Stihl mm55 starts and runs, but only at full throttle, then only at fast idol speed. Have installed new carb, fuel line, filter and spark plug, no change.

If it only works at full throttle it sounds like this is a fuel supply issue. Are you getting hard start issues? When was this weed eater last given a full service? A clogged fuel line, Clogged fuel/air filter or and old spark plug can have a bigger effect than you think. Bad, old or wrongly mixed fuel can also affect how the engine will run. (Stihl recommend a 50/1 mix on 2 strokes). Modern fuels go 'Bad' after a few months leaving a varnish like coating inside the fuel system. You often get this if it is stored over the winter with fuel in it.
The carburettor could also need cleaning as well. If you have no experience working on small engines or carburettors your safest option is to get a specialist repairer to look at it for you. Servicing small engines like these is usually quite cheap as it is usually only one spark plug, a foam air filter and a small amount of engine Oil.

pull the plug crank it with the plug hole downward no throttle no choke, to be sure all the fuel is out of the engine.. then let it site a couple hours plug hole down,,,

the lower the octane the hotter the fuel burns, the higher the octane the SLOWER it burns.. in my stihls i run low ocane and no boosters, in my race cars I ran lowest octane I could find... I am the nhra-vw points champion for 1978..
I also hold the record for all of nhra for the most points in one event...

higher octane slows the burn rate and volatility of the fuel, producing a more steady burn, like whats needed in small planes, there you want a a constant burn rate..

not sure of the age of your stihl but real old had points then they went to simple electronic then to pos electronic the go no go type, the early electronic rarely went out right dead without warning, you have spark so that shouldnt be the problem, the carb may be the one with flapper valve or the ballbearing valve or little plastic stopper, the flapper will let water through so it gets pumpout fast, the ball bearing not as well the dam stopper is even less efficient for that, .. no water they can all flood easy the spark plug get a little wet and being 2 cycle the oil shut the plug off, wont spark when you put it in of the engine has too much fuel... use either to blow it out it dries fast and you can put it back in after about 5 minutes... a trick to see if it will fire is knowings its all dry, hold the throttle full open and have some one spray a small shot of starter fluid just a spit right as you pull the cord, you should get a few hits, then you can do the choke and prime per the instructions for your model...

Sounds like lack of fuel. Try backing the air fuel mixture screw back 2 turns and see if it helps. Once it starts, see if you can keep it running by spraying carb cleaner into the carb. If you can keep it running long enough, place the palm of your hand over the carb inlet to choke the carb causing a sudden suction in the carb. Some times this will dislodge foreign material and help it run.

Could be adjustment if it has a fuel adjustment screw or maybe it needs the carburettor cleaning out.

Was any original fuel drained and replaced with fresh fuel?

If there are any diaphragms in the carb they could have perished in those 2 years or become hard and inflexible.

May need to have the unit checked. Could be something as simple as a clogged muffler, air filter, fuel filter or engine wear or damage.

Check the following, and then clean or replace:

• Fuel Filter - Located INSIDE the Fuel Tank
• Spark Plug
• Spark Arrestor - at muffler
• Carburetor - May need a rebuild kit

Watch this as well...

and...

Pumping the primer bubble while the engine is running should cause the engine to over fuel and stall (if the tuning is correct). Sounds like its not fueling enough, pumping the primer is forcing fuel through the carb causing the engine to pick up rev's, but this is soon exhausted which is why it wont rev on the throttle.
* Check that the fuel line is not damaged and drawing in air. (if this is intact try the next step)
* Strip and clean the carb. Replace any diaphrams if needed.
* Check gaskets seal between carb, air filter and cylinder.

Just out of curiosity what standard carb settings did you use.?

Since you don't indicate the make and model, I can't give you specifics, but when I had this problem it turned out to be a dirty carb. This can cause uneven fuel flow and loss of power. At full throttle, you may not be getting sufficient fuel to work properly. Try cleaning the carb a nd update this with the results.

Dan

check your muffler screen for being plugged, this will cause it to not accelerate
2StrokeJim

Stihl weedeater was running and stopped suddenly. Put in a new fuel filter and cleaner air filter with soap and water. Checked spark plug and it appeared fine. Started the weedeater, ran for a second or so and stopped. Spark plug wasn't gas fouled. Could it be the carburetor needing new diaphram gasket and spring.