Could be an air leak. Check EVERY part of the fuel delivery system, the head, the backplate, the carb and O'ring etc. Then see what happens. Also try the stock settings and see if it'll run, and adjust from there. It shouldn't be duff after just 3 gallonds of fule unless it's been overheated regularly or has eaten a lot of dust and crap. Check the piston and liner for marks ar dark burn residue.

Ian F.