Malossi 166 Gearing

Guys, on holiday next week and intend to take the engine out and open up the ports and intake on my 2001 PX125. While I’m at it I would mind lengthening the gearing as it’s hitting max revs in 4th.

I’m a newbie to Vespa’s, so please excuse my ignorance, but what do I need to lengthen the gearing?

My setup is Malossi 166
Dell’orto SI24 carb
Simonini exhaust.


Sure 21t will help with that and then choose a pipe that makes power further up the rev range. Scorpians seem to do this well but i’m not a fan of stainless exhausts. Also you could keep a little of both by fitting a p2 4th gear. This will give you a jump betwwen 3rd and 4th though.

Curares dead right about the inlet port. If you buy Norrie Kerrs book it shows you exactly how to do this with pictures. You can also increase the inlet timing by cutting the crank. This will also increase your top speed at a cost of low down power.

Sure even 21t would be enough in the real world. You are hitting a brick wall with revs because of the exhaust-simmoninis are good for low down power but dont rev on. Be very careful when modifying the inlet port. Norrie Kerrs book is excellent for useful advice on this.

Cheers Diablo,

what should I look out for when opening the inlet tract. I am quite conservative, so I’m likely to remove not enough rather than too much!

I wouldn’t think you want to lengthen it to much.Perhaps use a 22 tooth Cosa clutch.

Any other ideas?

The problem is that it is geared too much for acceleration and not enough top speed. Even with the cases as standard, on a straight the bike will rev to the moon in 4th. I could do with a bit extra top speed and tame the acceleration down a bit.

Hope that makes sense.


a 21 works generally well with the 166.
No Simonini with this kit, as diablo also said.
OK Scorpion, will let you rev much higher then the Simo, and if you are not a stinless steel person you can go for a Taffspeed.
For the inlet port:
the most important thing to be careful is its width. The width is responsable of thesealing of the compression plate with the crank shoulder. If you make it larger then the crank shoulder then… then the rotative sistem can’t function well and you have to use reeds.
So, play with the length but not do it with the width since in the width section you have about 1,5mm (on each side) before the disaster…
or the reed…