Better flow thru the venturi.
similar to adding the uniflo to the VM

-- more power=captain?
-- more responsiveness=anythings better than the cv
-- better mileage for a given amount of power=you want more power or what?
-- lowered operating expense=mpg?
-- less backfiring, popping, etc.=fuel map you want?
-- longer engine life=think natural gas
-- neater sound from the intake side of the engine=the unmuffled sounds of a savage heart
-- better looking engine=what? you don't think 10 miles of ducting look cool?
-- cool factor when hanging out = you wanna go fast or talk fast?
-- more reliability = think natural gas, how cool would a nat. gas tank look?

if you are going with a tm40-6, which i recall is a harley carb, you will need to change the ap nozzle jet to around a #40 or 45. thats about 1/2 the size of the nozzle jet for the harley. if i recall the slow jet is about right to start with and a main around 150 is good starting point. you need a smaller intake boot to fit the carb to engine and the air filter side does not fit the stock intake hose. details!

Im not tryin to lower the boom on ya here, but, th way Im readin your post, youre actually wanting more than you realize. Its just not that easy, to just spit out a carb & the jetting, even IF someone knew exactly what exhaust you want & what elevation you live at,

Its more of a "Change this, create a bit of a jetting issue, chase it down & solve it" kinda thing,,,

M<aybe someone will come along & kikk me to the curb, I could be wrong,, hang around & wait on the smart guys.,.

The TM 40 WAS one of the primary after market carbs used on Harleys when the stock carbs were replaced.  Now days the MARKETING  is for the newer 42, 45 & 48mm versions.  Why ?  Because they sell for more $$ and because the engines keep getting bigger.
The TM 40 can certainly be made to work on the LS650 engine, it has been done before, but the throat size will result in poorer performance vs a 36mm VM or Amal or Keihin or TM because of slower air velocity in the throat.
If you swap the stock carb for a performance carb then go with a 36mm, it is the best size for this engine.  It just WORKS.  In any state of tune from stock to max mod that is still STREETABLE, it is the best size carb for best performance for this engine.  
I choose the 36mm VM for the Performance Carburetor Kits because it was the best bang for the buck, parts and jets are plentiful/available, and our engine likes it.  The Mikuni VM is the most popular performance carb in the world for single cylinder engines, both 2 &4 stroke.

RocketScienceSmurf wrote on 07/27/19 at 13:39:11:



A 34mm VM will work fine on a stock engine and would perform very well in the low-mid range, high range not so much.  I would not go smaller than a 34mm though, i think a 32 is just too small.  I’ve never tried to run one so cannot say for sure.

It seems like if you go too small with a carb you still have a lot to work with, but if you go too big you might end up with a big fat zero. If the carb has a accelerator pump you may be able to hide low end response problems , but that's really not tuning.