Idle jet setup method?

[KTy]

I'm asking the question because my idle jet is 55 and it seems it's again too small...

The method I use is; The engine is idling around 1500rpm, I open full throttle, and I see if it revs easily. I turn the mixture-screw until the engine has a "fast" reaction.

I'm already with a 55 idle jet and the air-screw has less than one turn (and it's not satisfaying)...

Should I consider that there is a problem or is it normal given the spec of my engine ?

A 60 idle jet could be "normal" ?

Specs :

http://ktylife.free.fr/vespa/KTy133/CartersFrais%e9s1.jpg

and

http://ktylife.free.fr/vespa/KTy133/

Thanks !