Craftsman 12 hp snowblower runs rough

My older recently acquired Sears Craftsman Drift Breaker snowblower runs rough and cannot reach high rpm's. This is a 12 horse engine, I think it has a tecumsah snow king engine. Purchased used, model number begines 536. not sure of the year. Starts right up and will idle fine. If I increase the throttle the engine never really produces any higher rpm, or at least not enough to "throw the snow" any distance. Could this be a altitude adjustment on the carb? I live at 8,000 feet above sea level in Colorado. Been looking online for detailed diagrams and have not found any that have been helpful. not very experienced with small engines and looking for some guidance.

I'm thinking this blower is about 15 years old but not sure. It is a three stage model, has 14" auger blades, with the a chain drive small drift buster auger on the top, above the main auger.

 

You might try http://www3.sears.com/ enter the model number, the on the parts list find the motor model number then do a search on that. Have a nice day. Geo

http://www3.sears.com/

 

have you tried working the throttle manually at carb.? sound's like linkage
is binding if it will not rev up,make sure nothing is blocking carb linkage action
if it will throttle up at carb then you'll know which way to proceed..good luck