What is wrong with your Yamaha pw80 if it runs for 2 or 3 seconds and then dies?

Fuel may be turned off or the filter could be clogged.

But that sounds like the jets are clogged in the carb to me.

Just a tip to keep this from happening, shut off the fuel when you are ready to quit for the day ( and leave it for an extended amount of time ) and allow the engine to die, this will leave no fuel in the carb to go bad and mess things up for later.