Modern Vespa

Has anyone on here had any joy fitting a Malossi Power Exhaust to an otherwise bog standard Euro3 Px 150 ? Ive upjetted (main) from 96 to 104 and tried everything in between. Pulls like a train (compared to previously) under WOT but sounds rough as anything trying to cruise at low speed. I've ordered some more more Jets & Stacks to have a play over the weekend but any pointers would be gratefully received. What would be a realistic maximum shift in jet size just from changing the exhaust ? Plenty of advice for SIP exhausts but not sure how comparable this is.

What year is it ? It could be that you are hitting the rev limiter vs a jetting issue. Recent PX150 have a limiter. You can resolve this by replacing with a Readspeed Zeus CDI

Its 2013 - Its when it isnt revving that the problem occurs, You can feel the rev limiter top end and I probably will invest in the CDI at a later date but this is 1/4 -1/3 throttle

Don't buy any more main jets.

The main jet doesn't come into play until somewhere between 2/3rds and 3/4s throttle.

Pick up a copy of Norrie Kerr's Tuning Vespas manual.

To quote

"All carbs work in a set fashion. The system of operation is called progression, I.e. Basic law. As each step of the progression tails off, the next step commences. The culmination is in fact full throttle. The carb has four stages in it's operation I.e.

1. Closed slide, operating on the pilot system/mixture screw.
2. Up to 1/4 throttle, slide cutaway/pilot system.
3. 1/4 to 3/4 throttle, needle/needle position/needle jet.
4. 3/4 to full throttle, main jet.

Excuse the punctuation I'm on an ipad.

That's word for word what's written in the manual.

If your problem is in the lower revs it's not the main jet.

I can't help with what you need to do but I hope that stops you chasing a red herring.