gilera cougar 125 2003?

Hi,
Check the vaccum line to the fuel shut off valve for leaks, you may be suffering from fuel starvation if it is not opening correctly.
If your bike has a "prime" position on the fuel tap, use that to see if the engine continues to run normally (prime bypasses the vaccum valve on most bikes).
If your bike runs on "prime" or with another "slave tank" as a diagnostic method, then your vaccum valve is the culprit.

Cheers

westcountry
October 2013