hi all,
i am in the middle of tuning out some decel popping on 06 Dyna...as are many more of us, and am a little confused over
the cause of it.
Seems general cause of this is the extra fuel that is already in the system from just before the rapid decel. and this then not burning fully and igniting in the hot exhaust etc.
Fine, that makes sense.
But.....why is it that we all only see this condition rising its head AFTER we add free flowing exhaust and air cleaner???
surely this leans out the mix and should reduce popping?

Surely the ideal fix is to lean out the mix on throttle decel, as the SERT allows, but havnt the pipes already done some leaning and only made the problem worse?
In the case of Powercommader people, the main fix I#ve seen on this forum seems to be to add fuel to the low throttle positions. If the popping is from excess fuel already how does this help...or does it just drown the mix to disable it from burning.....in which case idling is affected too.

can anyone help on this????.......it would be good to know the ideal or theoretical fix...even if in practice we go a different direction depending which is easiest to get good enough results.

........

 

It is a lean pop, that is why you get it more after the upgrades.
The ECM shuts off fuel on decel (decel eleanment mode).
Lower the numbers in the decel enleanment table that are
above 0 by half...the higher the number the leaner it is
on decel.

 

I'm no rocket scientist or chemical engineer, but I think maybe why there's popping or combustion in the pipes after we lean the AFR out by putting on higher flowing air filters and pipes, is because a leaner mix, means more air and I bet a leaner mix flash point or combustion point is lower then a rich mix.

I came to this conclusion thinking how combustion works. First we spray gas and air into a close cylinder, then compress the mix (compression is the key here), then ignite it. Well there's no compression in the pipes, so a leaner mix must ignite more easily then a rich mix.

Anybody who knows what there talking about care to chime in?

 

This off the web...


It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.


Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

 

Good stuff.
Thanks!

 

Hey priceamp,

I AM a chemical engineer. Gotta a problem with that dude??? [:@]

I too have severe popping on decel after laying off the throttle (closed) even after a custom dyno.

My scoot runs rich for sure too. And I think a rich mix would flash at a lower temp???

Now are you guys talking about cutting the PCIII settings at "2%, 5% or 10%" throttle position by one half to reduce this popping???

Appreciate any tips.

THANKS (I should have been a rocket scientist, then maybe I could have figured this one out myself )

 

Actually I'm a EE grad of '84. God I hated chemistry in school. Never did figure out what the heck a mole is? Think I got 2 C's and 1 B in that sequence. A very humbling experience.

All moles and electrons aside, I'm going to install a SERT tonight and flash the ECM with a decel enleanment table of .75x from the reference table and see if the popping is reduced. Most say, reduce table by .5x, but I like to experiment, so I'm going to do .75x first, then .5x if there's no change with .75x.

And funny, you'd think with spending good money for a dyno tune, the dyno tech would try and tune out popping? Maybe they do and can only reduce popping by so much? Maybe they only focus on performance, 'cause typically that's why you're having your bike dyno tune?

 

Decel popping on the newer HD's is caused by lean VE table values at the 0 map (closed throttle) at above 2750 RPM's When you put a WEGO on the bike, a sharp decel-closed throttle run from high RPM's shows well over 20:1. So high that a wego can't even measure the air/fuel ratio.

I notice when using the sert-twin scan 88+ with the wego to tune my bike that the popping will start going away when you can tune those deceleration 'peaks' to about 16-17:1 by raising the VE table values in the right areas. I'll post some pics of an untuned bike and a tuned bike. A huge difference and the main reason why seat of the pants tuning can only get you so far with these FI systems.

 

barrygreen is right lowering the numbers in the decel table does stop the pop!

but why it does is beyond me??????????


and another ?????.....is the canned sert touring 1550 map richer or leaner than the 1450 canned sert map??? i've loaded both on my 1450 flh and the bike runs a hell of lot better with the 1550 map, more hp and NO decel popping....BUT the motor runs about 50 degrees hotter and i get bluing about 2-3" from the pipe headflange not at the head, which i think means its too rich and fuel is igniting in the pipe, but, other say if bike runs hot its too lean?????

one would thing that the bigger 1550 motor would need more fuel???

HELP?????

this does not make sense??

thanks

 

This log came from an '06 Dyna with a 1550 kit, SE 203 cam and V&H exhaust. The shop that SERT tuned this bike never bothered to tune anything under 80 map so there were issues with deceleration popping. This was a tough bike to tune because the base map was so far off and was extremely lean and erratic. Notice that even under gentle closed throttle acceleration, the bike's A/F ratio jumps off the chart. This map was over 20-30 percent lean at several points.

This is a great example of why using the Deceleration enleanment adjustment will not work if the bike is too far out of tune.