Okay, lets wind this back a bit then.
How full is the fuel tank?
They are almost impossible to drain in situ as a siphon wont really work and if it's over half full I wouldn't want to be using the hinge to support it so the only real option is to remove it and drain it properly.
If there isn't much fuel in there then the chances are the Ethanol has drawn in water and there is some swilling around at the bottom waiting to rust it out or intermittently block the outlet.
Moving on, as Utopia rightly suggests the diaphragms of the vacuum pump (and vacuum tap if it is not a manual one on a 600?) will likely be not at their best given the bikes' age- I replaced them on my 900 a couple of years back as both were shot.
If you can't drain the carbs in situ then you have no real idea if they are empty (likely they are) or if they still have fuel in which then poses the issue of that is a fair amount of stale fuel to shift or fill with fresh while turning the bike over on the starter.
A word of warning on this too, as already hinted at, the starter electrics are fairly marginal to say the least on the early bikes with the ground connections being a particularly under sized gauge for the job expected of them (it's amazing the difference fitting beefier cables will make to the speed the motor turns over) add to the this that if you run the battery down constantly with the starter it will mostly likely damage the Sprag clutch bringing another world of pain to you.
Only you know what fuel you used most when it was running but if like so many that have been fooled into thinking Shell's V Power is the best thing ever, those carbs will likely be horrible inside where it has evaporated and left its' residue and the chances are until they are cleaned out properly you'll never really know.
I appreciate that many carb'd bikes will start reasonably easily after a lay up but most are either gravity fed or electrically pumped whereas the Monster is peculiar with it's (in many cases) double vacuum system which, if not at it's best can manifest itself in all kinds of ways.
During my eighteen years of ownership (at time of refresh) my '97 900 (very) occasionally suffered from vapour lock, occurring on very hot days when filtering in traffic with an extremely hot motor which would eventually die through fuel starvation.
My fix was to completely refresh the fuel system and while doing so the effects of Ethanol coupled with age were obvious:
All the 900/750/600 models are getting on in years now and as such they all need that little bit of extra lovin' or owning one will just become disheartening.