I have a 350 husqvarna, starts up fine, runs fine when at half throttle, when u put into full throttle it lacks power and backfires, go back to half throttle runs fine

This is classic fuel starvation, start by removing the fuel hose from the tank and replace the filter on the end, if no better, it may be a tank breather issue, when the saw starts to die slacken the fuel cap, if the saw picks up you need to replace the breather, if no better you can make a small adjustment to the H screw on the carb, unscrew ( counter clockwise ) the H screw this will richen the mixture, you will need to pull the anti tamper sleeves back on the screws, this can be done by gently screwing a suitable self tapping screw into the sleeve, when it hits the screw head under neath the sleeve will start to pull back, once pulled half way off you can enter a 2mm screw driver through the centre if the sleeve and intp the slot in the adjuster screw underneath.