This works for 2008 S40 with 2300 miles at elev 1300 ft.
Rejetting and muffler modification was done to eliminate shotgun blast backfire and reduce the slight cold engine stumble for the first mile with a cold engine.  This was NOT done to increase horsepower.  With these changes, it will be a better ride.  Sparkplug was pulled after 800 miles and it is the perfect color.  Mileage is about 52 mpg riding at 50-60 mph.  Backfire is a slight whooosh at shutdown, not even noticable from 20 feet away.  Lots has been written about rejetting.  I know this works and works well.
Remove stock 145 main jet and replace with 150
Remove stock 52.5 pilot jet and replace with 55 - Do NOT use vented jet with holes in the sides.
Remove original white spacer thickness .112 inch and replace with 2 metal washers totaling .043 inch.
Install Harley Dyna muffler and drill one 3/16 hole in the center of the baffle.  This will vent gasses and reduce backfire.  It will not make the muffler noticable louder.  Do these changes and you will be happy.  It is not necessary to go overboard with rejetting.  A little tweek makes it run nice.  I know this has subject has been beat to death, but hopefully with these notes, you may only have to do the job ONCE.  It has a nice steady pull all the way through the power band. No stumbles or flat spots,  low speed or high speed.

Of course those changes make it illegal for street use...

Charon wrote on 02/22/10 at 18:43:13:

  If I drink 2 beers am illegal for street use

In the US, I don't know of any law that is broken by the OWNER adjusting or modifying a carb that is not subjected to smog emission testing for street legal.
Mfgs, dealers, repair shops etc are bound by law to meet emissions standards,......owners are not,.......to a point,....being we do not give reason for a state patrol to cite you for,...excessive smoking, excessive noise, etc etc.
There are laws that prohibit modifying an emmissions system, which as far as I know would not include carb or dist and cam tuning to improve performance.
Of course I could be wrong. But if so, lets see a list of emissions specs that us owners have to abide by.
One day when smog checks are required on motorcycles, then is when the standards will be set for us owners.

Thats one reason I didn't change the spacer was to get good mpg and keep the plug from fouling.Your on the low speed jet only at low speeds and high speed jet mostly if your at wide open throttle,the spacer is where you are driving most of the time.The high and low speed jets do help the mid range some.

So in US you they don't give you "a break" if your vehicle is NOT equipped with electronic injection, computerized ignition and O2 sensor?? They treat all vehicles under the same standards?
Strange if that is so. Here legal bracket for COx and NOx is higher for carburatted vehicles. Very strict for others.

If this is pertaining to my post above, I'm not understanding the point.
I can't find where I said emission specs were the same for all vehicles. I would sure think their not.
dasch wrote on 02/23/10 at 06:46:27:

thumperclone wrote on 02/23/10 at 06:18:39:


Pilot screw remains sealed.  I did not want to change too many things at once.  The jets, spacer and small hole in baffle were enough to get it running great. If I didn't get the results I wanted, the pilot screw was the next step.  Breaking the seal on the pilot screw is also an obvious clue that someone has been messing with the carb.  Also, I do not need any choke to start and the low speed cold stumble was more noticable when temps are 60 degrees or less.