Full of air

The air-flow meter has two important features, the diameter of the air passage and the tension of the spring on the air flap. You can't do anything about the diameter (And unless your modifications are REALLY extensive, you don't want to) but the spring tension is one of the main parameters in calibrating the ECU. The output-voltage/flow relationship is linear.

You can measure the actual opening of the air flap quite easily. Pry the cover on the air-flow sensor open very carefully (Held in place with silicone rubber, usually) and you'll see this:

The copper nose touches the rail that is given a pre-determined voltage by the ECU. (When the engine is running, naturally). If you measure the voltage with a voltage-meter you will get the opening. Hold it in max to see the max.voltage and in min. for minimum.

The spring can be adjusted by rolling the plastic gear with a screwdriver.

The effect on the injection period is explained as follows: If the flap opening at a given intake airflow (ie.power level) is smaller the injection period will be shorter, ie. leaner mixture. It also works the other way around, but if you loosen the spring too much, the flap will be at maximum prematurely. and the engine will run lean on any higher airflow-levels.

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