Modern Vespa

Thanks for the manual, I never thought of looking into that for some reason.
As I have it now I believe I'm close but yet still lean.
If I were to go with the suggested setup I would definitely be seizing!
Thats what started this whole topic was me trying to understand why it needs a 135 or 138 main to give me a decent plug when the starting point should be 122 as you pointed out.
Now in my opinion my scooter runs well , holds idle , starts on first or second kick. And is a pure joy to play old man racer on!!
I'm thinking of saying who cares what size main is in there and getting a CHT installed and continue practicing wheelies,,,
I just hate not being in control of it if that makes sense. Fine, it runs and doesn't burn up but I can't tweak it or tune it past this

Not sure if I thanked you sdjohn but thanks!

Before my last post I made a search on Malossi166 and found this one too - the spec sheet said it used a Denso W24 ES-ZU, which is 19mm reach
But looking at the spark I would say its the short one (HS), no much doubt about it.

Peeteboy2, you're halfway to solving your problem now that the big sharks dived into the thread ... respectfully

Before you finish dialing in the main jet, I think you should try to get closer on the mixer tube. Yes, the BE3 is leaner than the BE4 (three steps leaner). Put a BE4 in there and, unless you have an air leak, it should become apparent that your AC120/MJ135 combo is too rich.

Now I'm quoting myself, you know when sometimes you develop a discussion without knowing the facts? I visited the mechanic a couple of hours ago and "touched" the subject ... "- some people drill the bore on the carb on tuned engines...", his straight answer "- Sure, I drilled yours to 3mm"

Enough talk, here's a video I uploaded one hour ago of my yellow:

I need to make a correction, when I put the BE4 in yesterday I realized I was running the 150 air not the 120!!
Sorry about that.

So today I will do a proper plug chop and see where I stand!
Thanks

Thats funny but most discussions are developed without one or both parties knowing the facts. I find a discussion is just a long question and answer, right?
I hear what you saying.
Nice scooter by the way!

I just found the thread and copied this part of it. Its very lengthy!!!!

Atomiser/Mixer Tube

This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. *** Therefore, the BE4 is 'leaner' than a BE3.***

Hey thank you for all that information sorry if I didn't think you already I've gotten lost within my own thread LOL. It's very much appreciated

So, there have been so many threads on what mixer is leaner - I think it's a rabbits nest and possibly give people wrong info depending on which post is read. I always get confused trying to read those old threads

Scooter center explains the anatomy (in funny English) and why the numbers don't map on simply to "bigger = leaner": https://www.scooter-center.com/en/atomiser-dellorto-type-5353-si-be1-2560001

The takeaway I got from this link is that the mixer is NOT too be used as a primary tuning device. You should set main jet and idle (IMO) and then use mixer changes to tweak the mysterious in-betweens.

For example, excerpt from SC:
"""The mixing tubes have an effect over a very wide speed range. For example, a BE4 mixing tube greases strongly just above the idling slide valve position. A BE2, on the other hand, is relatively lean in the lower speed range, but greases strongly at the top.

The most common mixing tube BE3 covers 90% of all applications of the SI carburettor."""

If you are lean at WOT I think MJ changes must be the focus to cure it, whereas the mixer tube is more for fixing that pesky flat spot earlier or later in the throttle position - adding nuance the MJ alone can't.

As Rob hodge put it: "more actg as a modifier that act on the other jets in the carb, similar to the taper of the needle in a more conventional carb."

As an example, say you've got good WOT jetting but seem to have a slight leanness low the mix screw can't quite nail, you might bump to a BE4 from a BE5 (relatively flat)

I don't know where that came from, but it could not be more completely wrong. I regret that I do not have the motivation right now to unlock for you the enigma that is the SI carb mixer tube. Maybe tomorrow.

No I understand and im not trying to fixate just pointing out that what I read and what I'm told on here don't always jive! Hence some of the confusion.

Thanks man

Not asking you two im just pointing out that once I've thought I've learned something on here I then find out im wrong, more often then not.
Thanks for your input!!!!

Well, doesn't that just change everything. There is a world of difference between an AC150 and an AC120.
Put in a 120 before you blow it up

Too much labor, bend the ground wire away and use a loupe to see down there with good lighting.

Well I'm back to square one. If I were to go off of tip color reading I would tend to believe I was running just rich enough.
I got a fast flow tap, drilled my carb from bowl to MJ the recommended 2.5mm, rechecked my timing at 3000 rpm and visually nothing changed with the plug. I would have thought it would have richened it up.

One mile WOT with a softsieze putting an end to my fun this is my chop!