Gearing Polini 50 on a Special

Hi comrades,

a new question. What gearing do you advise on a 50 Special with a Polini 50cc kit?
I want to reach the maximum speed possible without loosing the opportunity to climb occasionally a mountain.
I will fit a 16/16 or a 18/16 (19/19 also available) carburettor, a mazzucchelli crank, electronic ignition, lightened flywheel (1,6 kgs), Polini righthand exhaust, piston porting, case porting and inlet porting.
I estimated 7-8.000 rpm and 4-4,5 hp.

Hi mates
I don’t want to increase the bore size. In fact this Vespa will be my son’s first motorscooter and he doesn’t want to have troubles with the local police which is not very tender. My son hates other motorscooter but wants a fast one. In fact I own a good 102 Polini kit and a straight cogs gearing to reach 90 km/h but…

Hi Curare,

thank you very much. As a matter of fact my next tuning for my son’s Vespa will be more an exercise of style than a real long term necessity. In a short period he will notice the lack of power and ask me the 102 kit. But I agree with you that it will be fun.
Just to fill my curiosity: how did your tuning go with the reed block? And what speed did you reach?

hi diablo,

it’s a 50 cc Polini.

Why are you doing all that work but not increasing bore size?

Thats a lot of work for a small kit what cc will that give you?

I agree, for all that work, i would at least fit a 102 kit, but for not too much more money you could put in a new crank and go up to 130, then you could climb all the mountains you want, as for cost, I just bought an et3 crank for 40 us dollars on ebay so they arent that expensive.


if you really want to up gear it just put the 16 sproket.
Even 18/67 will be too long with this set up.

Use a 16/16 carb.

I have done a 50 project like this with reed inlet polini reed block and special lip crank. You can see the picture of the cases on my blog. Yes the small frame cases you see are for a 50cc. The cases are worked as to fit a Polini 75HP, but they will go with a 50HP standard cilinder reworked which bottom transfer are made the same then the 75HP Polini. This was so could have the possibility to use another cylinder of bigger cc.
It was just an exercise of style, but it was fun.