Join Date: May 2010
Location: Calgary, Alberta, Canada
Actually combustion is almost always incomplete, strictly speaking. What happens, specifically when decelerating, is that you transition from a high volume to a low volume charge. In that transition you always end up with a fuel rich charge and that gets discharged into the very hot exhaust. If there is enough O2 in the pipe, it will ignite the fuel, hence a pop. This is far more pronounced and far more likely with the pair valve operational, or with an exhaust leak because some fresh air gets introduced.
There is only one way to stop this, but the methods vary, and that is to create a condition in the exhaust pipe that is unfavorable to ignition.
1) Create a situation where there is not enough O2 present to allow combustion of unburned fuel. That is done by rendering the pair system nonoperational and ensuring you have no exhaust leaks that may allow fresh air into the system to allow combustion
2) Create a situation where there is not enough fuel available to allow for combustion. FI bikes may try a strategy of completely cutting off fuel at anything much above idle when the throttle is closed. Can't really do this with a carbed bike.
3) Create an overly rich condition where there is not enough O2 present to allow combustion. Again, with FI bikes there is a strategy to actually increase fuel at anything much above idle when the throttle is closed. Ever wonder why you can have a fuel pump and wires inside of a fuel tank? The conditions inside of a fuel tank are so rich that even a spark cannot ignite the fuel, There simply is not enough O2 present. Same thing in your exhaust pipe. Raising your AF ratio in your idle circuit can push your high RPM / closed throttle exhaust fuel ratio past what will cause a pop.
I'm not sure if you know any of this, so if you do, you can just skip this. The perfect AF ratio is 14.7:1 (by mass). That is 14.7 parts air, 1 part fuel. This is considered the stoichiometric mixture where there is no O2 and no fuel left. Combustion is complete. An AFR (Air Fuel Ratio) of 8:1 and richer will not ignite (referenced to gasoline). And AFR of 18:1 and leaner will not ignite (referenced to gasoline). The addition of ethanol will shift these numbers richer BTW. Best power is at about 12.6:1 and best economy is at about 15.4:1. That's in the combustion chamber, of course.
So if you can create a condition in your exhaust when you would normally see popping where the AFR in the exhaust is richer than 8:1 then you will effectively eliminate the much hated popping. This is what you would be reaching for by changing jetting. Your bike would rich a bit richer down low, but would be really rich during decel. That's how I first dealt with the problem on my 1800, by using a Cobra FI2000, like an electronic jetting kit.
'12 Road Glide Ultra, Kuryakyn Alley Cat intake, Jackpot Super Duals header, Jackpot HiRoller mufflers, PV Tuner, Daymaker LEDs
Last edited by Harkon; 06-02-2017 at 12:28 PM.