Checking the Basic Air/Fuel Mixture

Photo later

Setting the mixture - but first read below...


The basic mixture is checked under the following conditions:
  • Engine warm
  • All mechanical systems of the engine in spec: valves adjusted, ignition system in good order, timing properly set.
  • If setting the mixture on a Lambda system, first unplug the O2 sensor.
The preferred method of checking the mixture is to use a CO gauge. Mixtures are typically in the range of a few tenths of a percent up to 3%. The actual numbers are given in the Porsche spec book for the particular year model of the car. If the car has a catalytic converter, it is important to place the CO sensor before the Cat. Photo later (CO meter)
Photo later (air box) An approximation of the correct idle mixture can be made by the folllowing:
  • With the engine warm and running, slightly lift the air flow sensor plate.
  • If the RPM immediately falls off, the mixture is probably too rich.
  • If you pull down, and the idle falls off, you could either be too lean, or about right. Do the step immediately below to find out.
  • If the RPM slightly rises, then falls off as you continue to raise the sensor plate, it may be close to right.
  • You are trying for about 14.7:1.
  • The required movement of the sensor plate is slight, on the order of maybe a 1/16" or less.

The basic adjustment is made with a 3 mm allen wrench as shown in the top photo. Clockwise is rich, CCW is lean. In this photo (actually a cutaway for a 4-cylinder car) the adjustment for a 911 would be where there is a vertical brass fitting between the housing for the sensor plate on the left and the fuel distributor on the right. Below this vertical brass piece is a small brass screw on the arm holding the sensor plate. The 911 sensor housing has a hole in place of the vertical brass piece, and the allen wrench goes through this hole, into the adjustment screw.