Motorcycle Repair: GL 1200 backfiring, lean mixture, fuel additives

Question
I have asked you about this before and you have sent me some info, but my computer crashed and I lost it all.

I purchase a '84 GL 1200 about a year ago.  It came with a backfiring problem.  100% of the time when engine braking it backfired (in about 2 to three seconds after starting the braking - if I engaged the clutch before it backfired during braking, it backfired) with a loud bang and at no other time i.e. acceleration.  Since I purchased it, I have replaced the spark plugs (the old ones looked fine, a tan color).  This had minimal effect on the backfire. I have replaced the exhaust manifolds.  The right side had a little leak. This had some significant effect on the problem.  Now it backfires intermittingly on engine braking and still backfires 100% of the time when engaging the clutch during engine braking.  The times it does not backfire during engine braking and the clutch is engaged below 2000 RPM, it will not backfire.  The dealer's service department suggested that I may have a lean mixture and check the pilot (mixing) screws to be at least 3 turns from light closed contact.  I did that with no effect.  The compression on 4 cylinders with the throttle wide open is 160 pounds.  I had someone follow me at night and did the engine-brake-engage-clutch thing. They said the fire was coming from both pipes. Any suggestions?  

Answer
Mark,

Send an email address and I'll re-send the files.

Which model GL is this?

What octane fuel are you using?

Is the OEM ignition installed?

When was the last valve clearance check and adjustment?

Carburetor synchronization checked?

Any fuel additives being used?

Reply and we'll go from there.

Respectfully,
Mark Shively