gas and air ratio?
 

Last night after filling up with gas my 99 boxster started backfiring and if i try to accelarate it back fires and tries to stall out. This situation is very bad when car has been sitting. When starting, massive amounts of smoke comes from the exhaust until i can get the car moving(always smokes when a/c on). When i get going it hesatates very badly,backfires(espeacially when changing gears) It runs smoothly when there is no acceleration but knocks and pings around 2400-2600rpms. Please help!! One more thing. I noticed weeks back that the car was a little rough getting going when cold.

Is the Check Engine light on? (it should be on given those poor running conditions. Maybe its burned out. Anyway, untill you download the error codes to post here or get it to a qualified shop (dealer, or independant like IMA in Fairfax or RPM in newington or Intersport in McLean, for example) you really won't know the true problem imho.

ratio
 

thanks for the reply. Went by my mechanics place today and is closed until 8/14-vacation