Just purchased an 1983 xt550 with the same problem. I removed the air filter when cleaning the carbs and but did not replace the airfilter for the test drive. It ran like a raped aped. I finished assembling and it started running like %!¤ again. The secondary carb in vacuum controlled so apparently it´s very touchy. I tried using some generic foam while I was waiting for the new filter. It acted like the timing was off, miss´n mid to top range. a new filter and it revs to the limiter with no missing. I´ve never seen an air filter make such a dramatic change but apparently the vacuum controlled secondary carb is touchy. Try running it with no air filter I guess