Like showcrop said, everything has to work together.
Before adjusting the carb be sure everything else is as it should be. A commonly overlook area is the air cleaner. There is a wire mesh filter above the oil reservoir that needs to be cleaned periodically. Also check for mud dobbers, bird and mouse nests.
There are many factors that affect mixture. Be sure the engine is up to operating temperature, the spark plugs are fresh and clean, the oil has been changed and not contaminated with gas. It may even be necessary to tweak the mixture according to the daily operating conditions, as in a hot dry day will need a leaner mixture than a cold damp day.
The top adjusting screw is strictly for idle mixture. It needs to be adjusted at low idle RPM, down around 400-500. It will have little to no effect on the top end power. However, if it makes no difference, something is clogged internally, or the engine is not healthy enough to idle at low speed.
The lower adjustment is the main jet. That's the one that fine tunes the mid-top mixture. The best way I have found to adjust it is to bring the engine down to idle, then suddenly open the throttle while observing the response and exhaust. Keep doing this while closing the main adjust about 1/4 turn at a time. The engine will begin to stumble or die. At that point, begin backing the main out 1/8 turn at a time, repeating the test until the engine will take sudden throttle without hesitation. A single puff of black smoke is desirable.
From there, operating under load, observe the performance, exhaust, and fuel consumption. Black sooting means still too rich, turn it in slightly. Loss of power means too lean. Fuel economy will be best when the mixture is right.
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Today's Featured Article - A Lifetime of Farm Machinery - by Joe Michaels. I am a mechanical engineer by profession, specializing in powerplant work. I worked as a machinist and engine erector, with time spent overseas. I have always had a love for machinery, and an appreciation for farming and farm machinery. I was born and raised in Brooklyn, New York. Not a place one would associate with farms or farm machinery. I credit my parents for instilling a lot of good values, a respect for learning, a knowledge of various skills and a little knowledge of farming in me, amo
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