I have a fs 90 weedeater, it want cut grass at full throttle

SOURCE: Stihl weedeater starts but won't run at high speed

This sounds like a timing problem.
It is too far retarded, and needs advancing a couple of thou.
There you go:)

SOURCE: Ryobi 25cc grass timmer, cuts out when full throttle applied

check gas cap vent check ball may be stuck

SOURCE: when i give my stihl

This could be from a bad fuel filter (has a dark appearance when dry), from the spark arrestor screen in the muffler being plugged up with carbon deposits (take out the spark arrestor screen and clean it by burning off the carbon deposits), or the carburetor needs to be adjusted. If the carburetor needs to be adjusted I would take it to the nearest Stihl dealer that has a Gold Certified Technician. That way the unit is adjusted properly and is in good running condition. You can find a local dealer with the dealer locator. The link is HERE.

SOURCE: Have a Craftsman LT1000 and

Inspect the inside surface of the discharge tube. If you do not find any hangups, then I would wonder how fast you are mowing. If you are going full speed, I would recommend that you slow just a little. I was just thinking, have you been doing the same thing for a long time and recently have this problem develop. So, give good inspection. I do not know what type of bagger you have, but is the air able to escape the bagger, if it is not then it sets you up for a clog. If you can not find anything on detailed inspection of bagger system, there are sprays for under the deck of lawnmowers to minimize the chance of grass clinging to the underside of decks, you could try applying that to the ducting of your bagger. I hope this is helpful.

SOURCE: can't start new stihl weedeater fs 45

The Stihl FS45 has a particular way of getting started as do all the stihl products. We sell Stihl equipment in our store and service and repair them also. The normal starting procedure is to:
Be sure the switch is turned to the ON position which is the negitive sign (-). The (O) is the off position.
Press the primer bulb 3 times.
Move the choke lever to on.
Pull the starter cord three times only. If the engine fires, turn the choke to the off position and pull till it starts.
If the engine did not try to start during the first three pulls with the choke on, you must turn the choke off and pull till it starts.
Remember:Pulling more than three times with the choke switch on will flood the carburator on every stihl product thats on the market.
However, if the units still refuses to start, being its new and still under warrenty, return it to the repair shop that services Stihl products and they will repair it according to warrenty. All parts and labor should be honored at no cost to you. Please provide your receipt and serial number located on a silver tag located on the unit or the mechanic will get it off the unit when he works on it. Either way, it will most likely have to have a new carburator put on it. I hope this helps. Please comment back if this was helpful. Thanks; George