[swifty2014]

So it shuts off instantly like you turn the key off? But starts and runs good for a short while.
Maybe an electrical problem, such as kill switch contacts or ignition switch.
Can you hold the throttle up to about 2000 RPM and let it run there for a while and will it still do the same shut down ?
If it stays running at higher RPM and doesn't die it may be just very lean idle circuits. Have the mixture screw plugs been removed so it can be adjusted?

 

[swifty2014]

If the plugs are black it is running too rich. Is the air filter plugged? Try turning the idle mixture screws all the way in and back out 1/2 turn to see if leaning out the idle circuit does something to help it run.