clueless colonial

Hello, I'm having problems with my fuel delivery on a '73 TX650. This bike was a former AMA racer and has been rebuilt for the street after its racing days. It has a custom exhaust and tank but is otherwise stock. At about 1/3-1/2 throttle it runs well until violently leaning out (or flooding?), enough to send me forward in the seat an inch or two. I have rebuilt the carbs and changed the jets but not much has changed. I went to a shop and they told me that I was running rich, while a friend of mine says it's running lean. Needless to say, I'm a bit confused. The only thing I can think of is that the problem has to do with the exhaust. It is straight headers into a small collector (loud as hell!) that backfires about 2-3 times when compression breaking. When the bike surges, I hear a distinct sniff or whisp of air coming from the front of the motor. There is no flange connecting the pipes to the block, only two springs that hold the pipe inside the hole. Is it possible that the pipe is rattled loose every so often, causing a lack of back pressure? I'm just a dumb Yank so any help you can provide is appreciated! Thanks dh