Have a fs88 brushcutter. Had a new fuel tank fitted (by stihl supplier) but now everytime I put brushcutter away the fuel tank has drained out onto the floor. I do not hang brushcutter up it is flat on the floor. Also after about 30 mins of running it seems to struggle for fuel, I open fuel cap and this seems to solve the issue for 10 mins or so.

If your brushcutter does not run or rev, clean out the exhaust, some of them have a small mesh baffle which needs to be kept clean. If it clogs up just sit down for aawhile with a very sharp neddle or something similar and unclog the little holes, this is a better option than buying a nes one.

Take it back to your stihl supplier they should back up there work.