Starting well without choke is a sure sign of a rich mixture. Dying when warm is another. Put it this way - I used the choke to start mine a few weeks ago and forgot to put it back. When it got warm, it ran rough at full throttle and wouldn't idle, and then wouldn't start again until I realised my mistake. That was a 'Doh!' moment. Get the idle mixture sorted out before anything else. Costs nothing to do this
If Jens' advice doesn't work, your next step should be to check the float height and the float needle. Float too high and/or a leaky float needle could give the same symptoms.
It might have a low mileage, but at 28 years old it's likely something has failed or gone out of adjustment.
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