Cold start problems

Suspect choke’s pull-off as mentioned above. You didn’t say if choke is electric or mechanical, both have a pull-off provision that as soon as vacuum is developed at engine start the device cracks open the choke plate about ¼” or so, and as engine warms continues to ratchet plate to full vertical open.

Mechanical pull-off is accomplished via a vacuum actuated diaphragm located on carburetor. I used to check for leaking by attaching a length of vacuum hose, sucking diaphragm fully retracted holding that position with tongue, and seeing if it can hold a vacuum. If diaphragm is good, you will need to make small adjustments to rod with a “U” bend connecting it to choke plate to adjust pull-off opening during a cold (overnight) start. Also there is a bimetal spring located in manifold beneath a tin cover with a projecting rod that pulls choke open as engine warms and pushes choke plate closed when cold. This spring can break causing choke plate to flap at will. To test spring action when engine is cold pull air cleaner, move throttle shaft by hand and see if spring forces choke plate closed, or if engine is warm see if spring fully retracts choke plate to vertical. If these conditions are not met, there is some adjustment at spring coil housed below tin cover, or the spring has failed, broken in two, and needs to be replaced.

A factory electric choke generally has an internal piston that is moved by vacuum, and unless it is stuck, will not need tending to, however choke articulation or pull off can be adjusted by rotating its round black cover containing heating coil by trial and error before a cold start. Or, as 67dart273 said; perhaps a broken bimetal spring internal to choke has failed and is unable to move choke plate open or closed.