Mcallister MGTP254 Strimmer starts and idles but won't fully rev

I've been given a Mcallister MGTP254 Strimmer as a "used once fine but now won't run". I've cleaned and checked the carb, air filter, plug, fuel filter, and used new fuel and oil. It starts easily and runs quite fast, but only just enough to get the strimmer head running but not much more. If I squeeze the throttle button, it doesn't speed up, in fact it slows down. I've replaced the carb gaskets, fuel tubing, and tried adjusting the main jet but nothing improves the high rev problem.
The carb is a Walbro WYJ look-alike made by Ruixing.
Anyone have any idea of what to try next please?

Hello what fuel mixture is it  50/1 44/1  40/1 33/1 25 /1 as if it's not exact it can cause starting problems ,also check the little rubber cap is on the needle valve  I have known them to come off in the bottom of the housing and prevent fuel getting through also check there isn't a groove round the top as the needle valve as it wont seat properly   jezza