Small Engines (Lawn Mowers, etc.): lawn tractor, injection valve, sized model

Question
i just bought a lightly used (2hrs) sears lawn tractor. it is last years 1000 model with the 18.5 b&S motor. every thing works as it should except the motor rpms seem to surge up and down. it is more noticeable at idle but occurs at all throttle  positions. what is the cause of this erratic running?

Answer
bill,

  your idle fuel air mix is surging into the engine adjust the throttle cable to stop the air injection valve from opening and closing basically adjust the cable forward about 1/8th inch in to the carb rocker arm.
what your wanting to do is give it just a tad more fuel at idle speed because as it seems now it is not getting quite enough it does happen in shipping  with sears the parts loosen slightly and allow slippage but  this should fix it for you.
that should stop the erratic running which i may add is abnormal to this sized model but quickly repaired as show above
Also the outer filter cover the foam one that goes around the air cleaner should be saturated lightly with 10W-30 engine oil this also keeps the airflow regulated