There are check valves etc inside of the these carburetors and at times the become stuck open or closed and cause an internal leak that will either supply to much fuel on the low side or not enough on the high side etc.... There is a testing procedure that stihl has trained us with that allows for testing of these check valves. In approx 10 minutes i can tell weather its worth rebuilding the carburetor or tossing it and installing a new one. Also if a welch plug has come loose or is leaking it will allow an internal leak in the carburetor. It has come to a point where it is at times not cost effective to put labor and kits into a carburetor and we just replace them as needed. The accelerator pumps also can become worn and leak as will throttle shafts and plates.I have never seen an internal leak in a carburetor before, normally you can just rebuild the carb instead of replacing it. The rebuild kits are around $6-10. I would also replace the fuel line and filter if the unit is really old.