I assume your Weedeater is an FL26 or similar. Also assume you have removed muffler and screen and cleaned any carbon build up. Excessive carbon will produce similar symptoms as lean carb mixture, such as low power, poor acceleration, and overheating. These have Chinese made Zama carburetors. They are set up to run very lean to meet EPA and CARB emissions requirements. Plus EPA requires carb to be unadjustable by end user to retain lean burn calibration. Both low and high speed jets are recessed and have splines but no slot or other means of adjustment. A special tool is used to make adjustments. Dealers are not even supposed to sell these tools to general public, but you can sometimes find for sale on Ebay. You might be able to turn the jet with a needle-nose pliers. Try adjusting the high speed jet out 1/8 turn (this is jet nearest aircleaner). This should provide enough enrichment. If that doesn't help, your next step is to have a tech look at it.

Shawn, what you have is the fuel pressure is dropping. When upright, the leak\lack of pressure is making the motor run lean being it is not being pushed up to the carb. When u turn it upside down, it doesn't have to move up. If you do not get any fuel leak when u turn it upside down, your problem is in the carb. You should just need to get a carb rebuild kit.

ok first off im a small engine mech i guess youd say in nc been that way since i was 6 the thing with these weed eaters you see alot of is they use a diaphragm carb so they get fuel at any angle as lets face it holding a weed eater level ell the time is not easy the diaphragm is thin and susceptible to dry rot and fatigue running it even once in a while during the winter is advisable but keep some fuel in it as far as additives stabilizer is ok id guess but most additives are "miracle cures" and just like their counterparts in the 1800's they are bs and do nothing more then degrade fuel and possibly cause adverse running conditions. the air fuel mix screw on most weedeaters is made so only a "trained mech" can do it however this is their downfall as no carb i dont care what kind stays at one setting every year year round humidity and conditions change and so do parts over time engines use less or more than the screw allows i try to stay with the 90's models or put a diff carb on them ive had the same two weed eater featherlights for prob 2 years then add when the owners got them and they have lasted a long time i operate them weekly doing lawn care (hey they were cheap) and occasionally i have to adjust or fix small issues another example of wear that changes amount of fuel used would be rings 4 strokes use oil in nthe crank case so the rings do not wear so bad however 2 strokes top ends and rings do not get as much oil staying on them 24/7 and wear faster so over time they lose compression power and burn more to run the same. im not an expert but these are just the usual suspects the best bet though is the carb needs a cleaning and poss a new diaphragm

Sometimes when you run completely "out of gas" and run the carb bowl dry, a small particle of grit, previously "barred" by the small opening afforded by the needle valve (under "full bowl" conditions).......will make its way into the carb bowl when the bowl begins to empty and the float drops (opening the needle valve wider than when bowl is full) as the fuel level drops. This particle (or particles) of grit might then make their way into the main jet, which can cause lean mix and surging. If the grit gets into a pilot jet, it might also induce some surging BUT the most obvious symptom of a clogged pilot jet would be that the engine will not idle at low RPM. With a clogged pilot jet, you might be able to get the engine to start and run at higher RPM (and it might surge), but when you tried to get it to idle at slow speed, you would reach an RPM (higher than normal idle RPM) where the engine would suddenly begin to stumble and then die like it ran out of gas. You may want to empty the carb fuel bowl (catch the contents in a clear, clean glass or jar so that you can examine what you pour out of the bowl). If you see some slugs of water (water can sometimes clog a pilot jet) you should spray some carb cleaner into all the holes and passages on/in the carb. If you see some gritty looking junk, you might have to disassemble the carb and probe the holes with wire and blow out with compressed air. As njdpo suggested, you may want to "add some dri-gas, or Methanol alcohol to disperse and transport the water out of the carb.