Modern Vespa

Is there any magic formula for dropping the MJ size when going from a be5 to a be4 mixer. Be4 looks to be the richest mixer according to the excellent jetting guide on this forum. So I'm assuming if I go to be4 from be5 to improve my half three quarter throttle it's likely I will have to drop the main down couple of points for three quarter to WOT . Jack mentioned to me in a PM about the air corrector MJ division ratio theory , is there one for mixers , appreciating their numbering doesn't greatly mean a lot as they are all over the place. I'm on be5 ac140 124 mj pilot 45/120 and appreciate when I put the be4 in I'm going to have to go high on the jet again and run and step jet down , but where likely will I end up? 118/120 say? Just checking I've got the right jets to hand .

There's not a step factor for atomizers. They were each introduced to match a specific mixture or emissions profile for which Piaggio didn't have an existing solution.

If you look at the number and size of holes, it might be possible to impute something across the throttle range, but I've never tried to do anything like that. It's not that it wouldn't be an interesting and probably beneficial exercise, I just never get that far down the project list.

You may even find that you don't need to adjust the main at all when you up the atomizer, because the mixture is so dominated by the main at WOT. It would be more likely to come into play at 3/4 throttle in that transition space from atomizer to main.

I would definitely start with your existing 124 main, which is not out of line with what I'd expect to need along side a BE4. What's your overall setup? Carb size, top end, exhaust, porting, etc?

Thanks chandlerman. It's a bolt on dr177 , road 2 , 24si, matched air box inlet, drilled filter , no porting . It all runs fine , been using two years but I'm not happy with the mid range. It revs out third etc but there's a point when you hit around 50 that I can only explain as sounding dry. Following a pm on here with another forum member he informed me that this is lean sounding. I was at time trying my polinibox again , but it was causing vibration. Hated it. Gone back to road 2 and drove today to work and it drives great, plug colour perfect having upped jet to 124 from 122. I've ironed out minimal throttle issues by richening pilot mix screw. But I'm feeling that the be5 needs to be richer in the mid throttle range. It's this sort of incline I can't explain , just dry feeling 50 mph which goes when throttle opened up

I'd just throw the BE4 in there and so long as it still revs out cleanly, not even worry about the main. If need be, you can always walk it down a jet or two. Changing jets in an si carb is so easy that I see no reason not to start with what you have and walk it down a jet or two.

Thanks for this, will have a go Saturday.