well, what I thought was good on the Idle Air/Fuel mix plug MOD didn't work out.

Still a backfire on DECEL and during shifting. I can't seem to find the sweet spot on this adjustment. There's a noticeable difference with adjustments, some worse, some better, but none that resolves the backfire completely.

I'm not one to go in and change a bunch of chit and then piss and moan about it not being fixed. So I like to take things one at a time. These mikuni carbs aren't like the old school honda carbs I'm used to messin' with, these only have one adjustment, honda has everything on the outside.

MOD's thus far:
1. DYNA muffler swap.
2. Idle Air/Fuel mix plug MOD. (3/4 turn CCW from stock position when plug was removed)
3. ????

what's next?

I posted this in another thread....but I will repeat it here so I don't have to type it all again.  It was a response to a fellow who said his bike backfires when the "snaps the throttle closed"....and it applies to anyone who allows the throttle to close completely while riding.

"This is not the motorcycle that you should be "snapping the gas off", as it is certain to make noise when you do that.  Carb jetting might make it a bit better - but it will not stop it from happening.  You need to adjust your riding style to suit the big, primitive single with a carb.....this is not a modern, fuel injected motorcycle - it is a primitive beast.

When you let the throttle snap closed, you have created a high engine vacuum that pulls a lot of air past the butterfly and slide - but the main and needle jets are closed off....only the idle circuit is providing fuel.  The fuel mixture goes very lean as a result, and the spark plug can not ignite the mixture in the cylinder - but when the lean mixture builds up in the exhaust it can ignite and make the popping and banging sounds.  If you jet to eliminate the noise when coasting - the engine will be running too rich when at a normal idle.

When you shift gears you should gently roll the throttle off - but stop just a bit short of being all the way closed.  Same thing when you are coasting down a hill or slowing down in traffic - roll the throttle off just far enough to slow down - but not all the way closed or the noise will begin."

It is all part of being a "good" rider - you need to adapt your style to suit the motorcycle and it's quirks.

Rat,

Mine backfired when I shut the engine down after driving the bike off of the delivery truck the day I received my S40.

Kenny G

Rat,

You will be able to improve the after firing with tuning and some gentle use of the throttle, but that is about as far as you can go.

I spent a lot of time experimenting with various springs in the Transient Enrichment Valve. And when ever I would completely eliminate the after firing I had trouble with the RPMs dipping too low during deceleration. Once the engine stalled when I was downshifting in a really tight corner and that can be dangerous.

One thing I have concluded is to make sure your idle speed is fast enough. When the idle speed is on the slow side you will increase the after firing.
Kenny G

Decel should rumble growl and occasionally pop some.
If it's backfiring during shifting, either you've got a leak, throttle control needs considered or maybe you should get the thing shifted and back on the gas. I'm not there to see.
From second on up it'll take gear shifts without touching the clutch, just open palm, slap the gas off with a quick lift on the toe and roll on the gas. It'll go right back down, too. Just don't do second to first, the RPM,jump is just more than I could ever master. Tried about six times, didn't jerk the crap out of it twice,, gave up on that before I hurt something.
Describe backfire.

Kenny G wrote on 10/11/15 at 20:09:56:

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Hey Kenny,

Thanks for the smart phone idea.
Here's a link to the iPhone App that works off audio from engines:

http://www.tunelab-world.com/tach

There's also some videos on YouTube that show the proper idle speed. You can tune your idle by ear with those.

Bubba wrote on 10/28/15 at 15:08:50:



Yes.......when you increase the idle speed you are opening up the throttle and allowing more fuel/air into the engine - so when you close the throttle between shifts there is more fuel available to correct for the lean condition that exists when the slide drops and closes off the jet needle and main jet.  You can accomplish the same thing by holding the throttle just slightly open when you shift or slow down - a more thorough explanation follows:

The fuel mixture goes really lean when the throttle is closed and the engine is either slowing down between shifts - or you are slowing down and have the throttle fully closed.  There is a high engine vacuum - but with the throttle closed and the slide down...the only fuel that is getting to the engine is coming from the idle circuit....and that is not enough fuel to allow the spark plug to ignite the mixture.  So the fuel/air in the cylinder is not ignited and it just builds up in the exhaust....and eventually the mixture reaches a high enough temperature in the hot exhaust pipe that it spontaneously ignites and makes a bang.

Suzuki tried to cure the problem with the TEV (Throttle Enrichment Valve) which is supposed to allow more fuel to flow during those high vacuum events - but it doesn't seem to work well with the E10 fuel.  Maybe it works OK for folks who can still find pure gasoline.  Some folks have fiddled with the spring in the TEV and claim to have made things better - but I don't feel it is a issue with the TEV not operating soon enough - I believe it just doesn't add enough E10 fuel.

There are folks who put in larger pilot jets and open the idle mixture screw, and this helps compensate for the lean mixture when decelerating.....at the expense of an overly rich mixture when the bike is idling or at low throttle settings.  Fuel mileage can suffer considerably, and the engine can belch black smoke when idling, and the engine can get loaded up with carbon.

You really can make a big difference in the amount of backfire by how you use your right hand.  When you shift gears just roll the throttle slightly back and don't let it close completely between shifts.  When coasting just open the throttle a tiny bit until the backfire goes away - but not so much that the bike is accelerating....just enough that the slide rises and allows more fuel into the engine.

For the "bang" after you turn the ignition off....the engine is still pulling air through the carb when it is spinning to a stop - and it is still picking up fuel as it passes through the carb.  Since the spark plug doesn't ignite the fuel mixture when the ignition is turned off - the unburned fuel mixture is pumped into the exhaust.  Once the fuel/air hits the hot exhaust system the temperature can reach the "flash point", and the fuel mixture can be ignited.  When you have a high idle speed you pump more fuel/air into the exhaust system prior to the engine coming to rest.....with a slower speed there is less fuel/air pumped into the exhaust.