Could be a few things:

- Pin Hole in your fuel line
- Fule tank location (to low maybe)
- Low Needle valve setting (get an experinced pilot to set this)


One thing i can recommend for a quick fix, is to get a one way valve to put on the fuel line between the exhast muffler nipple and the the fuel tank. This should keep pressure in the tank when u idle down.

Hope this helps.