Tractor Repair: Case IH 385 Injector Pump

Question
I own a mid-80's Case IH 385.  Two weeks ago, while I was operating the tractor, I pushed the throttle handle down to low idle while working off of the tractor.  When I got back on, I moved the throttle handle back up to speed, but the engine rpm's did not increase.  There was no response from the foot feed either.  

The linkage is still operating the throttle lever on the injector pump, and it looks to me like the throttle shaft is turning, but the engine will not increase in rpm. I haven't disconnected the throttle lever and tried to turn the shaft manually.  What do you think the issue(s) may be? Does it sound like I need to take it into a mechanic?

Answer
Duffy, it sounds like you have an internal problem in the injector pump. My guess is that the throttle shaft is broken inside....you might try to manually turn it. If it doesn't throttle up, you have a broken shaft. That component part IS available, I believe, but you might want to just replace the injector pump....BUT, if you don't, take the injector pump off the tractor and have it rebuilt. They are a hassle and it's money well-spent. Good luck!