Kawasaki Sportsbike Riders Club - Australia - Online Forum

It's hard to say without looking at it, but I'd say the shops have ripped you off because they're lazy. Replacing carbies without opening up and inspecting them is just plain dumb. Unless the carb castings are smashed or bent, there's no reason to just replace all of it. All the bits that wear are replaceable. Try hooking up to car using jumper leads and see how it starts. My GPX was hard to start for a bit and it turned out to be the battery. I couldn't even tell that it was struggling, but it was.
Irridium plugs are a waste of money for trouble shooting. You will never fix a starting problem by using irridium plugs. Use them if you really want to avoid changing plugs, other than that a fresh conventional plug does the job.
Since your bike is probably actually 15 years old, the rubbers holding the carbs could be perished and allowing air in which would lean the mixture. Try starting with and without the choke on, with and without a bit of throttle etc.
Have you had the carbs apart and checked the float height and whether they're flooding? You need to know where the fuel level is meant to be, raise the drain tube (put one on the drain nipple if there isn't one already) next to the bowl, open the drain screw and see where the fuel level is.
A more remote possibility is that your valve clearances are rubbish and you've either burnt valves or there's just too much duration and overlap due to zero clearance (it's not such a large effect though).