Getting rid of 4-stroking - The quest

Overview

In my last post I said that I had been trying to get rid of my scooters 4 stroking for over 10 years. It's actually quite a lot longer than that actually, and it's really starting to but me. I've decided to jot everything down in this blog in at attempt to make it less baffling (no pun intended) and possibly help anyone else that has a similar issue.

In the last post I went over the port timing, this time I'll go over the carb jetting.

Descriptions of symptoms

The thing that I can't get my head around is that everything I read tells me that 4-stroking is due to being too rich (we actually the cause is that the scavenging effect of the 2 stroke process isn't getting a chance to work on the amount of gas in the cylinder, so we could look at retarding the timing - more on that later). The thing is, if I lean out the idle any more it is just too lean., very slow to die down to idle speed, uneven, no power to pull off. But if I get the mixture "just right", the idle is nice and constant at around 1500rpm. The problem is as soon as I am moving and I position the throttle "just" off idle, the engine 4-strokes.

Current settings

I have a Dellorto 28 PHPB with a Ram-Air foam filter. 
Main - 128
Pilot - 55
Choke - 70
Slide - 60 (modified)
Needle - x2 (middle ring)
Atomiser - AV262
White float


Today I tried once again messing around with the air mixture, and took the air filter off to see if that was causing the bike to run rich off idle. Neither helped.

Next I retarded the timing by a couple of degrees to around 17 BTDC, this actually seemed to make the engine more rough in other places (which I can understand, the low down torque would be better with more advance, my variable timing module is on the way). Anyway, the timing retard didn't help the 4-stroking issue.

Conclusion

I'm beginning to think that all kitted Vespas do this.

EDIT

I started to think that I might be overcompensating for the lean slide by an over rich idle mixture setting (it was around 3 turns out). The two circuits to overlap quite a lot, especially in higher idle screw settings.

So I changed my over lean 60 slide for a 50 (unmodified) and swapped the 55 pilot jet for a 50. 

At first it was over rich everywhere, but I after some adjustment I was able to get the idle pretty nice, screw is around 2.5 turns out which is correct for the PHBH carb. Middle openings where very rich so I lowered the needle to it's most lean position (top notch).

The engine pulls well now and the 4 stroking is much better. It is still there though, but at least I know the engine is not too lean!

Depending on the weather I might need to change the atomiser for a AV260 and raise the needle right up but I want to play it safe for now.

I did a plug chop and it's nice and brown. Max CHT was 149 degrees C after a good WOT blat.

I'm hoping that the variable ignition timing unit will help even more.

EDIT 2

See next post when I finally solve this!