First, be sure the air cleaner is properly serviced, free of caked dirt in the wire mesh.
Be sure the thermostat is in place and the engine is coming up to temp.
Be sure the ignition system is in order, distributor good, advance working, points set. If the plugs are fouled, they may need replacing or cleaning.
Have the valves been adjusted lately?
Is the oil clean and gas free?
Now you can adjust the carb, providing it is in good order, not flooding or clogged with gas deposits, and the gas is fresh.
With the engine up to temp, choke open, air cleaner attached, engine idled down to 400-500 RPM:
Do the idle first. The idle screw is the one up top, just below the mount flange. Turn it out slightly, (which leans the mix), until the engine falters, then slowly back in to best idle, then just slightly more in.
Then set the main. That is the larger screw down lower. Set the engine at full governed speed. Locate the governor link, in front of the carb. Pull the governor back to idle and release. The throttle should instantly spring wide open and the engine rapidly come back up to speed.
Now, turn the main in about 1/4 turn, repeat the test. Keep doing this until the engine begins to hesitate when it is coming up to speed. Then back the main out until the engine will take throttle without hesitation. A single puff of black smoke is desirable.
At that altitude, you will find it needs less choke to start. There will also be some power loss, no matter how well everything is adjusted.