You definitely should be able to spin the tires and do wheelies :) . Others have covered the KIPS system pretty thoroughly, so I'll mention some other things to check: If you have an aftermarket silencer like an FMF Power Core or Turbine Core, it needs to be repacked periodically, or you will lose low end when the packing has blown out over time. I once messed up when repacking the silencer and left the packing material way too loose. I had to rev it to the moon and drop the clutch just to get it moving. Great top end, zero bottom, so check your silencer. Note: This *does not* apply to the stock silencer.
Clean the air filter. Need I say more?
When it gets to be way overdue for a top end job, these things lose power. A riding buddy on a bone stock '95 200 had this happen. He had several seasons on it (the KDX is a really tough bike) with just the occasional oil change, when gradually he noticed that hills that he used to torque up, he could no longer ride up without taking a really good run at them. A top end job (with new piston & rings) fixed him right up. It just may be time. If it has been a long time since your bike had a top end, things can get pretty gummed up. After two seasons, mine looked like
THIS.