My first post!

I purchased a 2007 S40 earlier this year with about 350 miles on it.  After replacing the bad gas and cleaning the carb, the bike ran very well for the next 100 miles.  I got some backfire, but not much.  However, I decided to remove the plug to check the burn.  And, the plug was bright white - lean.

So, I adjusted the idle mixture out a turn, and replaced the white washer in the carb with one about 2/3 the thickness of the original washer.  

The result:  the bike has a bit more power at low speeds, but now the backfire is deeper in tone and there is an engine knock or ping that didn't exist before when under a load.

Any thoughts on these new conditions that occurred after I did the washer mod?  I have tinkered with the idle mixture taking it out as many as 3 1/2 turns with the same results.  Considering the bright white plug, I was also planning to change the main jets.      

Thanks -- great site, lots of information!!

thanks, the real puzzler for me is that i would expect that making the mixture richer, the likelihood of knock/ping would be less.