Most likely cause... The issue shows when the choke turns OFF. This results in several areas to investigate...
The choke may be physically opening (turning OFF) too soon. A cold engine does require a 'rich' mixture. So there may be some mechanical issue (dirt, grass clippings, etc) that need cleaning, or a minor choke adjustment.
OR, the choke may be operating 100% correctly, but the mixture presented by the normal carb jets is abnormally 'lean' due to minor Ethanol clogging. Thus the choke turns OFF as it should, but the super-lean mixture is too lean for the still not-fully-warmed engine.
If you allow Ethanol fuel to remain ANYWHERE in the fuel system during storage (like winter), then the second option becomes more and more probable.
If Ethanol is the issue, then I highly suggest you start using a cleaner/stabilizer, such as StarTron. From now on.... pour 1/2oz into your 5gal main fuel can, and allow that to work its way into all your small engines.
