look in the tech section, under white spacer mod. RPG i say take out the spacer, it's not gonna cost ya anything but yer time.

I struggled with the shutdown/decel  backfire (anywhere from a dog fart to howitzer) until I replaced the petcork vacuum hose and set the idle circuit.

I dont know how many turns, I just kept bumping it slightly richer at stoplights until I got the low pitched growling on decel.

I used fuel line for the vac hose, that took 10 minutes and cost $2. It took about a week or two to get idle circuit right. I keep a small screwdriver  in the pocket of my riding jacket just for tweaking the  idle. After a couple of days, I didn't even have to look for the screw, my hands just knew right where to go.


I'm stock jets, muffler and spacer. Since then, I never had a backfire, but I have developed the weird habit of reaching down to the carb at every stoplight

HTH

The petcork is a major source of problems, including backfiring, poor idle, low gas milage.  

There's a writeup here on how to convert it to Run-Res-Off just to eliminate these issues. It a few hours of work, but low cost.

The vac hose replacement is a quick fix that rules out a weak part.

The stock vac line to the petcork is basically a soda straw that wraps around the intake, runs along the head to the petcork. The throttle bumps in to it constantly, and over time, it develops cracks.

These cracks reduce the vac to the petcork, slowing the fuel flow and more air is allowed  thru these cracks in to the cyclinder making the engine run even leaner.

The backfiring increases in both frequency and volume to the point whenever you shut the thing down,  expect to see people duck for cover.

Yup, $2 of fuel line, some idle tweaking and no more backfiring.

If the vacuum line is causing the backfire would riding in the PRI position make the backfire less?  I'm thinking this since on PRI fuel flows by gravity.  Might be an easy way to test if the vacuum line is the issue.

I have mentioned before that I have a Jardine slash cut Muffler that I am trying to sell,it's just a tad to noisy for me,it is in perfect condition and comes with a jet kit and heat shield,and hardwear,it is a perfect fit
for our bike nothing else to buy,I will sell the whole kit and kaboodle for $100 ,you pay shipping,PM me if you are interested.