Modern Vespa

Anyone got a ballpark mainjet size for a premix 1987 PX125 Lusso with Sip Road 2 and drilled airfilter. Using a 103 now and it looks/and feels (four strokes on WOT) waaaay to rich.

With the undrilled filter I used a 101 main jet. Went up two sizes for the drilled filter.

Jetting now:
Mainstack : 103 BE5 140 - Idle jet : 45/140 (adjusted to good throttle response and quick drop in rpm). Slide is without cutouts. 20/20 carb.

Try 160, be4, 105. From my notes I've got the stock main jet for a PX125 was 100? 4 for the exhaust plus 1 for the air filter. 106 MJ would work too.

98 main is standard. 140 AC as well. So my logic was like yours. 102 for the SR2 and 1 for the filter = 103, but it is way off. Black plug and four stroking all the way. Will try with a 102 and see if it clears up. I have been running the SR2 for months with 102. Slightly rich, so I thought a 103 would be good for the filter. Back to the 102 to see if that gets me in the ballpark.

If you have a 160 AC try it out.

Is your choke plunger retracting all the way?

I went back down to 102 and now it pulls nicely. Still four strokes a bit on WOT, but I can live with that. Plug is still dark, but insulator has got more of a chocolate color now. Not the pitch black coal it had with the 103 (maybe SIP branded jets are inaccurate?). It seems it runs good now.

I have a 160AC. Would I get some benefits by going up to 160 and a bigger main?

The 160 leans the whole stack out a bit which might clean up the bogging (too much fuel). Your idle will be a little crisper too. I wouldn't up the main jet. Just switch the air corrector. Easy enough to change and go for a run.

I've heard some chatter that the sip branded jets are inaccurate and favor the lean side. Ie. 120 isn't really 120, it's a 118. Search the forum and you'll see some remarks about it.

Whatever you decide, as long as your plug color is chocolate brown or darker, you're good. Rich enough to keep you from seizing/engine longevity.

I'm done chasing the perfect bisquit colored insulator. I really don't care if it four strokes at if I rev it all the way out in a gear. It's a Vespa not a race bike. I have my motorcycle for going fast. More than 50mph feels horrible on a vintage Vespa anyway

Drilled filter gave it more power on hills and it feels nicer to drive now. So I'll just leave like this.

Ended up with AC140 - BE5 - 102 and a 48/140 idle. Close enough for comfort