A stock S40 Boulevard is tuned very lean to meet EPA emissions standards. It also pretty much guarantees a significant amount of afterfiring (or backfiring to most people). To correct the afterfire problem, you'll have to remove the brass plug that Suzuki installs to cover the idle mixture screw.

The process is very simple and the results are remarkable. In the picture above at about the 2 o'clock position on the right side of the carb you'll see a small brass plug. What you want to do is remove this plug to expose the idle mixture screw beneath it. Do not confuse this with the idle "speed" screw, which sets the idle rpm's higher. When you adjust the idle speed screw you're making a lean fuel mix idle faster.  When you adjust the idle "mixture", you're going to add more fuel to the mix, thereby running more "rich".

The way you'll remove the plug is to drill a shallow pilot hole, then drive in a screw for a few turns until it wedges in firmly, and then you'll pull out the screw and the plug together.

Step 1: Drill the pilot hole with a 3/32" bit. Be carefull to limit the depth to about 1/8"-3/16" deep. You can wrap tape around your bit at the desired depth so you don't end up drilling thru the idle mix screw.


Step 2: Screw a fine drywall screw into the pilot hole until it is firmly gripping the brass. You do not usually need more than a full turn or two.



Step 3: Grab the screw with a pair of vice grips and pull it out. Takes a little effort.



Step 4: When the plug is removed, clean out any brass shavings that may remain inside, and then give the exposed idle mixture screw a half-turn counter-clockwise .  A half turn works well for most people as a starting point. You may need more if you are at a different altitude, temp or humidity.

If you have a tachometer, you can fine tune it further by adjusting the screw until you reach max rpms. Turn it 1/4 turn at a time, wait 20 seconds and see if the revs go up.  If they do, give it another 1/4 turn, wait 20 secs and repeat until the revs drop. Turn it back as needed to hit the peak.



Also, after-firing can also be aggravated by air inleakage at the exhaust header, Be sure to check that your header nuts are tight before you test out the new setting.

You're set. You should see a big reduction in afterfiring and for normal driving you should not expect a penalty in mpg. If you're idling in stop and go traffic a lot, it may reduce your fuel efficiency.


One last thought... it is possible that you may have to adjust your idle SPEED screw, on the other side of your carb,  after this mod.  I had mine tuned to idle at 1000 rpms with the leaner fuel mix. After I richened the mix the bike cut off at idle when I went for a test ride.  So if this happens to you, don't go in there and adjust your fuel mixture first. Just crank up the idle speed a bit and you should be fine.  

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