Tuning is simply adjusting your engine for peak performance. The factory-fresh settings may be a good starting point. However, you may need to compensate for higher altitudes, or for temperature changes with the seasons. Changes in fuel type or engine wear, may also require a tune-up.
Relatively more fuel to air is known as a "rich" mixture, while the opposite is known as a "lean" mixture. A lean mixture generates more power, but also generates more heat. Too much heat could damage the engine. A rich mixture generates less power and less heat. Too rich and the engine might quit while you're flying.
The popular twin-needle carburetor has two needle valves. The high speed needle controls the total fuel flow into the engine, and affects the engine's behavior from half to full throttle. The low speed needle tends to have more of an effect from half throttle down to idle. Both the low and high speed needles can be richened (counterclockwise turning), or leaned (clockwise turning).
Tuning Procedure: 1) Start with the factory recommended needle valve settings.
2) Start the engine.
3) With the engine or aircraft secured, open the throttle to full.
4) Lean the high speed needle slowly and watch and listen: Too rich: lots of smoke, low pitched sound Leaner: reduced smoke, sound starts changing to higher pitch Leaner still: little smoke, max rpm reached Too lean: very little smoke, max rpm can be heard falling off. Open the needle valve up 20 or 30 degrees from this point. If you have a tachometer, open the needle about 300 rpm less than max.
5) Still at high throttle, perform the pinch test. Briefly pinch then release the fuel tube going into the carb. If the engine momentarily speeds up, then slows back down, you're done. If the engine does not speed up, or quits, then it is too lean; richen the needle some more.
6) Now throttle back to idle; let the engine idle for a couple minutes.
7) Now perform another pinch test. This time pinch and hold the line. The engine will want to speed up and die. If it speeds up immediately, it's too lean, richen the low speed needle. It should take 4-6 seconds to speed up and begin to die. Any longer and it's too rich.
8) Also check the throttle transition. With the aircraft secured and engine idling for a couple minutes, advance the throttle quickly to fully open. If the engine hesitates, the low speed mixture is too rich.