Okay here is the Problem. I put on a Mac slip-on and Jetted the carb and now I have even worse backfire than before. Before the Jet kit was installed it only backfired when I turned off the bike. Since the install the my bike backfires on almost every shift and upon deceleration. I've taken it back to the shop that installed the jet kit and they said the backfire was from the muffler not being sealed correctly. I had them re-install muffler and still no luck they said there is nothing they can do? it makes no sense. I bought the muffler directly from Mac so I know it fits right. the pipe sounds awsome when its not backfiring.

please help, what can I do to fix this mess?

Well it could be sucking air at the header pipe muffler joint - it can also suck air at the head/head pipe flange - might need a new crush gasket there - thay can look good and not work very well -

by the way what jet kit did you have installed?

There are a lot of posts about the backfiring.
Usually the jet kits don't change anything about the pilot jet and usually the backfiring is caused by extreme lean condition in the idle cirquit. It can be caused by too much air/vacuum leak/, or by too little fuel /too small pilot jet/
Why don't You try one size up on the pilot jet ?
It is  a very cheap and easy try.

But didn't earlier bikes (ie my 1986) come with bigger jets?  Therefore is it still necessary to swap jets when putting on a different muffler?

Which models have the adjustable needles

I am getting backfiring after the muffler install and i'm getting crap gas mileage.  Under what circumstances could a HD muffler richen the air/fuel mixture?  Could backfiring/flames out the muffler be caused by a rich condition?

toddlamp8 wrote on 12/31/69 at 16:00:10:

A rich condition is simply too much fuel or too little air in the fuel/air mixture.  Achieving a rich mixture where none previously existed with only a change of muffler would mean the new muffler is more restrictive.  This is usually not the case but it answers the question you asked.  As for backfiring from a rich condition, yes it can happen - though it's really post-ignition.  Heated unburned fuel can be ignited when it mixes with air in the exhaust, either via a leak in the exhaust allowing air to be sucked into the stream or nearer the opening of the exhaust.  More common with the LS650 (and when going from a stock muffler to a freer flowing one) is backfiring from a lean condition if the bike was not rejetted/retuned when the muffler was changed.  Exhaust leaks are also pesky.