Yamaha 250 xt looses power in second gear it is as if there is no fuel getting through from the carb

You have a mikuni carb. there are curcuits to the carb. the first that works from idle to 1/4 throttle is the pilot cicuit, from there you transition to the needle jet/and jet needle. That controls the 1/4 to 3/4 throttle and then the main' jet which controls WOT. you may have a air screw that would be before the throttle plate if it is after it is a fuel screw find the factory specs for the jet sizes and make sure all is correct. if not you isolate where the problem is and correct in a rich or lean adjustment' I race yamaha 2 stroke road racing bikes, it is not an easy task sorry to complicate you so much

it wont hrut to have a tune up on the carbs but to set them back in sync you need a tool for it and you would need to reset the fule mix after that as well,