I had a TL125 for many years.Everytime I would properly tighten the carb to the inlet manifold the slide would stick.I tried many things including changing the manifold ,all without success.
Eventually, because of a rich running condition I changed the needle and nozzle in the carb and the sticking problem disappeared.
Upon inspection I saw, under a magnifing glass,the tiny burr in the nozzle that had been the problem all along...worth a check as your case sounds as mine.