Just to add to the body of knowledge already here I can give youinformation on my set up.
I have a 1995 Vespa PX200 Arcobaleno Elestart 200cc. It was running the standard carb and jets until my upgrade.
I bought a S.I.P Exhaust and a Mallosi 210 kit with head and gaskets.
Did a complete rebuild, and matched the ports to the new 210 kit and slightly polished the inlet manifold but did not open it out. It was reassembled smoothly and with the standard carb but with just the main jet changed to the 130 which was supplied. However, it was spluttering terribly once running.
I checked Timing with a strobe scope and it was smack on the 18 degrees BTDC which is set at the I.T mark on the stator, correct for my model. There were no air leaks or splits in casing or crank and fuel was getting to the carb OK.
The carb had the 130 main jet which came with the kit so after much frustration i started to investigate main jet settings. I went down to a 120 main jet and low and behold that did the trick.
No spluttering, fast acceleration and will get up to 70 MPH easy.
As a standard based set up this worked for me but making sure you have done a fault diagnosis on other causes which cause bad performance early i think saves a lot of extra effort.
Hope my information is some help
Eventualy i changed just the main j