B&S 18HP opposed twin model 42a707 idel problem (surge)?

it idels good for a few seconds then it revs up to 2000RPM then idels back down for a fewmore seconds before doing it again and now here lately its been running sickly missing,backfireing,running rich, but still has alot of power (does not sound good with stright pipes when you have this probelm) so can anyone tell me how to adjust the govener or fix this problem?