Carb size debate: does bigger really hurt low end?

So I have a 21mm PHBG on my 75cc polini v1, and I've been noticing a bunch people saying that smaller carbs = more low end while a bigger carb is better for top speed. I start thinking, maybe I should get a smaller carb since I mostly care about low/mid range (not that my current accerelation is bad). But what I started to question is, how is having a 21 mm carb and opening it 3/4 different from having a 15mm carb and being WOT? Seems to me that if you have a big carb and want good low end, just don't crank it to full throttle right away, do half or three quarters. Then open it up once you get going. I mean I guess there would be slightly less velocity through the intake and everything, given that those would still be the full width. Thoughts?

Re: Carb size debate: does bigger really hurt low

i understand your philosophy. ive ridden both a 16mm phbg and 19mm phbg on minarelli v1 polini 75cc kits and they definitely feel different. i think the 16mm's throttle range just more accurately hits the lower and mid ranges than the 19mm while obviously not taking in as much air up top. i could be dead wrong with this...idk...i can assure you its a different ride (although the 19mm one didnt necessarily take off slow). ahhh its hard to say.

Re: Carb size debate: does bigger really hurt low

Perfekt Timing Angel /

The great thing about PHBG's... they are awesomely tuneable.

If you know how to get them dialed in, your kit will have amazing power in all ranges.

Stick with the PHBG's in my opinion. SHA's will make you run inconsistent with that kit and could cause a seizure.

Re: Carb size debate: does bigger really hurt low

a smaller intake and venturi create a grater pressure difference which increases intake air speed. this is why in fast four cylinder cars that aren't turbo you will see four small intakes and never one huge one. Even with the throttle only partialy open the ventuti in the carb and the volume of you intake remain the same.

Re: Carb size debate: does bigger really hurt low

ITB's!!!!

Re: Carb size debate: does bigger really hurt low

the term would be over carbing.

to big of a carb could make your idle stink

but isnt a 21mm fully closed only like 2mm.

and isnt a 16mm fully closed only 2mm

whats the diffrence.

well the diffrence is when you gun it right off the line. you go from 2mm to 21mm in like half a second. and the motor is like oh fuck here we go and its racing to catch up starving your low end.

but on a 16mm set up. the carb goes from 2mm to 16mm and its ok with that its a little less. and it helps it get going

just ease on the throttle and give yourself a pedal or 2.

Re: Carb size debate: does bigger really hurt low

Perfekt Timing Angel /

You can use a different throttle slides to achieve different venturi effects, among the hundred bazillion other things you can change inside the PHBG.

I've run a Polini with a 16:16 SHA. My current set-up is a 21 PHBG. The PHBG gives the bike more torque and a much smoother acceleration than the 16mm. All around better performance. The gearing has not changed, the pipe has not changed.

Minarellis are magically torquey stock. Try switching pipes or gearing if you are unhappy.

Re: Carb size debate: does bigger really hurt low

) Cupermcnewbster ( /

bigger carb generally makes for more low end (when tuned of course). A 15 bing and 21 phbg both go fast, the bigger a tad more, but not much.

Re: Carb size debate: does bigger really hurt low

Perfekt Timing Angel /

It's not the size, it how you use it ;)

Re: Carb size debate: does bigger really hurt low

only over carbing a bike will result in a loss of lowend from having a larger carb. there are many other factors as well such as the type of intake and the type of carb, size of the intake etc....

Re: Carb size debate: does bigger really hurt low

The largest I could get the intake port on my polini was an honest 19mm, so a 19mm carb (that I run) seems just perfect. I have it 19mm from carb to port.

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