Modern Vespa

Hi guys! I have a mostly stock PX125E engine on a VBA frame. To fit the 10in wheelbase conversion I had to upgrade the exhaust because the stock one would not fit around the tire. The engine has a SI 20/20 Carb with a 160/BE3/98 main stack and a 45/140 idle jet. I can't seem to get rid of the bog/power loss that happens when I reach 1/4-1/3 throttle no matter where I set the idle adjust screw (from 1/2 turn out to 4.5 turns out). The aftermarket exhaust I bought here: https://www.scootermercato.com/Scooter-Parts/Simonini/SIM40301BL. Does anyone have a suggested starting place for an upjet? And do I need to upjet both, or just the main jet?

The idle jet will have its biggest impact from "closed" to ~1/4, where its impact becomes dwarfed by the main jet circuit. If you just changed the exhaust you can probably leave the idle jet size alone and compensate with the mixture screw to ensure safe closed-throttle behavior (stock should be ~1.5 turns out, so you'll be looking at at least that much), but you'll definitely want to update the main with a performance exhaust. Start high and work down

If you're ordering jets, pick up a BE5 atomizer as well. that will help with the lean transition at 1/4 throttle as well as the mid-range, which is probably also lean as well. This matters because when you roll off WOT, if that mid-range is too lean, that'll kill the top end and leave you stranded on the off ramp.

And if it's too rich at mid-throttle, you can go back to the BE3, but for $5, it's cheap insurance since you're already ordering.

Bogging just past 1/4 - 1/3 throttle points to the BE3 mixer tube being too lean. Try a BE5 instead.

Let me confirm, did you leave the 10" tire on the PX motor, or did you change it to 8"?

I think the OP said he changed to 10", that's why he had to change the exhaust to accommodate the bigger wheels

Correct, I think the front fork I swapped in is from a stella, but I still haven't found a way to properly mount the mudguard, if anyone has an idea that would be appreciated. BE5 atomizer and a 102 and 104 main jet are on the way.

Pictures of what you're working with would help a lot. Basically, expect to drill holes and possible fabricate some mounting brackets. Nothing you can't solve with a drill, angle grinder, and metal stock. Bonus points if you can weld rather than bolt the brackets to the fork.