Which size to use? The answer is: the right one.                             It happens that when you expect to gain more power just using a bigger carb what you get is nothing or worse you loose power. But the contrary is also true.

Sometimes a bigger carb is better just because it makes the engine  less nervous or makes it easier to set up.

So don't assume, but try. Trying is the only way to know.  From where to start ?

Here you have an indication for the carb size to use with some vespa kits.

DR 177 24/24si

Polini 177 20/20 - 24/24si; with reed intake 28

Polini 208 28 ; with reed intake 32

Pinasco 177 20/20 - 24/24si with reed intake 28

Malossi 210 24/24si with reed intake 30 to 35

 

Polini 75 16/16 - 19/19

Polini 102 19/19

Polini 130 19/19 - 24

Polini 130 d.i. 20/20 - 24

Pinasco 130 d.i. 20/20 - 21

DR 130 19/19

Malossi 135 25

 

Some kits are able to work well with different carb sizes. Others are more restrictive.

If with some kit it is not shown a smaller size carb as an option it is because some cooling issue has been experienced.

The carb is the only way in from outside air to the engine and this helps cooling it, the bigger the carb the more cooling potential you have. More so the bigger the carb the more fuel/oil can be used and the fuel/oil mixture has cooling capacity too.

A thing to remember when setting your carb:
Air/fuel flow controls the engine power, the fuel is also used to cool the engine. Richer mixter are common since the initial power delivery fades up to 20% on an air cooled engine and up to 10% for a liquid cooled one. Increasing the richness of the mixture will make you loose some initial power but it will reduce the power fade.