Modern Vespa

I'm trying to understand the logic behind the float chamber main jet bore hole on si24 p2 carb. From another post I made it was mentioned that the 1.5 mm bore hole is restrictive if putting in large main jet. I've just got my dr177 pretty much spot on with an ac140 be5 and 120 mj running on sip road 2.0 plug is chocolate brown on all running conditions, wot runs and pottering around town etc. I now want to move to a polini box I have. Jet suggested is three over sip road 2.0 so if I start about 125 and work down figured I would be safe. But will my carb be able to feed a 125 mj with that standard 1.5 mm feed? I don't want to drill it as it looks a right faff and don't have the tools. A 125 mj is 1,25 mm so is under 1.5mm right or does it not work like this? I have sip version 2 fast flow tap fitted so will this help? I'm not gonna do polinibox if I can't run with what I've got to hand so your help appreciated. Thanks

Yes, important. If you imagine putting a 150 jet in series with a 125 the resulting flow will be less than either alone. Like resistors in series but nowhere near as simple to work out.
Only need a 2mm drill bit. Not difficult to do.

Thanks for reply. But pragmatically, will a 125 jet work? I've got .25mm free in theory if its 1.5 mm . I likely only need a 123 or 122 jet. My 120 jet is working fine so will a 125 really not work on a 1.5 mm bore? Its easy to do if you've got the equipment to do it and clean it out etc. Compressed air blower, drills etc. I don't want to buy stuff just to stick a polinibox on.

Yes it will work.

Thanks mate. I will work down from 125 . Much appreciated

And if you choose to drill in the future.. its really simple with a small hand drill and bit. Cheap and easy.

https://www.amazon.com/Bastex-Precision-Vise-Drill-Twist/dp/B072M35LDZ/ref=asc_df_B072M35LDZ/?tag=hyprod-20&linkCode=df0&hvadid=309813767497&hvpos=&hvnetw=g&hvrand=13519169150075003767&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9012431&hvtargid=pla-569740741362&psc=1

Thanks Swiss. Thats great. I found this excellent video on how to do it https://www.youtube.com/watch?v=K7TJXHEAfaA

I put a 5/64" drill bit with a speed hex end in one of those screwdrivers that has four bits and did it by hand. Total control and nice and easy. You got this!

As everyone has said, easy to do by hand and just blow it out. Is easier than it looks.

Putting in a bigger jet than needed will work. However, only at WOT. When at half throttle or less the bigger jet will have less flow and will work like a jet that is too big. Fluid dynamics, eh.

Is there any advantage to enlarging that 1.5mm hole with his 1.22mm mj?

That’s the video I followed when I drilled mine.

Make sure you protect rim of float chamber with tape, as shown in video.

I also put a little bit of tape on drill bit to ensure you don’t drill in too far. I used a needle to gauge the distance into jet hole and put tape on drill bit same distance.
The needle is also good to show you the angle the drill bit needs to be so you don’t veer off.

After that it was actually surprisingly easy.

You've got a kick ass 225, what main jet are you on? I'm using a rather tame dr177 so trying to establish the optimum main jet you can run on an undrilled Si24 as I'm likely not going to be higher than 124 with adding polinibox. Thx

By far the biggest advantage to enlarging the 1.5mm hole is that, the mpg gets more economical. Does exactly the opposite of what you might guess.