I found this in another forum. Can anyone add to this?

There's one thing to remember about tuning. Rich is good and lean is BAD!
So, how can you tell if it's rich or lean? First, start the engine and go to full throttle with the main (big, high speed) needle turned out 2 or 3 turns from closed. The chances are it'll probably be a bit rich (safe) at this setting so now you have to be sure. While it's at full throttle, slowly open (richen) the main needle and listen to the sound. If you can hear the engine losing revs then it's already rich and getting richer. This is good. If you go rich enough it'll blow lots of smoke and start to sound a bit rougher, more of a growl than a high pitched sound from the exhaust. No problem, that's what's called 4 stroking and you don't need to go any richer than that but it's good to be able to recognise that sound.

OTOH, if after you've started the engine and you begin opening the needle and it starts to pick up some revs before slowing down then you've started off too lean (BAD!) so just keep opening the needle until you hear it sound growly and rough.

Now start winding the main needle in very slowly and keep listening to the sound. You'll hear a point as the revs go up where it makes that growly sound then suddenly goes all high pitch then usually back to that growly sound. Most engines will keep switching between these two sounds if you leave the needle alone and that's called the 4-2 break, meaning it's going from 4 stroke running into a normal 2 stroke then back to 4 stroke. That's about the setting you need to run in a new ABC engine but your 50SX has a ring so it needs to be run in while 4 stroking (really rich).

At this point you know the engine is rich and quite safe to run all day long but for more power you have to lean it (turn in the main needle) even more. Do this slowly and listen carefully to the sound as the pitch from the exhaust gets higher and higher. When you reach a point where you're not sure if the revs went higher then stop and wind out (richen) that main needle until you can definitely hear the revs drop. Don't worry about how many clicks you turn it, just make sure you've heard the revs drop. Then give it a bit more for luck until you're really confident about the tuning.

It all sounds a bit complicated but it won't take long before you're doing all of that in a few seconds.

Did I mention that rich is good and lean is BAD?