I'm having a little trouble setting up my carb. The bike is a 2017 with 3600km I'm at 3500 ft altitude. As all S40 are real lean from the the factory so was mine, it was doing a lot of popping and such. I drilled out the plug and set the idle which helped a lot, but in 2nd and 3rd at say 3000rpm the bike would surge like it was running out of fuel. So I did the white spacer mod I took the spacer out and put in two washers now the bike runs like a top, no popping smooth as all get out only one problem it wants to load up at idle to the point it will stall at a stop light. I tried setting the low speed idle again but now I can turn the screw all the way in with little affect. One other thing I was getting 67mpg and now I'm getting less than 50. What should I do now?

Well I can turn the idle screw all the in and it still runs, the jets are stock but I can't tell what they are as I never looked, I turned the idle down to 800 rpm to set the idle screw but then I set it at 1300 I have a tach installed so it's easy to see the RPM.

That's kinda what I was thinking My gas mileage might also go back up a little. But it sure runs nice off idle smooth as glass and no popping or afterfire. also when I let it idle for say a minute and hit the gas it bleches black smoke.

I had a similar issue that ended up being a result of raising the needle too much. I was consistently having the bike stall at idle. The longer it idled the lower the RPM got until it choked, and on restart I had the same poof of smoke. Aside from this, my bike ran perfect as well.

I'd agree, start with adding a washer to push the needle back down and see if that makes it better.

I wasn't to worried about the backfires it was the surging in the 3000 rpm range that was most worrisome. Making it richer solved that problem, I'm going to add one more washer and see if that clears up my idle problem. I will report back when it gets done.

The bike is new and the slide is smooth,it just surged a lot in the mid range. Even running as rich as it is I still have lots of power but the gas mileage is way off. I will add one more washer and see how that goes, if it still loads up I guess I will put the white spacer back in and live with it. I have a five year warranty, but when I talk to the dealer he says that's just the way they are, so much for warranty doing anything.

cygnet wrote on 09/18/17 at 21:10:13:


If your idle speed is high when you attempt to adjust the idle fuel mixture, you won't get a response as the needle will be supplying fuel.  You need to drop the idle speed down to 800 - 1,000 rpm to get in the range....don't leave it in this range after the adjustment is made, and don't spend a lot of time with the bike leaned over on the side stand while making the adjustment (but a board under the kickstand to level the bike up a bit).

The ideal place for the idle screw to be is between 1.5 - 2 turns out.  A pilot jet that allows the screw to be in that position is the correct size.  If you have to go out more than 2.5 turns to get the smoothest idle - the pilot jet is too small.  If you turn the screw out less than 1.5 turns to get the best idle.....or you can turn it all the way in...then the pilot jet is too big.  (There is a bypass hole that flows fuel around the fuel screw, and even if the adjustment screw is turned all the way in, fuel is still flowing...too big of a pilot jet and too much fuel flows).

Don't forget to turn your idle speed back up once you have completed the adjustment of the idle fuel screw (pilot screw).

At your elevation - the #47.5 pilot jet is probably going to be a good place to start.

In the diagram below, at #2, you can see the bypass holes prior to the pilot screw.  This is where the fuel is flowing when you have the adjustment screw turned all the way in.



The surging you were having at light throttle settings is related to the needle being too low - so you need to raise the needle to correct that.  The 2 or 3 washers that works for us at lower elevations may need to be 4 screws at your elevation - to correct the surge you need to raise the needle a bit to provide a slightly richer mixture.