hi all, never had a 250 before, bought a 1986 XT250 for a daily rider and am struggling to tune it. It's got stock exhaust and air box. Had been sitting for years. I've pulled the carbs three times now for cleaning and have a rebuild kit on the way. When I clean the carbs I have removed all jets, needles, floats etc and used copper wire through all tiny holes as well as carb cleaner. I have checked the float heights and they are spot on. It has always idled fine. Used to die with ANY throttle when I first bought it. After cleaning the carbs it will rev fine up until about 2500 rpm then start to struggle and die, lots of popping and backfiring until eventually dies. With the choke on it will rev up fine til about 4500 rpm then die same as above. When I pull the plug it is black and sooty, but most of the other symptoms suggest it's running very lean??? I've checked for airleaks around the manifolds and can't find any. Any suggestions for what to try next?