Usually the need for slight choke "ON" for improving run ability is caused by improper air fuel mixture. Most commonly, a carburetor idle mixture tweek will cure this problem.
Other mixture problems can be intake manifold leaks, carburetor gasket leaks, carburetor throttle shaft leaks, leaks at the distributor vacuum advance unit and attaching tubing and let's not forget if you have a vacuum operated windshield wiper system a small leak within this unit or hoses can affect the air/fuel ratio.
PLEASE NOTE!! Don't try to adjust the idle air fuel mixture until AFTER you have ensured there are no intake air leaks and AFTER you have your electrical system adjusted which includes point gap (dwell angle) and ignition timing. To attempt air fuel mixtures before adjustment of engine electrical system settings is a waste of your time. FIRST THINGS FIRST.
After setting your ignition point gap statically with a feeler gauge to specs, check your dwell angle (duration of time in degrees that the points are closed)using a dwell meter according to specs with the engine running at or near idle rpm.
Next, adjust your timing according to specs using a timing light and the timing marks on the flywheel/pointer.
Attach a tachometer to the primary circuit and monitor the idle rpm. Adjust the idle RPM screw to obtain idle rpm's per specs (usually around 600 rpms)
Using an arousal can of carburetor cleaner with a plastic straw nozzle, spray small amounts of cleaner around any and all vacuum connections, vacuum diaphragm and carb throttle shaft, carb gasket, and intake manifold gasket. If a leak is present you will notice a drop in the RPM on your tachometer caused by the ingestion of carb cleaner into the air fuel mixture via the leak point. Repair as necessary.
With no leaks noted, ensure the choke is FULLY OFF (OPEN), and adjust the idle RPM screw to obtain spec idle rpm's.
While watching the RPM's on the tachometer, slowly turn the IDLE MIXTURE SCREW in about a 1/4 turn.....if the rpm's drop....turn the screw out and continue to back the screw out 1/4 with 1/4 turns until the rpm's climb to there utmost highest reading. When the optimum air fuel ratio is achieved the rpm's will be at the highest point and the engine will be running smooth. If you back the mixture screw out too far the rpm's will begin to drop due to an over rich condition. The key is to slowly and methodically adjust the screw either in or out and achieve a maximum rpm gain. Once mixture is set, you should readjust the RPM screw and lower the idle rpm's back to specs.
If this does not cure the "CHOKE ON" to run best problem, most likely the carburetor is in need of internal cleaning or overhaul.
Stumbling upon acceleration, and erratic idle is commonly caused by LEAN air fuel mixtures when idle/power/accelerator jets get dirty or clogged and when the accelerator pump plunger circuit begins to fail due to wear. LEAN problems within the carb are masked by choking off normal air flow with the choke valve causing an over rich condition (and excessive fuel consumption) during all ranges of engine operation.
