Re: carburetor tuning...
Let me see if I can explain this clearly.
When you pull open your throttle its pulls the slide open in the carburetor allowing air to flow through the venturi. The venturi is the point in a carburetor where it narrows down to increase airspeed and atomize the fuel for proper combustion.
There are three tuning points in a carburetor. And transition points between them. Different carburetors have different methods for adjusting these.
The first is idle. This is when the slide or throttle plate (slide in the case of these carbs) is completely closed. On this carburetor there is no mixture adjustment for this. It is what it is. On many other carbs you have some adjustment....on this you do not. Do not worry about it. Set your idle speed with the thumb screw and forget about idle.
As you open the throttle two things occur. The slide moves upwards allowing more air through, and the needle moves with it allowing more fuel to enter the air stream to be atomized. The needle fits down into a jet. when you raise the clip on the needle it sits lower, hence it opens slower allowing less fuel in the mid or partial throttle. This causes for a leaner condition.
As you continue opening the throttle fully and air speed increases the needle becomes fully raised and fuel metering is controlled completely by the hole size in the main jet. You can buy different jets or drill your main jet as necessary to adjust the mixture on the top end. If the mixture is already too rich you will have to solder the jet and re-drill smaller.
What does this mean in the real world...
The only way to tell what your engine needs where you live is to learn to read the plug. In general leaner will give you more power. Too lean will give you a dead engine. Too rich will lead to plugs fowling and reduced power. The general consensus seems to be that most of these engines like the needle on the second groove from the top.
You want your plug to be a nice dry chocolate color at all operating conditions ideally.