LANCER wrote on 12/31/69 at 16:00:06:


Got a poor welding job done today. Had to go back to have it redone and even then it's not 100% sealed but it is much better than it was. When doing the soap & potato test there are at least two pin hole size leaks that I can see where the weld does not seal completely. The backfiring is reduced but not eliminated. It seems there is less poping during upshifting and just as much if not more during deceleration and engine braking.

I'm getting a #55 primary jet any day now. Hope that helps some, we'll see.

There's so much been posted about the backfire problem others may have already noted the following.  Backfiring is inherent to single cylinders exhausting through one pipe.   Especially when that pipe has a restrictive muffler to accumulate unburned mixture.  Fiddling with the mixture and exhaust too much overcompensates in other areas of tune even if it eliminates the "backfire prone" zones.   Multiple-cylinder engines don't suffer as much when they have a header that uses the exhaust from one cylinder to help extract the flow from the others.  This "tuned extraction" effect makes gas flow more efficient for better running.  All my old 4-stroke singles (Matchless, Velocette, Yamaha, Ducati) backfired to some extent under certain conditions of throttle / load.  Certainly the SooZook 650 is the worst thanks to EPA jetting and restrictive muffler, but don't overcompensate by going too rich on one jet.

Believe it or not, I have finally reached Nirvana (I hope)!!!  Just as you, Rob, I only have pops on engine braking/gearing down now.  I can snap the throttle closed with no backfiring at all.

I found my situation similar to Brewbrother, where my problem seemed to be in the rubber boots at the carburetor.  Those things don't have to be torqued down to get a good seal, and I believe I was overtightening them and getting a slight air leak there.

I'll say this.  It is darn near an art form to get these thumpers dialed in correctly.  At least it is for me!    But, it is nice to finally get there.

I guess I'm one of the lucky ones. I have no idea what kind of  carb mods might have been made before I got my '86 this year, but I just get the popping on gearing down and the backfire on shutdown - which has been virtually eliminated by allowing some transmission tension during shutdown.

Congrats to all that remove the backfire. Keep pluggin' away and it will end all together.

  I love carb tuning!

Damage from backfiring is one of those "over time " things, not an instantaneous event.  The Suzuki warranty is 12/12000, and they are probably figuring on the fact that if there is damage from this,  it will be out of warranty by then.  Besides, if the backfiring gets really bad, the customer will bring the bike to the dealership and want it fixed ($$), so they will do something to reduce the backfiring.  My dealership pulled the plug off the pilot adj screw and turned it out a bit at the 600 mile checkup ... because I had been complaining about it everytime I went in prior to that, and at the 600 checkup I demanded something be done.   I do have a better than average dealership, they actually listen (the shop supvr is a really good guy) and the mechanic knows what he is doing (he is about 40 and has been around the block a few times).
Anyway, it is a cost/return thing.  Suzuki will make a lot more money on the bikes than they will have to spend on the remote possibility that there is engine damage during the warranty period.
I don't know of anyone who has left the bike bone stock with the backfiring and not done something about it.  There are a few stock bikes that do not seem to backfire much... OF COURSE THAT MAY BE BECAUSE THE PEOPLE ON THE ASSEMBLY LINE MAY HAVE ACTUALLY ADJUSTED THE PILOT SCREW CORRECTLY IN THE FIRST PLACE.    I know, but it COULD happen.