I bought my wife an 09 S40 thinking it was a good deal. It's been ok minus the typical backfiring. I much larger issue has risen lately. I rode it the other day maybe 20-25 miles. I rode kind hard and a friend following noticed the rear tire was covered in oil. Pulled into a gas station and noticed oil liberally coming from the overflow line. It did this to friend who rode it with me 50-75 miles once and actually shut off. It didn't start it til it cooled down alittle. I don't know the first thing to check for this issue but, don't trust it for my ol lady. Any suggestions?

 

if it is engine oil. I would change the oil and filter for starters.

I have given up on trying to tune away the backfiring. If I set the idle mixture rich enough to reduce backfiring, it does not want to idle anymore. I just try to close the throttle more smoothly and hope for the best.