If you are referring to the jet seen on the picture, it's the starter jet.
If not I'm wondering how did you know I'm running 150 mains now?

Well anyway, I had 145 mains as a starting point, but changed to 150 because the AFR seemed a little lean. But I can get a proper reading on the spring when roads are not so slippery and I can do proper full throttle sweeps thru the rev range. The AFR meter is a bit too slow to react in these conditions.