Poor starting when not started for over a week.

[-ralph-]

My XT600E 4PT starts first push of the button from cold, if it has been started within the past few days. But leave it a week or more, and it's touch and go whether the bike will fire first, or whether the battery will run out of juice first, and getting it started often requires jump leads and more than 30 seconds on the starter button.

Any ideas what could be causing it, and if anything can be done to fix it?

So far I have put it down to fuel evaporation and/or degradation within the carb bowls, and the bike can't start until fresh fuel has been pulled through, but this is a guess.