Decel Pop!

Decel Pop
Understand that it is the performance parts we use that cause popping on deceleration. Mostly the exhaust systems. So popping goes hand n hand with high performance race engines.
Any thing you do to the TUNE (Fuel & Timing) out the decal pop is a band aide not an actual fix.

First thing is to try and fix the problem.
Second thing is to band aide with tuning.

Exhaust popping can be very different bike to bike. There is some decel popping you just cannot fix.
We found that a few little things could help most of it.

Addressing the problem

Decel  pop is un-burnt fuel igniting in the pipe. generally caused by oxygen being introduced to the exhaust causing it to get hotter and ignite the fuel. Exhaust leaks, and/or open baffles allow this extra oxygen into the system.

  • Use factory exhaust gaskets on pipes you can. Not the thin ones like SE has as they leak as well.
  • It can be common for exhaust nuts to come loose at the head during Dyno runs and even normal riding.
  • Make sure the exhaust is properly Installed, aligned and tightened.
  • With all the exhaust installed and all the fasteners finger tight. Torque/tighten in 2 or 3 stages starting at the head and working your way down to the brackets at the end for each stage.
  • Screamin Eagle riv nuts: Dont use them. A hex shaped rivnut does not seal proper in a round hole. (Must be a member to View)
    • Use these from our store with the Scream Eagle kits.
    • More important than the leaking affecting decel pop, it affected your calibration causing you to calibrate the bike too rich. On top of the riding issues experienced in other Air conditions (Temp, Pressure, Altitude)  caused by this being too rich in open loop areas can cause more fuel to build up in the pipe which will cause more decel pop.

Tuning tips to reduce pop. “Band Aides”

  • Spark Tables: Advance the timing in the decel column from highest rpm to lowest that won’t change the timing at idle. This will vary based on the spark table columns and what kpa the bike idles at.
    • Example: If the lowest column on your Spark table is 15 kpa and the bike idles at 35 kpa you can advance the timing in the 15kpa column all the way to 1000 RPM
    •  Example: If the bike idles at 25kpa (middle of 20 &30kpa columns) and the lowest column is 20 kpa then you should not change the timing for decel pop any lower than 1750 rpm. 1500rpm and below may effect cold to hot idle.
  • Air Fuel Ratio Table: Fuel wise you have 2 choices.
    • 1. Lean the mixture trying to reduce the amount of fuel in the pipe when the injectors come back on.
    • 2. Richen the mixture trying to cool the exhaust to prevent the un-burnt fuel from igniting.
    • Harley’s as a rule respond better to richening the fuel for decel. However, if that doesn’t give the desired results try leaning it out on decel
    • Like the spark table the Decel column will be the lowest kpa on the Air Fuel Ratio table.
      • Same idle rules as we discussed above for the spark table apply. Be careful not to make it idle too rich or lean chasing decel pop.
  • VE Tables: Last Resort. I rarely have to use the VE tables for decel pop band aiding. Only use it as a last resort… please fix the exhaust leaks first! Richen with bigger VE, Lean with smaller VE
    • Throttle based VE tables be careful not to change the idle area.
    • MAP based: VE tables Same idle rules here as for the spark tables.
      • Caution: Motorcycles at a low load cruise in the mid rpm may be running below 30 kpa and 3500 RPM. If you are changing the 20 kpa column on a VE table you may cause issues with cruise.

You will find more Tuning & Dyno tips on our Members Only Site as well as Tutorials, Videos, Downloads,…

Mike Daniels
Dyno Tool Store


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