I am having trouble getting my carb tuned right. I have an '05 S40. I have already performed the White Spacer Mod and replaced the main jet with a 155. I installed both a Sportster Muffler and a K&N air filter. No matter how I adjust the mixture it seems to backfire whenever I shift or when coasting down to a lower speed. The poping gets worse when putting back pressure on the engine such as coasting down a steep hill. I started the mixture at 2 turns out and adjusted both ways using 1/4 turn intervals. Do I need to change the Pilot jet as well?

Hello - I have a 05 boulevard 650 and it has been in the shop 3 times so far for such backfiring that it shakes the bike.  I also backfires about 8 times per day.  The shop tells me that there is nothing else they can do about it due it being a single cylinder and all single cylinder bikes do this.  Does anyone else have this problem with there suzuki single cylinder??

A Savage backfiring?  Hmmmm, let me think if I've ever heard that mentioned before.....

I just joined this site and asked about my 05 backfiring.  I have got replies like they never heard this before and roflao and I don't get it. Is the backfiring common or what.  I am being serious that this is a big problem for me.  It has caused me to wreck once all ready. I was going around a bend on a gravel road and it backfired and shook the bike and down I went!!  I have a friend who has the same bike and hers never backfired once!

Better yet.  Do a search.  This site's search function works well.  Just remember to set it for more than the default of 7 days back.  I usually use 500 for mine but for a topic as well covered as backfiring, 60 should do it.  Paladin's post above should've explained a lot if you click on the link and check out the post it leads to.

I wouldn't go to a bigger jet.  I have the K&N direct swap air filter and a MAC muffler and the 152.5 is doing very well for me.  One of the prime causes I've had for backfiring has been exhaust leaks.  Make sure you don't have any between the cylinder and the header or between the header and the muffler.  There are gaskets for both connection points.  That's not to say you couldn't have a problem somewhere else but it's a fairly easy thing to fix and, at least for me, was the primary cause for problems with backfiring.

I verified that I have no exhaust leaks. What I find confusing is that when running the stock filter everything runs great!

Tony wrote on 12/31/69 at 16:00:10:


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Tony, you have provided the information needed for the fix in what you have already stated..."backfires with the K&N .....does not backfire much with the stock air filter".  
What this tells you is that you are running LEAN with the K&N, because the K&N flows much more air which leans out the fuel/air ratio.  When you put the stock filter back on, it does not flow as much air which makes for a richer fuel/air mixture.  
So, in order to use the K&N (which is preferrable) you need to compensate for the extra air flow, and the way to do that is to provide more fuel...in the pilot circuit...so you need to go up a size on the pilot jet.

This does assume of course that there are no air leaks (as stated before) and that you are correctly tuning the pilot jet adjustment screw.

Keep in mind that a main jet change affects 3/4 -full throttle.
A white spacer or clip position change on the needle affects 1/4-3/4 throttle.
A pilot jet change and screw adjustment affects idle-1/4 throttle...AND THIS IS WHERE THE BACKFIRING ON DECELERATION  ISSUE COMES FROM.

***Note that I said "throttle position"...not speed or rpm.  Throttle position is what changes where the slide is in the carb.  You will find that almost all backfiring on deceleration occurs when the throttle is at or near the idle position, and this is when the carb is operating in the pilot circuit...regardless of the mph or rpm.
The problem is caused by an incorrectly jetted/tuned pilot circuit...and the FIX must be within the same  pilot circuit.