I have a Stihl FS85 Weedeater that I am having some serious problems with. I have been a mechanic for small engine equipment for 13 years now and never had a problem like this. Here is what it is doing. It will run at idle and at Max speed but will not go from idle to high speed on it's own. I have to manually choke it to get it there but once it is there it runs great until you drop it back to idle. It will then idle good but when you try to pick the speed up again it dies. The funny thing is that I have an Echo Blower that is doing the EXACT Same thing. Here is what I have done to them. I have rebuilt the carburetor, I have cooked the carburetor out 4 times in carb cleaner, the good stuff, I have tore the carb. down 4 times and the last time I took them all the way past the aluminum plugs that are in there and then cooked them out again. I have repalced the fuel filters, I have replaced the fuel lines. I have checked the cylinder for scoring, I have removed air filters and tried running as well with them in place. I have adjusted the carb. every which way possible. I have changed gas, Octanes, and oils. Used fresh mix. I have checked for bad seals in the crankcase. Nothing seems to fix this. The only thing that the 2 have in common is that they have Zama Carbs. The weedeater is a Stihl FS85 and the blower that is doing the same thing is an Echo. These machines have come from the same owner to be fixed with the same problems. I have racking my brain about this and I can't figure it out. The only thing I haven't done is replace the carb. with a new one. Anyone got any ideas.