Can you start it with throttle closed, then rev it sightly as soon as it starts or hold the throttle slightly open
At mousejunks tried starting with the throttle fully open it wouldn’t start then tried without the throttle open and it started and died a few seconds later as usual
Whiskers, what country?
If in USA, it's not the gas.
Not your plug, if can crank for long time it's not your battery. You hear fuel pump, so not that.
Starts and dies....
One crazy thought: disconnect battery please. Turn key on, don't try to start. . Wait. Reconnect battery. Now try to start.
Thought #2: turn key on, don't try to start, wait 10 seconds, now give full throttle and hold 10 seconds. Let go, wait 5 seconds, turn key off. Now try normal start using no throttle....just key on, pull both brake levers, push start button. Any better?
I'm thinking ECU (brain) needs clearing. If you have good battery and good plug, good coil connections - Something electrical is happening, telling fuel to stop flowing through injector.
Thought #3: open battery cover. Turn the key on, do not start, touch the pink wire to the frame to ground it (use a short piece of wire to do this if necessary). Turn off the key. Now start.
In my opinion, letting it sit for weeks should only effect battery strength....nothing else. But, maybe repeated start attempts has confused ECU.?
I do not think it is a serious fault ....unless ECU has died. Highly unlikely.
Give us report back please.
Stig