Popping on decel. Explanation requested

[ViennaHog]

so why would you target a specific exhaust AFR during decel? On a stock carb it is impossible to turn idle fuel off, so it by design. In the old days some cars had a decel fuel cut off, that used an electromagnetic idle jet. So again what is the reason behind adding fuel on decel?

[Hilly13]

so why would you target a specific exhaust AFR during decel? On a stock carb it is impossible to turn idle fuel off, so it by design. In the old days some cars had a decel fuel cut off, that used an electromagnetic idle jet. So again what is the reason behind adding fuel on decel?

Adding a small amount of fuel raises the chamber pressure slightly so a little more mixture is burnt before the exhaust valve opens and dumps it into the exhaust pipe where the after fire takes place. After-market pipes are often a big factor in making this bang louder as a lot of them have significant reversion bringing fresh air to the burning ejected mixture. The stock headers and mufflers hide this reaction and also lesson the the air available to the ejected mixture.
Thats my understanding of it anyway.
Hilly

[ViennaHog]

Why wouldn't I cut the fuel entirely and avoid decel popping altogether? Bring it back on-line when the engine gets closer to to idle speed

[Hilly13]

Why wouldn't I cut the fuel entirely and avoid decel popping altogether? Bring it back on-line when the engine gets closer to to idle speed

If you let the throttle off completely and the TPS is registering zero then you have shut it off, its perfect, still goes bang though, the vacume created when you back of sucks in anything thats there, limited fresh air as the butterfly is closed, reciculated exhaust gasses, whatever fuel that is around, this mixture does not burn well, so the mixture is ejected into the exhaust, on a stock engine this is still happening its just covered up.

[blusmbl]

Why wouldn't I cut the fuel entirely and avoid decel popping altogether? Bring it back on-line when the engine gets closer to to idle speed

It'll ride poorly if you shut the injectors off on every single tipout, and it's not good for emissions either.  The new bikes do cut fuel when off throttle for more than a brief instant, but there are specific entry conditions for it, and when you reintroduce fuel at a lower engine speed it's just as easy to get pops through the exhaust.

[FLTRI]

3 things need to be present for a decel backfire, pop, or gurgle.
We, as tuners can control 2 of these ingredients...Fuel and Timing.
We can retard or advance closed throttle timing...which can help in certain circumstances.
Fueling adjustment is really the most effective method we've found for minimizing, if not eliminating the irritating decel backfire/pop.

The above said, there are 2 schools of thought with fueling:
1) Cut all the fuel.
Upside: Eliminates 100% chance of decel backfiring.     
Downside: As mentioned by blusmbl, when the rider tips back into the throttle the fuel gets "turned back on" which almost always creates a hesitation in that transition further irritating the rider.

2) Add to the decel fueling.
Upside: Greatly reduces/eliminates the loud backfiring and leaves a bit of a mellow gurgle as the engine returns to idle.
Downside: Uses a minute amount more fuel to enrichen the decel mixture.

There are other techniques used by OEM/emissions calibrators that can help to control decel backfiring but for most of the tuners out there using the above mentioned techniques have been most effective to reduce/eliminate decel backfiring.

A question some ask about possible damage caused by decel backfiring. AFAIK there is nothing but muffler packing, if present, that would be affected by backfire/pop.

Bob

[ViennaHog]

Bob and bluesmbl,
thanks for letting me in on the details of decel fueling, makes sense now. Would one change the AFR at closed throttle or rather VEs at low maps (205 level cals)? And I think I will do the freeze plug fix and see what that does to reduce fresh air leakage.

Great advice as always