I bought a 1978 dt400 several years ago, It seemed to run decent at the time but before I got to play with it much life got in way and it ended up in barn for about 8 , it idle good and pull decent on bottom end but when it hit 2500-3000 it would bog out awful. Played with air/fuel screw atw in or atw out it made no difference in performance at all. It seemed very loose and actually fell out once while I rode. The only way it seemed tight enough to stay put was atw in. Was thinking new carb but figured I would run it by you guys first. I checked for leaks, not sure what main jet but almost positive is stock, needle set half way, checked gas line, float and  

 from near Salisbury, UK.

Check that all jets are clear - ideally dismantle the carb completely and clean all parts in an ultra sonic cleaner. If you've not done this before, take photographs as you dismantle the carb.

There is an O ring on each of the tickover and air screw, so make sure that these are ok.

Also check that no wildlife has taken up residence in the air filter!