07S40rider wrote on 09/18/08 at 14:30:38:


No,  the mixture was set for pollution emissions off the factory floor.  Being lean to the point of becoming inefficient.    Inefficiency means that you could get a tiny bit more mpg by adjusting it a tiny bit  ....... key word.... tiny bit...

For most,  the "fun" factor takes hold and most people tweak it to most instant power (means centered or slightly rich)........  AND then "fun" factor causes more WOT and that lowers MPG....

Note: the super lean tune will, over time, cause the engine to start polluting more as the parts get damaged from the extra heat produced..... but it passed the EPA std's when made...     Now, I will say that it is a very fine line to get the perfect tune, and most people don't have it perfect.  I know mine is running slight rich......  avg city riding 47-49 mpg.

only to say, as you probably already have read elsewhere, that you don't want to be more than 3 full rotations out.  Any more than that and you risk the screw coming loose.  To reduce the "backfire", which is really an afterfire, you might also try increasing the idle speed a little.  See if that helps....

And,..an afterfire is a "ka-bang!" kind of gunshot sound.
Little "whump,..pop..blat...wooble, wooble" sounds on deceleration are normal for a big thumper, and are harmless, and kinda' cool.

I got mine screwed out one and a half turns,it still backfires,but runs great

Clarify the start solenoid adjustment conditions, please.    Too tight is ????   Too loose is ????  Solenoid adjustment doesn't play, mis-assembly was the culprit.  Reason why this is so ????

Your post is interesting, but I cannot connect it to the "normal" solenoid adjustment parameters which may affect many bikes.

I bought a 2005 S40 on September 12. Very nice motorcycle with low mileage and no corrosion. Made purchase with no test ride (cautious seller). Have put on 150 miles since purchase and very happy. But noticed the backfiring. Started research, including this site. Today, I removed the brass plug and adjusted low end mixture. Also, removed the spacer on the slide needle. All went well.

I removed the brass plug by drilling it out with a drill bit slightly smaller than the casing around the plug. I was careful not to let the drill bit touch the head of the adjustment screw. It really was a 'piece of cake." By the way, there was a small hole in the middle of the brass plug. I stuck a needle through the hole and determined that there was about a 1/4 inch space between the outer face of the plug and the face of the adjustment screw.

As suggested, I turned out the low end mixture screw until idle speed peaked. A half turn counterclockwise. During test drive, noticed an increase in power and smoothness in acceleration. Backfire is less but still offers a minor "pop" now and then just to let me know it's still happy.

Other than having to remove the seat and the tank, this was an easy fix. No need to remove the carb. Just be careful working in and around the diaphragm, especially during replacement of the spring and cover.

Two day later - In my previous post, I recounted my "success" with removing the spacer on the jet needle and adjustment of the idle mixture screw. In the past two days, my "success" has proven less so. My engine started running lean, not rich! Yes! That's correct! Leaner! More pronounced backfires and weak acceleration, especially from idle. I progressively richened the idle mixture with no improvement.   So, today, I replaced the spacer (Smart not to have thrown it away!) and now it runs just fine. And the idle mixture adjustment seems to make a substantial difference. Consequently, I don't recommend removing the spacer. Leave it alone. BTW, the brass plug in my carb was 9/64 thick.