4 Stroking - Solved

I had a bit of an epiphany moment when I realised that the idle to 1/4 is actually adjusted in three ways, not two.

Once I looked at the diagram a little closer I noticed that 

1) The air/fuel screen controls idle to 1/4 movement through a hole down stream from the slide

2) The slide cut out effects the mixture in that range as well. 

3) However ALSO the size of the idle jet effects fuel coming out of a hole just inside the slide. It is not regulated by the mixture screw at all.

I had been thinking that the mixture screw would be enough to compensate for the difference between 55 and 50 pilot jets. In fact there are 2 sizes between those, being 53 and 52. 55 was too rich, and always was. 50 was too lean and I couldn't make it rich enough. It's obvious when I think about it now, but I thought the screw adjustment would be enough.

I have since ordered the in-between sizes and had a chance to install 53 today and the 4 stroking is almost completely gone.

Now the changes I make to the mixture screw are apparent and obvious.  I am able to find the 4 stroking by increasing the idle speed, lean out the mixture till the revs increase and even out, and then back it off (richen it) by 1/4 of a turn which helps pick up off idle.

I'm going to try a 54 when it arrives from Italy, but I am so close now I am happy. 

Happy days!