Ya for a fuller sound, new mufflers and at least some 2 1/4 tailpipes would do the trick. Picking the right mufflers the first time is the trick. LOL A friend would buy mufflers and install them with the band clamps. If he didn't like the sound back to the store he would go. LOL
Are the secondaries coming open too quick creating a bog? Just thinking out loud. A lean bog will usually cause a backfire. Backing off the timing is worth a try. It's easy to bump it back up if need be.
