The Tecumseh 8-18 HP manual is a great one. You could be having carb issues as those carb wear the butterfly shaft or bushings and they suck air. Bet you spray some carb cleaner around that shaft and rpm's will change/smooth out.
Hope you didn't remove the center main jet when you cleaned it. They are screwed in carb, then drilled at factory. Once removed you can't get them lined back up and carb will be too lean and you will get rpm "surge" or "hunting". There is a way to cut a new cut a new slot in it, can't rememebr where I saw it at, or replacement ones can be bought.
Start with your adjustment screws on jets at 1 1/2 turn out, then fine tune from there.
You called it. I did remove the centre main jet/emulsion tube.. who knew. I just ordered a Genuine Tecumseh carb rebuild kit from a local shop. I know I could have found one cheaper on eBay, but. I think it's best to support local shops. A bit more money perhaps, but, so what.