You can really tell a lean condition if the engine likes to surge at a constant throttle position. It's almost as if the clutch is slipping. I upped the jet sizes in my carb just slightly and it runs sooo much better. A little goes a long way! She ran great before but runs excellent now!

In the 7 days I've had my S40, I've ridden almost 300 miles and noticed almost every single time I slow down, there's a backfire. Loud, subtle or somewhere in between, but pretty much every single time I come to a stop. I just assumed it was normal for the Thumper to sputter a little bit.

Hello from NC.  Just jumping in here as Im new to both this forum and my 2007 Savage. My bike backfires on deceleration(and one little "woof" out the tailpipe when I shut it off!)  Also a blue pipe.  Would like to correct (backfiring)this to some degree as it drives my wife bonkers.  The bike seems to run ok, although it seems to work very hard accelerating, as if the thing is trying to make power, but can't.  Seems as if it could run with a bit more spirit like some of my past bikes.  I read the discussions about shim kits, slide needles, blanking plugs, etc, but I really don't know what all that means.  My real need is to find a great source to teach me some of the basics so I can take advantage of the knowledge this forum seems to have.

Squack! Yes both of my Savage's had the popping issue and both required rejetting the carb. I used a 52.5 pilot , 150 main and 2 washers on the needle for both bikes. I bought 2 kits form Lancer. Make sure the exhaust is completely sealed up and it removed 95% of the popping. I'm using a Harley Dyna muffler also. Sounds good and accelerates smoothly.

I'm at 200ft elevation here and not humid with fairly hot temp 90s.  

Here's some good info on backfiring, scroll down to the 'Burn Baby Burn' sub heading. I've seen bits of this post all over the internets but I tracked down the original post.

http://tinyurl.com/hgsasoo

Look in the Technical part of the forum.
Serowbot, I think, posted a good, Readers Digest level
How to understand what the bike is telling me about the carburetor
explanation.
It helped me.. can't remember it, but it made sense.. And I am not a carb guy..

One other method not yet mentioned: modifying the Transient Enrichment Valve (TEV), a small vacuum-operated valve on the right side of the carb.

I could not get the backfires on my 2007 S40 to stop by any of the standard methods: eliminating exhaust leaks (there weren't any), adjusting the idle mix screw, changing jets. What finally worked for me was taking a small amount off the spring that is part of the TEV. If the standard methods don't work for you, I suggest taking a very small amount off the spring (such as 1/4 turn), then test for backfires for a couple of days. If you still get backfiring, take a little more off and test again.

If you weaken the spring too much, the valve will open too easily and you'll get a surge at idle, but if you weaken it just a little, the time it stays open on decel will increase but idle will be unaffected. In my case, removing 1/2 round of the spring weakened the spring just enough to almost completely eliminate backfires. Just driving around town, it never backfires. On a recent 300+ mile trip, on a variety of roads and speeds, I had a total of 3 backfires. Before this mod, I'd have more backfires than that on my 6-mile trip to work.

My two cents.

David