Guessing using data provided. Stuck power valve piston. Easy fix. Runs ok with choke on cold. As it warm up the rods are not moving up and down and as choke opens it goes flat as you open throttle.
20 mins to remove carb
20 mins to clean bench off
20 mins to open carb and remove and spray power valve and all passages
20 mins to reinstall carb.
(Generous time scales)
If carb was rebuilt as recently as you say, I bet the top gasket is still reusable. Spray straw all the emulsion tubes and passages w carb cleaner while apart. If you have not worked on Qjets feel free to ask any questions. Hidden screws etc.