My stil wipper snipper starts and runs but only on low revs.it will not rev up

Most Stihl's are the same...Sounds like you need to adjust your mixture screws on the carby. you will need a small flat blade screwdriver (usually these are supplied with the wipper snipper from new and they have a little arrow on the top), H is for the High speed mixture screw, L is for the Low speed mixture screw. Turn the screws clockwise in until they seat (do NOT tighten!!) then turn them anti-clockwise the recommended amounts of turn on the cover around the holes. Careful not to lean the mixture out too much, but if it is the newer type they have a restictor on the High Speed that stops it at 3/4 of a turn. start it up and hopefully it will fix your problem, if not you will have to take it to a professional. Further info majority of STIHL high and low mixture setting are fine tuned via the respective rev settings , you'll have find out the particular setting from your STIHL dealer(sorry)