The smallest air leak anywhere in your exaust will cause backfires as well.  I doubt if raising the needle will make much difference in how the idle circuit performs.

Backing the fuel mixture screw out more than 3 turns does nothing.  Once you reach 3 turns the needle has backed out of the orifice and will no longer restrict flow.  You either have something plugging up your pilot jet - or you need a bigger pilot jet.  You may have had the idle speed set too high to properly set the mixture screw...it cannot be set if the idle is so high that the slide and jet needle have begun to open.

Clean the carb thoroughly and make sure everything is clean.  If the bike has the stock air filter box, I would start with a #50 pilot jet. 2 washers and a #150 main.

First warm the bike up thoroughly and set the idle mixture screw at 1.5 turns out.  The idle should be a bit lower than normal so you can accurately set the mixture.  Adjust the idle mixture screw as shown in the video below.  The ideal setting listed by Mikuni is 1.5 turns.  If you have to back the screw out more then 2 turns - you need a bigger pilot jet.  Not just for correct idle - but also for a proper mixture as the carb transitions to the needle jet as the fuel source.  If you can turn the mixture screw all the way in and the bike still runs...the jet is too big....more than 2.5 turns out to get optimum idle smoothness - the jet is too small.

https://www.youtube.com/watch?v=zm5mB3R8Ucw

Once you have the pilot jet set correctly, adjust the idle speed back up to 1,000 - 1,100 rpm.

Next the main jet is chosen by making full throttle acceleration runs.  I would start with a #150 and see how it runs at full throttle.  If it runs well then install the next larger one.  If it runs better great....if it burbles or runs slower....go back to the #150 or a #147.5 and see how it runs.

Once you have the main jet set the needle is the next place to go.  The bike should run smoothly when you just begin to roll on the throttle slowly and accelerate.  Ride like you have the Police watching you closely.....or have your wallet sitting on the seat behind you and you don't want to lose it.  As you slowly roll the throttle on the bike should accelerate very smoothly...without any hesitation or flat spots.  If the bike is lean in this transition the bike will "surge", as if it doesn't know how fast to accelerate.  Take washers out from under the needle to make this go away....start with 3 washers and if you feel the surge go to 2 washers.  If the bike burbles and sounds rich...add a washer.

Backfires - If you have done all the carb jetting properly....then the bike will run properly and any backfires that remain are either part of owning a big single....or you have an air leak in the exhaust system.  Backfires do occur if you let the throttle close completely when shifting gears or closing the throttle completely to use engine braking.  Loud or unbaffled mufflers make it worse.  The backfire comes from closing the throttle completely when riding - which shuts off the fuel flow through everything but the idle circuit....the engine is pulling a lot of air but cannot get enough fuel and the lean mixture cannot be ignited by the sparkplug - but is ignited in the hot exhaust system.

Trying to jet the carb to eliminate backfiring is not proper.  You jet the carb so the engine runs well.  The Carb does have a TEV valve that is supposed to add some additional fuel during the high vacuum situations and help reduce backfiring - I don't think it works well enough with the ethanol fuel that is used today.   When you clean the carb make sure the TEV is clean...it is the spring and diaphragm thingy with 3 screws on the right front top of the carb.

Thanks a bunch for the replies.  Follow up questions:

•  What's a good source for the different jets?  Online or auto parts store or Suzuki dealer?

•  The bike has rash on the exhaust pipe, so we're thinking of switching to a Harley pipe per the thread in the stickies.  I'm guessing it would be best to swap pipes first, and then go through the carb cleaning/jetting/tuning process?

•  This feels like a dumb question, but when you say "clean the carb thoroughly" are we talking about completely removing and disassembling the carb, or running some sort of cleaning product through it while still on the bike??

As you can tell we're not very experienced with motorcycle maintenance or engines in general, but we have tools and can follow directions and YouTube videos.

Thanks for the source for jets.

I just went out and checked for exhaust leaks.  Not knowing any other way to do it, I started the bike and held my hand around where the header pipe plugs into the cylinder, and where the muffler joins the header pipe.  I felt no leaks.

Is there a better way to check?

When we got the bike the one of the header pipe bolts to the cylinder head was pretty loose, and I re-torqued them to specs.  (The bike came with a service manual.)

So, I'm thinking my wife will keep riding the bike, which really runs okay except for the backfiring, until we can obtain jets and a Dyna muffler (or equivalent).  Then we'll dive into removing/disassembling/cleaning the carb, reinstalling, and following the jetting/carb tuning instructions.



I'll be rather amazed if we put it back together and it runs without a lot of mistakes and cussing.

Does the carb have to come off the bike to change out jets?  

(We live in Gig Harbor, WA)

Hey Alienbogey, I also live in Gig Harbor WA and own and ride a 2012 S40. My S40's workings are all stock, except the air-mix idle screw and petcock. So here's what I have discovered:
1. After-firing and crackling on decel are much worse in cool weather, and before it is thoroughly warmed up. 2 oz of STA-BIL to the tank will reduce this somewhat. If it's +/- 50*F, I just run half choke and that eliminates any issues.
2. The little gas station/store in Ollala, on the waterfront by the bridge, sells non-Ethanol unleaded exclusively. This gas makes my S40 run a whole lot better, and with choke off just "Puffs" on shut-off. Still crackles a bit on a quick roll-off, tho. Suggested mods may be needed to completely eliminate.

HappyDan....Alienbogey.

You both need to go into "My Center" by hitting the icon at the top of the page.  Update your information so that your location (City,State) shows up on the left side of the page.

You can connect with a whole lot of good folks that way..........

It'scalled JIS - Japase Industrial Standard. Motorcycle shops should carry them or check Amazon.