Throttle Stuck? Craftsman 917.372270?

I recently got a used Craftsman model # 917.372270 4.0 HP, 22 in cut self propelled mower. The mower has literally not been used for a couple of years and only used a couple of times. I know gas stabilizer was used in the fuel mix. The problem is the engine throttle has a hi and low adjustment and bulb pump for use when starting cold. The mower starts fine but it seems to make no difference in engine speed if I have it in high or low position and it sounds like it is running "wide open". I don't want to casue excessive wear on the engine. I am not a mechanic of any sort. can someone help me out and explain how to fix the problem and tell me like a 2 yr old. Any help apppreciated. Thank you.