with Mattax.

The transitions before main jet actuation is absolutely the most complicated and difficult area to tune. And is the area most likely to need attention out of the box. Depending on how far off it is will determine how much attention. Most of the time, the main jets and their bleeds are not far off and can usually be brought in by testing at the track.

In order to clean up your low speed, you have 3 routes to choose from;
A)have someone else re-calibrate it,
B)have someone else try to tell you how to re-calibrate it, or
C)buy the drill bits, taps, bleeds, jets, and books and start experimenting.

I will offer a shortcut that has worked well for me. Strip out some individual strands from some electrical wire. Mike them and assort them by diameter. Since your carb is too rich, you probably need to reduce fuel flow somewhere. You can try a wire in the T-slot restrictions and then a wire in the idle restrictions to see how it responds.

Now wires in restrictions is a bit like using and axe for a scalpel job, but will quickly show you what needs attention. Then you can buy the drill bits, taps and bleeds to fine tune those areas.