Modern Vespa

Too rich I'm guessing, go down a couple of main jet sizes?

It's off my lambretta. Brand new 500 miles ago on a new 150 cylinder, head and piston. Never feels like it's really achieving its highest revs.

Is it telling me anything else?

Pilot 42
Main 114
Atomiser -2
Slide 5914-1

That seems like a big jet. Which carb do you have on it ?
Here are some comparisons :
http://www.lambretta.co.uk/downloads/carbjetting.html

SH1 20 carb. I've seen that table before but could not link it to my set up. Series 3 LI 125 with a 150 kit on.

It's not terrible. A bit rich if high rpm chop. If it goes ok I'd leave it.

So, it's basically an S3 150. The SX 150 with a 20 on it shows a 102. Why wouldn't you try sneaking it down a bit ?

Think I will, think I'll go down to 112 and creep down further if all well. I just feel like there are more revs in there that I'm not getting. I have a 42 clubman exhaust on also and the standard airbox has been modified a little.

4 pts for clubman, 1 to 2 for airbox, so about 108 if 102 is the norm but you don't know if the porting, heads been modded etc do you? I reckon round about a 110, but dropping to 112 sounds a good plan, could be spot on.

The head is a brand new one. SIL I believe. Bought it from a reputable dealer and has no modifications on it port wise. I'll go down to 112 and then 110 should I feel the need. I'll stick my CHT gauge on and gauge what happens with each jet change.