Small Engines (Lawn Mowers, etc.): lawnmower, briggs and stratton, speed selector

Question
I have a yardman 5 hp rear wheel drive self propelled lawnmower. Model is 125 148c501. It has a Briggs and Stratton motor. I have the owners manual for the motor which lists lots of models all starting with 12..... The self propelled moves quite slowly. I took in in for repairs and they changed the belt but that did not make a difference. Then they tried adjusting the carburetor, I believe turning the idle mixture screw counter clockwise. This seemed to help somewhat but it burnt a lot of gas and would stall when I was cutting the grass. There are 3 different speed settings and I am not sure the speeds are any different. Of course, I have it set on the fast setting. The lever to set the speeds does not seem to be properly aligned  to the markings for the 3 speeds but I can move the lever and there are 3 different places where it "clicks in to place". I would greatly appreciate any suggestions you might have.

Answer
 Hello Terrylalande:

 Check the Speed Selector Cable for Stretch. It the Cable is Good, then there is Most likely a Worn Gear in the Transmission. From your Description It Sounds like the Repair Shop did the Usual Things to Correct the Problem, so a Bad Gear(Worn Gear) is the Most likely cause for the Problem. Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John