380sl Fuel Regulator?
 

While I was finishing a tuneup on my '83 380SL, a neighbor who is a mechanic took it upon himself to try adjusting the allen nut adjustment which is accessible through the air filter cover...my guess is that its a fuel regulator of some sort??? In any event, the car runs great but the mileage dropped to only 9-10 mpg...UGH!!!
My question is this: is there a way I can adjust this back to optimize the mileage again? (if I try to adjust it while the engine is running it dies out right away)
Many thanks,Jeff