My 98 Summit 500 became hard to start this season as well. In fact, your starting description sounded just like mine. I would check your high altitude compensator (H.A.C.) too. 99% of the time it fails on the rich side. Essentially the HAC is a sealed bladder inside a small can that expands and contracts with changes in altitude and temperature. It's mounted on the airbox. The bladder controls a needle valve that varies the fuel pressure inside the float bowls. Long story short, my bladder was deflated, it was running overly rich. I fixed it via instructions from another doo forum - google "H.A.C. unveiled". Now, I'm back to normal starting, good performance and my spark plugs look great.
If your HAC has been disconnected and discarded and many have, I'd agree with the others, your carbs should come off for disassembly and cleaning. I've done five sets for myself and friends this year and only one required new parts. Most were gunked up in one way or another and all are running and starting great again.
Send me a PM, if you want more insight on the HAC fix.
Good luck,
Dan