Rich and lean refer to the fuel/air mixture entering the engine. A "rich" mixture means there is more fuel than optimum for the amount of air. A lean mixture has an excess of air. A lean mixture causes the fuel to burn faster, and consequently hotter. There is an optimum mixture where the engine will run at peak rpm when at full throttle, and this mixture is controlled by the high speed needle valve, with a small contribution from the low speed mixture.

The objective of tuning is to adjust both the high speed and low speed mixtures such that the engine achieves peak performance in the air, does not run too lean and damage the engine at full throttle, and idles reliably at a relatively low rpm (typically 2500-3000 rpm).

Tuning is not magic. It is methodical, and once you understand how to do it, you will be very satisfied.

First, understand where the high speed and low speed mixture adjustments are located. The high speed needle is the larger knob, normally on the left side of the engine, and is close to where the fuel inlet line is. 2-3 turns out from the full in position is a good average starting position for a new engine. Turning the knob out (clockwise) will make the high speed mixture richer (more fuel). Turning it in (counter clockwise) will make the high speed mixture leaner (less fuel).

There are two types of low speed adjustments, depending on your carb type. Most of the higher performance engines have a low speed needle valve, generally as part of the throttle barrel, and is adjusted by a small screw inside the throttle arm. Like the high speed needle, adjustments out (clockwise) add more fuel to the mix (richer). Adjustments in (counter clockwise) provide less fuel (lean). This low speed adjustment controls the mix up to about 3/4 of the throttle, so its very important to be set properly for a reliable idle and good transition to full throttle.

The second type of low speed adjustment is an "air bleed screw". This is readily identifiable by a small hole in the front of the carb with a small screw that opens or closes across the hole. In this case, rather than modifying fuel flow, it is modifying air flow at low speeds. Turning the screw out (counter clockwise) adds more AIR to the mix, thus LEANING the low speed, and turning the screw in (clockwise) reduces the air flow, thus making the mix RICHER.

To tune the engine properly, start from the factory settings. Start the engine, warm it up, and remove the glow igniter. If the engine stalls immediately, its probably too rich. Turn the high speed needle in about 1/4 turn, and try again. Once the engine is running without the glow igniter, go to full throttle, and turn the high speed needle in (clockwise) until the engine reaches peak RPM. Give the engine time to respond as you approach the peak. Use a tachometer. It will make the job much easier. Once you have found the peak RPM, turn the high speed needle back out (counter clockwise), until the RPM is reduced 200-500 RPM below the peak. This ensures a slightly rich mixture at full throttle to prevent damage.

Now its time to check the low speed mixture. Reduce the throttle and adjust your throttle trim setting until the engine reaches its lowest reliable idle speed. This may be between 3000-3500 rpm to begin with. Don't worry. It will improve as the mixture is adjusted. Allow the engine to idle for 15-20 seconds, and then push the throttle open quickly. If the engine hesitates, stumbles, and then goes to max rpm, your low speed idle is slightly rich. Turn off the engine, and lean the low speed mix, BUT NO MORE THAN 1/8-1/4 TURN at a time. After you make this adjustment, restart the engine and recheck the high speed needle. This is important, as the adjustment in the low speed mix will affect the high speed mix. After readjusting the high speed needle as before, do the throttle transition test again. Repeat this process until the engine transitions smoothly from idle to high speed without sputtering, and it reliably idles between 2500-3000 RPM.

If you do the throttle transition test, and the engine stalls immediately, the low end mixture may be too lean. Make the adjustments as above only richening the low speed mixture by 1/8-1/4 turn at a time.

An alternative to the throttle transition test is to pinch the fuel line at low speed. If the engine rpm rises slightly before stalling, the mixture is rich. If it dies immediatley it is lean. If there is no change in RPM, its very close to optimum. I prefer the transition test, as this more closely mimics performance in the air.

If you have to adjust the low speed mix more than 1 turn from the factory defaults, there may be a problem. Check your fuel lines, make sure the needle valves are clear, that the air bleed hole is not clogged, that there are no air leaks, and that your glow plug is good.

Once your engine is tuned properly, you shouldn't need to touch it again, other than to check the high speed needle at the beginning of the day. After running properly, if it begins starting hard, transitioning rough, or won't idle, your glow plug is probably going bad. Change the plug before you make any other adjustments.

Good luck!
Brad