I'm a brand new rider. Rode my first motorcycle at the MSF Beginner Course in July and now I'm the proud owner of a used 2007 S40.

The previous owner put different jets in (0.055 pilot and 0.155 main), a HD Dyna exhaust on, and turned the idle screw way down.

The reason I'm here is the infamous backfire (I've seen it called afterfire on some websites).

Backfires happen. Quite often. Most of the time it's a solid pop, with the occasional vocal bang. The previous owner said it backfired LESS after he made the changes. WOW. I can't imagine what it sounded like previously.

Back on topic, am I correct to assume that reducing the "white spacer" is probably a good way to reduce the backfiring? The previous owner said he didn't do that adjustment because he had read about some bad results.

At what point should I consider the idle mixture screw a required adjustment?

Thanks for your help.

-Drunken Dwarf

You can get "backfires", afterfires from being too rich also.  And your jetting is on the rich end.  Seviersavage is referring to the idle mix srew on the right side of the carb not the idle speed screw on the left, by the choke.  Your idle mix screw may be blocked off by a brass plug, if so it must be drilled out to make adjustments.
"Pop, blat, wooble-wooble, blat" type sounds are quite normal for a big thumper engine on deceleration.  "Kaa-BANG!", gunshot type sounds are not good and you should try to reduce those.  

My jetting is 150#/50# and I have similar mods to you.  I am at 2500ft elevation in a dry climate, and that makes a huge difference in jetting.  What is your elevation and climate?  Heat, humidity, and elevation will make you run rich.  Dry, cool, sea level will make you lean.

Feel free to ask for help, search past posts, and welcome ,
Serowbot

verslagen1 wrote on 09/17/08 at 19:13:55:

I guess I need to buy a torque wrench.  

-Dwarf

seviersavage wrote on 09/17/08 at 18:08:30:

It's a stock air filter, but the cover has been removed. Not the 'outside' cover, but the 'inside' one. I actually haven't taken it apart to look at it. I just know I've got the parts sitting on my desk. (I think it's proper name would be "Air Filter Access Cover" or "Air Filter Compartment Cover" or "Air Cleaner Case Cover"...it's the plastic one.)

-Dwarf

verslagen1 wrote on 09/17/08 at 19:13:55:

Header Bolts = where the exhaust connects to the cylinder?

I hope so, 'cause I just tightened them to 15ftlbs (+/- 4%).

I got the brass plug out, managed to ding the lower lip of the fuel tank in the process. Where can I get paint?

I haven't rode enough to get a gas mileage measurement. The previous owner said it got over 50mpg.

I think I'm starting to understand this carburetor stuff.

With a big main jet, I'm probably running a bit rich when I'm 1/2 to full throttle.
With a big pilot jet, I'm probably running a bit rich when I'm idle to 1/4 throttle.

The idle mixture screw could help me adjust the pilot system.

Changing the spacer would make the it run richer 1/4 to 3/4 throttle. Which I may or may not want to do.

How would I go about determining the correct course of action? Is it just a matter of preference if I don't have access to a dynamometer?

-Drunken Dwarf

verslagen1 wrote on 09/21/08 at 08:38:24:

OK.
What if my main is already too big?
What about 1/4 to 3/4 throttle?

-Drunken Dwarf

[quote ]

I don't know of a test for too big, only too small.
Drop it down a size and test it.  My guess is a 150 will do you fine.
I don't know a fool proof test for the mid range. . [/quote]

In general, the following apply

TOO RICH

-engine noise dull/intermittent
-condition grows worse when choke opeded
-condition grows worse when engine gets hot
-removing air cleaner will improve condition somewhat
-exhaust gas heavy
-fouled plug


TOO LEAN

-engine overheats
-condition improves with choke opened
-acceleration is poor
-spark plug burned
-engine surges, lacks power

Excessively lean carburetor settings can contribute to backfiring. If the mixture is too lean, it may burn very slowly and unevenly. This condition, in turn, may result in burning mixture remaining in the cylinder until the beginning of the next intake stroke when it can ignite the incomming air/fuel mixture.

Getting back to the original question. In many cases, the idle mix screw is all that needs adjusted to eliminate backfire so if that is the desired result it should be attempted before changing jets after adjusting the idle speed. (There's a reason it's a screw adjustment even if it's covered by a plug to make the environmental people happy.) There's also a reason the idle speed is on a thumbscrew. It's nice to have a tach to adjust by, but if the engine is running real slow and rough and even "loping" it probably needs adjusted.

As far as rich or lean running conditions, Lancer has a nice summary there. Others have also put in valid points. In your case, you may want (different than need) larger jets because of the change in exhaust however you may not have the right sizes.