I too have a Gasgas fse400ec and have had exactly the same problem.
1st I was told the battery discharges and I ought to buy a trickle charger which i did and its a nice thing to have but it didnt stop my problem.
Next I'm told to buy a new battery, I did but as it happens that was not the problem.
So I gave up on the "cowboy" motorbike repair shops and took my Gasser elewhere after she just refused to make a sound other than just "click" when the start button was pushed.
The issues were the standard electrical connectors/fuse boxes that are not waterproofed at the factory.
These are found under the seat/fueltank and on my 2002 fse400 are small red
fuse's. After 4 years they had corroded inside and when the key is turned no power gets to the fuel injection which is the familiar "whine" noise you hear when its turned on. Pressing the start button only produces a "click" sound of the solenoid under the seat..
Replacing these fuse's ought to do wonders for your bike,,,,,,,one more point the engine kill switch on my Gasser is also a bit finicky but I had it re-soldered and fixed as much as can be but the switch has a habit of turning the engine off at unfortunate moments so replace that if yours is a bit "loose".
Hope this helps you, I can say my Gasser is finaly giving me untroubled performance after 9 months of intermittant electrical woe's.
Lex P
