After playing around with modified Holley carb jets in an effort of process and elimination to determine why my 16 year old's 250 LTD was running lean, I found the problem. It turned out the carb "holder" that attaches the carb to the cyl head was badly warped. I have no clue why I didn't spot it right away because I sprayed carb cleaner around the area where the holder meets the head while the engine was running and found no change in RPM when I did. The only reason I spotted it was because I had already removed the carb to inspect it more carefully and decided to remove the 2 screws for the holder while I was at it to take a better look at it as well. Once removed it was obvious it wasn't making a good seal to the head and determined it was the flange of the holder that was warped badly. I then grabbed an exacto knife and some gasket material and cut a gasket for it, applied a thick coat of Permatex "Ultra Blue" silicone to both sides of the gasket and installed the holder. At first I gently snugged the 2 screws allowing the excess silicone to ooze out the sides then wiped off the excess that had oozed out into the intake port and outside the flange then I walked away from it for an hour and let the silicone set. It wasn't until after the silicone had set that I finally tightened the screws, installed the original main jet that luckily I never messed with and the end result is that the bike runs much stronger now. Yeah I know the holder should be replaced but as long as the gasket I made continues to seal, I'm not messing with it.