Here's an interesting read on decel popping.
Re: Exhaust pop on decel. Rich or lean?
«
Reply #9 on: Jun 27, 2010, 06:49:42 pm »
I'll reprise my explanation from an earlier thread:
Expect some popping. Even a perfectly tuned engine will pop on decell if it has free-flowing exhausts. Some say it is from a lean engine, some say rich. I think it is a combination of tuning, valve timing and scavenging that results in combustion occurring in the exhaust pipe and/or being carried over to the exhaust when the exhaust valve opens.
If you think about the mechanics of what is going on upon decel, it makes perfect sense:
1. You start out with an engine that is turning high RPM with throttle that is open, flowing freely.
2. Next, you close that throttle. This has the effect of creating a vacuum in the intake manifold (CHOKE!).
3 The vacuum pulls air and fuel from wherever it can--including the ISC (or pilot jets on a carb)
4. Air will also take the path of least resistance--including sucking exhaust gases back into the combustion chamber during valve overlap (the brief period of time that the exhaust and intake valves are open at the same time. This is sometimes called reversion and can mess up what your autotune tells you. If you have free=-lowing exhausts, this is more probable than with fully baffled exhaust.
5. Now you have a rich mixture heading for the cylinder AND what little oxygen is drawn in is further diluted by gases that are coming back into the chamber from the exhaust valve. The result is poor, incomplete combustion! that lasts beyond the point where the exhaust valve opens. Depending on the temperature of the exhaust pipe and gases, reignition is possible once this mixture is introduced into a more oxygen rich environment. This produces the "pop."
Why does a lean running engine have a tendency to excessively pop? The exhaust gas temperature of a lean engine is substantially higher than one that is running on the rich side ( an EGT probe will show this). Therefore, the high EGT serves to heat the pipe more and the gases themselves become a much better source of re-ignition. By enriching, you are cooling down the exhaust temp and therefore reducing potential re-ignition sources. But at the same time, you are presenting a rich mixture to get the whole thing started.
Any 4 stroke engine--even those that are flawlessly tuned will pop on decel. It is harmless, but can be annoying.. However, I would never alter the tuning of an otherwise performing motorcycle to get rid of it. Go by the the color of your spark plug rather than the popping as an indication of the tuning of your scoot! It is normal. Annoying, but normal. Can be reduced, by trial and error, but normal.
