my bike runs great..but when I'm out to the store and it sits for a few minutes
it will back fire thru the carb when I start it and unseat the carb causing a major vACUUM LEAK. I have to tear it all apart to reseat the gasket to the intake . I've already check for vacuum leaks while bike is hot , none found. Any suggestions?

 

Coughing through the carb is an indication of a lean condition.
What jets are you running??

If you had an S&S it bolts to the manifold so there is no chance of it coming loose.
Just a thought!!

 

stock jets, only coughing(backfire) is on restart when hot
no cough while running.
no backfire when cold starting

 

Idle jet is probably too small, go up a size and try again.

 

It also sounds like something is not right with your gasket. I think your saying the carb to manifold seal becomes unseated when it backfires?

Replace that seal and check alignment of the carb. Is the carb mounted properly with no play, gaps, or movement to that seal at the carb to manifold?

I agree with the others, your slow/idle jet is lean. Pick up a couple of sizes larger than what you have. If your stock is 42, then get a 45 and or a 46 in there. YD

 

That's what I was thinking. If the backing plate is installed correctly, no way it can come loose.

 

Something is causing cylinder fire to come out intake,maybe hanging valve.

 

About the carb coming unseated....what do you find when you take it apart or are you assuming it came loose because it works when you take it off and put it back on again???

 

I was curious if you happen to have the old compliance type fitting instead of the newer style