If you take off the cowling and the black plastic piece covering the carb throat you'll see what the primer knob does. Pulling the knob out closes a butterfly like flap, limiting air into the carb/motor.
its the carb
pulling out the choke (or primer knob as it called on this one) closes the intake tract and pulls more fuel through the venturies in the carb and thats why it runs better tear the carb down, dip it and reassemble with a new carb kit
dont run it like this anymore untill you rebuild the carb or youll burn up a cylinder or a piston because as it is now its running lean
Did just that levi. Sort of. Didn't do a complete rebuild, but found a couple of screws on the side plate of the fuel pump loose and the gasket on the bottom of the bowl was iffy. Tightened screws and replaced gasket.