Jim:
If you do not see an problem with the linkage I woudl start easy and then head to harder problems.
1. Bad gas - while running the engine spray some starting fuid into the air intake, if that kick up your RPMs you probably have a fuel problem. I also suggest trying a new tank or different tank of gas.
2. Make sure that your fuel lines are all sealed tight. Often the clamps will get just a little bit loose allowing some air in the line, lack of vacuum means lack of fuel with means lower RPMs;
3. Check your throttle cable - it may have stretched. Disconnect the cable and push the throttle by hand (use a screw driver or pliers to move it). If the cabel is streched then you are not getting full trottle;
4. Next, check the plugs - re-gap or put in new ones.
If all else if OK and still having problems re-build the carb - you can get a re-build kit pretty cheap and it is not hard. Use your digital camera to take pictures of teh linkage and such as you take it apart - makes re-assembly easy.
If you need help you can always give me a call - I have fixed lots of these (hence the steps above) and I know someone who actually knows what he is doing.
