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Date Posted: 12:29:47 03/12/02 Tue
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Detonation is one of those problems that can be aggravating to diagnose and correct. There are many possible causes and often more than one of them are at work.
In our experience, after literally thousands of diagnostic phone conversations with our customers, the most common reason for detonation in Mikuni-equipt Harley motors is excessive cranking pressure. High compression ratio pistons and mid-level cam designs can combine to elevate cranking pressures beyond the engine's tolerance.
The second most common cause, again in our experience with our customers, is too much timing too soon.
Harley's "Y" manifold causes mixture variations in the 2700 -- 2800 rpm range; the air/fuel mixture can vary enough to cause intermittent lean running in either cylinder and, sometimes, detonation.
Completely open exhaust systems, especially long straight pipes, tend to have short rpm spans in which they affect pressures in the intake system. These pressure variations, like those caused by the "Y" manifold, can bring about lean mixtures and detonation. This problem typically appears in the 3000 -- 3600 rpm range.
Pre-ignition, although uncommon among current Harley engines, can cause detonation. Pre-ignition describes the mixture being ignited before the time set by the ignition timing. In fact, the usual first sign of pre-ignition is detonation. Pre-ignition is cause by some hot object in the chamber. If something, like a small piece of carbon or an over-heated spark plug ground strap, is hot enough, the added temperature of compression can bring about detonation. Suspect pre-ignition after checking cranking pressure, timing and carburetor tuning. Ignition systems that complete the ignition advance too early can cause detonation. A highly-tuned Harley engine with cranking pressures near the maximum defined above, should not reach full advance before 3000 rpm, 3500 is usually better.
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