Px125 Sito plus exhaust and main jet

First of all, sorry for my bad English. I have a question. I'm about to order a Sito plus exhaust, and my Vespa px125 comes standard with a main jet 96, air corrector 140, and a mixer tube BE3. Is it mandatory to change the main jet by one of 98?? Do I have to make changes in the mixer tube or air corrector?

Thank you very much!

Hello ElectricSun. Thank you very much for your answer! You are right, it have a BE5, I made a mistake when typing the post.

I've increased the main jet to a 98, and after a plug chop, the spark plug was white, so I increased it 2 points more, to a 100 one. After a new plug chop, the plug is a little more dark, but not brown. Do you think that it's necessary increase the main jet 2 points more?


I'm a little surprised you've got a BE3 – they are usually BE5 – unless you have a much older model (but then it wouldn't have a 96 main).

The general opinion is that you only need to increase the main jet by 2 or 4, but all vespas run differently. You'll need to do a 'plug chop' at full throttle to see if you've got it right.

Hope it works out OK.

Hi! Let it run for a while with the 100 main, better too rich than too lean. How dark is the plug? If the plug is a Bit oily it is not a real problem, the center of the spark must be chocolat-brown Not only evaluate your plug on one ride, but give it some km's.

I just installed a SIP road exhaust, which is namely very nice, on a PX 125 (with P200 airfilter) and put a 100 main jet giving good results. (same configuration 140/BE5/100 ; 45/140, but now I'm going to try a 160/BE3/100 50/160 with the new exhaust and drilled airfilter)


104-106 likes me very high main for a stock PX125 with only a non-catalyzed exhaust (no drilled airfilter he?), keeping in mind that every Vespa is different. But Ok, your plug is indeed not good now.

Try 2 times plug test: one time drive at low revs and immediately shut down your vespa and look at your plug. Repeat this also at high revs. If there is a big difference maybe your PX is too lean/rich on High/low revs...

Everyone forgets the throttle slide was different for non cat exhaust, the current throttle slide has cut outs to make it leaner for the cat exhaust.

The VNX1T-VSX1T service manual indicates that the standard setup for the P 125 X was set up with the following configuration with the SI 20/20 D;


IDLE: 48/160


This should be your starting point and then up jet to 100


Hi Yoel! With the 100 main jet, after 1 km at full throttle, the plug's colour is this:  




I'm waiting for a new order to change it for a 104-106 main jet, because I think that I'm riding too lean... I hope that this change fix the problem...!