17.5 vs 19mm dellorto

[bluesi808]

Is there a huge difference in performance between a 17.5mm Dellorto and a 19mm or even a 21mm? Also im keeping the stock air box and i'm getting a 70cc polini sport on a 140qmb keeway matrix.

[reveeen]

You are kinda asking: how high is up?

Your engine is an air pump. You should be *thinking*: "how can I maximize air flow through this thing?

So your question: Is there a huge difference in performance between a 17.5mm Dellorto and a 19mm or even a 21mm?

Is hard to answer. On a stock, RPM limited, engine: no there is no "big" difference.
Add a 70cc kit: the difference grows
Port, and case match, that kit: it get's bigger
Add an exhaust: bigger still
Add a larger intake, reed block: bigger

Typically, on a 70cc, with an exhaust, 19mm seems to be "the choice", the 17.5 will work, be a bit easier to tune, but gives you no growing room.

[90GTVert]

For a sport kit you probably won't see much, if any, difference in performance of the 3. In theory you should see gas mileage decrease as venturi size increases, but I doubt that will be a major difference between those three either. The 19-21mm should be sufficient for any future setups within reason, that would be a big reason for me to go larger. I use a 21mm on my 96cc and 2strokd even uses a 21mm on the infamous 103cc Skeedr drag scoot.

[reveeen]

I have some 24mm "stupidity" going on here..... when really, all that is needed, is 20mm.

(I'm having to use the big stuff to match up with the reed blocks/manifolds)