These extracts were taken from this page Tuning
It actually for a weber/dellorto, but i assume the theories are the same.
If the engine stumbles when you open the throttle, it could be a pump adjustment, but it could also mean a main jet change. Pay attention to STEADY throttle position when testing in these areas! It's easy to confuse the two. If the main jet doesn't solve it, try the pump. If it runs good at 3000-4000 rpms and stumbles elsewhere, your main is right and the problem is somewhere else. Try to do main jet testing from 1000 RPMs up to 4000 or so. This eliminates the accelerator pump issue from your testing!
Spitting/sputtering/cracking is an indication of being too LEAN. Now, what RPMs that it's doing this tells you which jet you need to change! 2000 RPMs and lower, and you need to richen up your idle jet (go bigger). 2500-4000 RPMs and you need to richen up your main jet!
We want as little accellerator pump squirt as possible to remove the stumble from the engine.