According to the manual you have an early style pump, meaning before EPA regs were applied. That gold nut hides the fast idle screw. Remove the nut and under it should be a lock nut with a screw that is adjustable to accomplish the correct high idle, which is 3350 +/- 100 rpm's.
However, I would be concerned with the cause in the drop of rpm's. You didn't give us any history on the machine. No. of hours? Did you acquire it with this problem or has the problem creeped up over time. Is the air filter clean? Forgive me for asking this, but when you adjusted the valves did you know how to properly do it - engine cold, intake and exhaust clearance .008", no lifting action on cam?
Another thing is bleeding the fuel system. This is likely not the problem as much as you have run the machine but its easy. Air can get trapped between the injection pump and the injector. This can happen when you run out of fuel, change the filter, etc. Because air compresses it doesn't get pushed through the injector. With the engine running, crack the fuel line nut at the injector and bleed it (one at a time). It only takes a moment. Start with a cold engine and be careful of obvious fire hazard!