performance, roller cam: 90% - 93%
All-out race engine, 95%-105%
Blown engines, typically over 100% or higher, depending on amount of boost.

Instructions
1) Find the displacement of your engine in cubic inches (CID). Determine the maximum RPM for your car. For example you may have 450 CID with a maximum RPM of 5,000.
2) Check the volumetric efficiency (VE) of your engine. If you don't know, you can assume that it's about 80 percent since most street cars are designed to that level of efficiency.
3) Multiply the CID by the maximum RPM and divide that product by the product of 3,456 and the VE. For example you would multiply 450 by 5,000 to get 2,250,000 and then multiply 3,456 by 80 percent to get 2,764.8. Then divide 2,250,00 by 2,764.8 to get the CFM required: 813.8.
4) Select a carburetor that will produce 110 percent to 120 percent of the required CFM. For example with a required CFM of 813.8 you would select a carburetor that can produce between 895 and 975 CFM.

Another formula to use is:  CID x RPM Divided by 3456 = CFM

Suzuki Savage Engine:

652cc - 40 ci
VE - 75% - performance cam - 80%
Engine redline - 6500 RPM

Required CFM:    Stock engine: 62 to 79 cfm (calculated)
Required CFM:    Performance modified  engine: 66 to 84 cfm (calculated)
http://www.carburetion.com/calc.aspAprox  
Approximately 71 CFM Needed for a 40 CID Mildly Built Engine

36 Centimeters (cm) = 14.173228346457 Inches (in)

A Mikuni 36mm carb flows approximately 120 CFM.


Carb CFM calculators
http://www.csgnetwork.com/cfmcalc.html
http://speedmaster79.com/tools/carburetor-size-needed/
http://www.bleachburnout.com/blog/2008/10/17/carburetor-cfm-calculator-with-g...
http://www.4secondsflat.com/Carb_CFM_Calculator.html

Mikuni Carb size (IN CFM)

34mm MIKUNI Carburetor 109 CFM
35mm MIKUNI Carburetor 114 CFM
36mm MIKUNI Carburetor 119 CFM
38mm MIKUNI Carburetor 129 CFM
40mm Mikuni Carburetor 140 CFM

If your numbers are correct then a 34 mm Mikuni would be a lot more responsive. Maybe the stock carb's size is one of the reasons why this bike runs so lean from the factory.

so do I  
but I still want the wind protection, and storage capacity oif a scoot
Can't have it all Art  

Super Thumper wrote on 01/19/16 at 22:44:15:


In stock form (read: lean), I was getting nearly 80 mpg but it was a totally stock engine.

Nice setup Badwolf, I know for a fact my underseat storage will hold more than those saddlebags, but the top case might make up the difference (my underseat will hold 2 FF helmets, or a full backpack AND a few hardback books) the Yamaha 650 seat might come close to the scooter seat in comfort
You still have no leg protection, can't stand on the pegs to relieve butt burn (I can stand up on the boards) and unless you've re geared, it's likely not smooth at 80  
to be honest, if I'd had a few day with the scoot, instead of a few miles, I likely would have kept the S40 and modded it
One thing I can say, forward controls or haighway bars are a good idea

you've definitely done a heckuva job there, and it do make me miss my S40
part of the trade was to get some credit history going, so I'm not balked buying things I want in the future
I can say I have no problem with the idea of getting another S40 / savage, and sort of wish I'd kept mine (I could have, no down payment was required)
question is, would I have ridden it enough to keep it running?
I test rode a C50 while I was at it, there is no way one of those will ever come home with me