It sounds like your choke is not being used. If you are setting the choke, the carb is venting air somewhere. Take the cover off the motor and look at what is happening to the choke plate when you set the choke.
The choke plate should have either a vent on the butterfly plate if it closes tight or the butterfly plate should be ajar about the thickness of a credit card if the plate is unvented. Then check the mounting nuts on the carb base for tightness. If you want, you can put some ATV around the outside base of the carb to ensure its sealing.
Also note if there is a stepup cam for the choke RPM. This would increase the RPM with adjustment for cold starts and reduce the need to Throttle so much to cold start the engine. If it does not have this part thats Okay too, just a feature to be used if it is there.
Most likely your low speed idle mixture needs adjusting and its common to have the throttle capable of shutting off the engine without a killswitch. But try Cold starting it and see if sealing the carb base and adjusting the butterfly plate has helped. Then do the mixture adjustment if you are not happy with it.
If all this fails to help, have the engine running and try moving the throttle shaft with your fingers. You want to see if the throttle shaft has play where it pivots in the carb body. Its hard to explain but the extra venting of air can occur where the shaft rubs the casting. Depending on your talent it means you need a new carb or you need to rebuild the shaft.
Good luck on your motor.
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