3.0 Fuel Injection 121

Let's talk about adjusting the idle mixture. On conventional carburetor engines, we check the idle mixture by manually leaning it with the cockpit mixture control.To adjust, move the carburetor mixture control smoothly towards the idle-cutoff position. Simultaneously, watch the manifold pressure gauge for a decrease before an increase as the engine stops firing.We know the optimum mixture is reached when a manifold pressure decrease immediately precedes the manifold pressure increase as the engine ceases firing.The amount of decrease varies by engine. Generally, a manifold pressure drop of approximately 1/4 inch indicates proper adjustment.If your installation lacks a manifold-pressure gauge, use a tachometer. We aim for a 10-50 r.p.m. increase just before engine stoppage.For fuel-injection engines, the mixture change during manual leaning with the control is often too rapid to note any momentary RPM increase or manifold pressure decrease.Therefore, we set the idle mixture slightly leaner than best-power on these engines and check it by enriching the mixture with the primer.To check the idle mixture on a fuel-injection setup, first, we set the throttle to obtain the proper idle speed.Next, momentarily depress the primer switch while observing the tachometer and manifold pressure gauge. If the idle mixture is correct, we'll see a momentary increase in engine speed and a drop in manifold pressure.If the engine speed increase or manifold pressure decrease exceeds specified limits, the mixture is too lean. If RPM drops when enriching with the primer, the mixture is too rich.