Pretty common to se that. If it uses any oil that may be what you are seeing.
Before adjusting the carb, be sure everything else it properly tuned, plugs fresh, distributor working properly, points set, valves set, air cleaner clean, (especially the wire mesh inside the canister, often overlooked), thermostat installed and working, carb clean and float level set, fuel tank clean and delivering clean, fresh fuel to the carb. Be sure the oil is fresh and not contaminated with gas, especially if it has been run rich or cold for a while.
Good information John posted. The most important is the throttle response test when setting the main jet. Let the engine warm up, then idle down to slow idle, then suddenly open the throttle, listen to the response and watch the exhaust smoke. Start leaning the mixture 1/4 turn in at a time, repeating the test until until the engine falters. Then start slowly opening the main screw, repeating the test until the engine responds crisply without hesitation. A single puff of black smoke is the goal.