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Lean mixture

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Many people are under the (apparent) misconception that popping on deceleration is due to the mixture being too rich. I assume that the rationale behind this is that the popping is the combustion of unburned fuel as it comes into contact with the hot exhaust headers. This would tend to make sence since it is more likely for there to be unburned fuel in a rich mixture than with a lean mixture.

From my own experience, my XR4 was running over-lean with undersized (and clogged) jets, and popping was prevalent. A re-jet and carb clean and I'm now considering entering my bike into the 2007 MotoGP series and giving that Valentino Rossi a run for his money (that'll teach him to leave Honda!). But I digress.......

We all understand however, that popping is a symptom of leanness. Can anyone explain the mechanics behind popping (how and why it occurs)?

Meatbomb.........

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Too lean OR too rich can cause popping on decel. The "note" or "character" of the pop is different though. Lean pops (the more common of the two, in my experience) is a much "shorter", sharper sound--often several very close together. Rich pops sound quite a bit different. It's hard to describe, but it's more of a "wet" "blap" type sound. Rich backfiring on decel is less of a POP and more of a BLAP.

What's happening is that when a mixture is too rich or too lean you're getting incomplete combustion in the combustion chamber. Spot-on jetting will consume nearly all the fuel in the mixture. Too rich OR too lean and the mixture does not burn reliably. This causes unburned fuel to pass out of the combustion chamber (yes, even when lean) and collect in the header. This unburned fuel will occasionally ignite causing the backfire...

A properly jetted pilot circuit will normally produce a mild pop on decel. This is because the mixture being fed into the combustion chamber under higher RPM decel conditions IS a little lean. That does not mean that under small throttle opening acceleration conditons (when the pilot circuit is operating normally) that you are too lean. You see, under decel conditions the vacuum signal in the carb is very high and pulls a reletively large amount of air through the small opening in the carb. This causes a little bit of leaness--hence the pop. But that leaness goes away when the carb is subjected to the "normal" vacuum signals during acceleration. That's how, techincally speaking, you're too lean under decel but spot-on during acceration.

This business about vacuum signals is also responsible for a bunch of other carburetor "anomalys"--it's a fundamental deficency about the way carbs work.

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Air leaks in the exhuast system can also create conditons that cause backfireing. But don't let anyone tell you that is the ONLY thing that can cause backfiring (as some tried to tell me back when I was researching this very topic).

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