Scooterotica

Can anybody tell me which will give the richer mixture with a PHBH30 Dellorto please. Assuming both clip positions are the same, an AV264 atomiser with an X13 needle or an AV266 atomiser with an X7 needle.

Thats a bit of a tough question really. Each setting will give slightly different results between various throttle positions between 1/4 and 3/4 throttle, mind you saying that, the av266 atomiser will make it richer as well between 1/8 and 1/4 throttle as well.
You need to jet to what your bike needs so for example if it was a bit lean running an av264 between 1/8 and 1/2 throttle you would need a bigger atomiser, say the av266 but you may still need a richer needle in as well.
You need a good selection of needles and jets to jet a bike properly.