I had a similar problem with mine mate, ended up gettin really annoyed with it, taking the injection system off, putting an XR2 carb on it and straight away it ran beautifully. I could beat Xr's too with the single carb.

Another thing to consider, if its a MFi XR engine, they have bigger valves than standard CVh heads, think thats how mine went well, i.e decent carb and big valve head.

Best of luck whatever you decide.