I have a LB Dura-Force powered 21" hand mower, SP. It is about four seasons old, but has been run pretty hard. It has been through some difficulties (no need to spell them out here), but it ran pretty well last season ... until the last month. I did not spend much time, but lived with the problem as it did not impact productivity in any great way. The primary problem was lack of speed control with the throttle. After a little searching, I found the throttle butterfly to be worn, the shaft passing through the throat of the carb, with the fin on top catching air flow from the flywheel, and a blind pin on the bottom. The real problem was the pin on the bottom was badly worn, out of round and not fitting tightly into the hole. The movement of the entire shaft caused the bottom of the fin to interfere with the slider mechanism. This mean uneven speed control. I replaced the shaft, and the blind pin fit pretty well into the hold. Apparently most wear was on the pin, not the hole. The new shaft now moved freely and did not interfere with the slider. The shaft is the only part I replaced. However, the speed control was not working well. Movement of the lever on the handle made no difference in engine speed. Sometimes the engine speed would race, and then settle to the proper speed. It was if the throttle was catching someplace, but that place was not obvious. My thought was that the disc that slides through the slot in the shaft was misshapen from my removal and reinsertion. This Winter I got new parts, a new shaft with fin (same one replaced in the last month of the season), a new throttle disc, a new throttle spring. I put all the pieces together yesterday, and got the engine running. However, the problematic characteristics remain. I am unable to get proper speed control with the thumb wheel. I have no tachometer, so must judge speed by ear. I am sure it is running a bit too fast, but any movement on the wheel makes no difference. Also, the speed control lever (moving the slider on top of the air box) makes no difference in engine speed. The shaft with the fin seems to be free, has spring tension, and the disc fits well in the slot (don't think it is providing any interference). At this point, I am at a loss what to try next. I have removed and reinstalled this carb many times, and did likewise on another LB of former years. My point is that I am working with familiar parts and mechanisms. I have had successes in changes before, mostly making spring replacements. Obviously, none of my explanations will make any sense, UNLESS you have had some experience with these engines. Anybody have a suggestion for next step? I am not sure if an entire carb can be bought as an assembly. If so, maybe it is time to trash this one in favor of a new assembly. My patience is pretty thin with this engine. However, I do want to have it running in top condition when the season begins. It is a workhorse for us, often running 6-7 service hours per day (more than a full tank of fuel). Thanks.