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To complicate matters, a lean fuel mix burns faster, a rich one slower.
Where did this idea come from ? This is incorrect, in fact the opposite is true. Lean mixtures burn slower. One demonstration of this is the lean intake backfire, which many CIS 911 mechanics are familiar with. The cold running mixture is too lean, for a variety of reasons, and the mixture is still burning when the intake valve opens and the fuel in the intake ignites. The fact that lean mixtures burn more slowly is the reason vacuum advance, not retard, is used. The less dense mixture at part throttle, low load cruise needs more time to burn completely and the engine can tolerate the extra advance because the cylinder pressure is low. This results in higher efficiency. Vacuum retard is strickly an idle H/C emission test device, it works mainly because of temperature, and has nothing to do with mixture formation. Ignition timing does not effect mixture preparation, it is adjusted in response to it. Conditions in the cylinder vary considerably under different loads and many absolute statement are false because of it. What happens at rest under idle conditions is largely irrevelent to what happens under load. Unfortunately, many misconception are created by playing with engines at idle.
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Paul
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