How do you secure the muffler head pipe seal?

I think I have found my back fire problem. It runs like a whipped racehorse, but when I let off, I get a couple of very large backfires, not crackling, but loud explosions.  After spending a couple of days adjusting everything under the sun, I have found that the muffler header pipe leaks from the underside of the junction from the head pipe to the muffler.  I went today and bought a new muffler gasket, and have tightened the clamp in multiple different positions at the junction and I can not get it to not push exhaust out the end of the junction on the bottom side.  What do you guys do?  I have to be missing something simple........

That's prob not your issue. A slight leaks not going to cause that. You should prob go up a few sizes on your pilot jet. You could be running lean! Bury the pedal on a long stretch turn it off and pull your plug snd check it.

Pilot jet is 50, main is 180 (assuming its a dynajet) for sure it's not a mikuni.  idles best with a/f at around 2.25-2.5 out.  It has a cdi(rev box) that came on it when I bought it.  Just did the 686 and have a snorkle on made with the mcmaster/carr? hose.  K&N Filter with outerware sleave.  Modified sheave with 16 and 14 weights staggered.  It takes off with authority and can get 55-58mph on the speedo.   Seat of the pants feel is great.  I think I might be a little rich on the main, but dropping to 175 seemed to really limit top speed. to 50.  It is a decent pulse coming from the junction.

Mine has done that on occasion when the two nuts securing the muffler clamp into the head have come loose Allowing exhaust to come past that junction. You can actually hear it under the engine cowl. Only then does it backfire. If you had a minor leak there it probably wouldn't back fire so other problem might exist