Hi

A good possibility is that some of the reeds in the reed box have broken (this was my problem or at least one of them), thus letting the fuel escape back out of the cylinder while the engine is at low RPMs. because the choke is on when you start the bike, the fuel can't escape so it starts and will idle but once you let it go and slow down, there isn't enough suction from the cylinder to maintain fuel flow into the engine. It's simple to check and cheap and easy to fix so take a look.
Another possibility is that the throttle cable is just not sitting right. It could be wrapped around the frame strangely. Or the throttle is just not letting enough fuel in when you take your hand off it.
It could be lots of things but it's not anything big, it will just take some tinkering. I'm guessing you have a broken reed.

Also if it starts with the choke, but will not idle with the choke off, check to see if the idle jet is plugged. If it will not idle, but seems to rev up, down, and then dies when you try to adjust the idle. You got an air leak in the carb manifold or gasket. If your ignition timing is way off or if the automatic spark advance is stuck. These things can cause erratic idling and strange running. If it starts and idles good, but loses that good idle as it warms up. Check the valve clearance. If a valve is just a bit tight, it can leak a bit after things warm up and expand. Once you know the engine will start and run, I would turn it off and change the oil and filter. You don't necessarily have to do this. If the original oil looked and felt good, you could run it for a while.

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