Uprating PK125 on the cheap

I’ve just started running a 1983 PK125 and would like to get a bit more speed out of it (use it to commute part dual carriageway/part busy city traffic).

Problem is I can’t justify spending too much in one go, so I’m after advice on best way to go. Eventually I hope to fit a 135 kit (is Malossi best?) but I assume I’m best to start by uprating the exhaust and carb.

If I fit a Polini or Simonini exhaust should I upjet the carb? These exhausts are available in left or right hand. What’s the difference? (other than the side they come out, obviously)

When I get to fit a kit do I need to uprate other components like the crank? I presume some adjustment in gearing is preferable.

Recommended carb for use with 135 kit?

I am completely new to Vespas but run and Lammy so I am not a complete spanner with a spanner.

Any tips gratefully received.

Thanks Curare.

Main reason I want to change the exhaust first is that the standard one sound horrible[:’(]


if you are not after extreme tuning, I can’t see a better kit then the Polini 130 (normal intake. There is one with double intake that is more difficult to carb and the small improvement over the standard version doesn’t justify the effort.)

You can use it with your crank, with a Simonini exhaust, or with a Sito (Sigar style or banana, I don’t know its actual name). The Sito is not a real expansion, but works well with the Polini for a city - touring engine.
For the carb, if you are not going to do any modification to your cases and to your crank, you can use 19 or 21.

Has far as the step to get where you want, I thing it is alwasy better to start from the kit, expecially if you are not thinking about doing some cases work.
Then get the carb and then the exhaust.

If you were thinking about bigger improvement then you have to do much more work and spend more money.
It will be : match the barrell to the cases, using a racing cranck, 24-25mm carb, and one of the more exotic expansion exhaust that are all more expansive then the kit itself.

For the other questions:
if you want to use first an exhaust (what I don’t think is the right way to go), you should change your main jet, and sometimes the grade of your spark.

If your are after a mild tuning and not spending that mutch, you can stay with your gearing.
If you bolt the Polini kit with no cases work and no racing cranck, it would be OK, and you could always change something that you find of not your liking, later on.

Whoops clicked to quick - more to message above

It will take me a few days to fit the kit and I don’t want to take the scoot off the road until winter so thought I’d change the exhaust first to get rid of the horrible sound of the standard one and then look to fitting kit later.

I do fancy doing some porting work to get the most out of a kit but I believe you have to get some extra metal welded on to ensure the casing is thick enough in places?

Thanks Curare.

Main reason I want to change the exhaust first is that the standard one sound horrible[:’(]

You can safely port casings for a polini 130 without any welding being needed.

My setup on my engine is :-

polini 130 (matched casings)

25 mil dellorto, race crank, 4 plate clutch, standard gearbox all running through a humble leovinci exhaust ( the dearer pipes don’t pull too well unless more tuning is done). The motor is sweet and can sit at 55 all day (no joke) with more power on tap to challenge the p2’s.

expect to top out at 65-70 depending on final exhaust choice and carb set up.

go for high load bearings and get your flywheel lightened (12 quid at beedspeed)