ENGINE RUNS BUT NO POWER

Question

KT 100 YAMAHA ENGINE RUNS PERFECTLY WHEN KART IS OFF THE GROUND (ON ITS STAND),BUT YOU CAN STOP THE ENGINE BY GRABBING THE REAR WHEEL WITH YOUR HAND.AND IF YOU PUT THE RUNNING KART ON THE GROUND (ENGINE CONKS OUT) I HAVE PUT IN A COMPLETE NEW GUSKET SET IN THE CURB BUT DOES NOT HELP (PLEASE HELP)