hey, bought my mom a sears/briggs mower about 1-2 yrs ago. most poorly designed POS mower. (who puts the spark plug up front?? now it shorts out everytime she mows under a bush.) It runs, but is always reving up and down, up and down. constantly. I am going to change the oil out, because its black, and so was the plug which i cleaned, but there is NO carb adjsutment on mowers anymore? no a single screw?? the muffler is also black, would that effect it running smooth? besides a good clean out, anythign else I should look for / do?