Friends...
The diameter of 1/2" is in the part of the union with the admission.
You are right, is difficult to take the measurement within the Venturi, reason why I suppose that that measurement is the one that it will have to be but great for a greater power.
I completed the ignitoin based on which it indicates the manufacturer (Falkon) in its Web page (www.falkon.cz), in addition, I was proving different positions until finding the greater number of rpm.
I will try to find walbro of the characteristics that have mentioned to me.
Thank you again

Sergio