Popping through the exhaust on decel
Banned
Joined: Jan 2009
Posts: 1,551
Likes: 6
You guys are chasing all around the problem yet of none you know exactly why it pops on decel. Pop in the exhaust requires heat, excess fuel and 02, right?
A quick decel leaves unburned fuel in the exhaust train......Unburned fuel mixed with the right amount of air can be triggered to explode if provided enuff heat and air, right?
There can be NO exact determination of the cause until you control 2 of the three things that make decel pop happpen. You either have a leak that allows fresh 02 to get sucked into a hot/rich mix, or it is too rich to start with and ignites when engine is at high vac and air gets in thru the back end of an otherwise tight exhaust system.
A too lean condition will cause decel pop when it raises exhaust gas temps enuff to explode unburned fuel.....There is ALWAYS unburned fuel in the exhaust when engine braking, but it will NOT ignite if either too lean or too rich....it will only ignite if all conditions for cumbustions are met....those conditions require heat, fuel and 02.
Too lean provides heat, too rich provides fuel.....Leaks provide air....Get the right combination of each and you have pop.
First fix any possible leaks, then provide enuff fuel to keep temps down, then tweek the map to remove any REAL excess fuel.....from there live with the results all short exhaust systems produce because all 4 stroke engines have a hole in the cylinders due to cam overlap where the engine sucks and blows thru the exhaust...It blows out, then sucks back because of cam overlap when both valves are open and piston is still moving up n down.......Picture it and you will get the idea.
Stock exhaust today are very restrictive...they are restrictive in both directions, plus they do a really good job of muffling sound. You might never hear decel pop with stock exhaust.......But you will fersure hear it with aftermarket pipes.
In the normal course of events, expect SOME decel pop on long engine braking coastdowns with throttle shut tight, but never accept it when performing hard throttle shifts when you roll off the throttle to click another gear. If you get pop then, you got a real problem and need to look for leaks, excess fuel or a too hot exhaust gas temp from too lean condition.
A quick decel leaves unburned fuel in the exhaust train......Unburned fuel mixed with the right amount of air can be triggered to explode if provided enuff heat and air, right?
There can be NO exact determination of the cause until you control 2 of the three things that make decel pop happpen. You either have a leak that allows fresh 02 to get sucked into a hot/rich mix, or it is too rich to start with and ignites when engine is at high vac and air gets in thru the back end of an otherwise tight exhaust system.
A too lean condition will cause decel pop when it raises exhaust gas temps enuff to explode unburned fuel.....There is ALWAYS unburned fuel in the exhaust when engine braking, but it will NOT ignite if either too lean or too rich....it will only ignite if all conditions for cumbustions are met....those conditions require heat, fuel and 02.
Too lean provides heat, too rich provides fuel.....Leaks provide air....Get the right combination of each and you have pop.
First fix any possible leaks, then provide enuff fuel to keep temps down, then tweek the map to remove any REAL excess fuel.....from there live with the results all short exhaust systems produce because all 4 stroke engines have a hole in the cylinders due to cam overlap where the engine sucks and blows thru the exhaust...It blows out, then sucks back because of cam overlap when both valves are open and piston is still moving up n down.......Picture it and you will get the idea.
Stock exhaust today are very restrictive...they are restrictive in both directions, plus they do a really good job of muffling sound. You might never hear decel pop with stock exhaust.......But you will fersure hear it with aftermarket pipes.
In the normal course of events, expect SOME decel pop on long engine braking coastdowns with throttle shut tight, but never accept it when performing hard throttle shifts when you roll off the throttle to click another gear. If you get pop then, you got a real problem and need to look for leaks, excess fuel or a too hot exhaust gas temp from too lean condition.
Last edited by oinker02; Aug 17, 2009 at 11:24 PM.
Tourer
Joined: May 2009
Posts: 315
Likes: 1
You guys are chasing all around the problem yet of none you know exactly why it pops on decel. Pop in the exhaust requires heat, excess fuel and 02, right?
A quick decel leaves unburned fuel in the exhaust train......Unburned fuel mixed with the right amount of air can be triggered to explode if provided enuff heat and air, right?
There can be NO exact determination of the cause until you control 2 of the three things that make decel pop happpen. You either have a leak that allows fresh 02 to get sucked into a hot/rich mix, or it is too rich to start with and ignites when engine is at high vac and air gets in thru the back end of an otherwise tight exhaust system.
A too lean condition will cause decel pop when it raises exhaust gas temps enuff to explode unburned fuel.....There is ALWAYS unburned fuel in the exhaust when engine braking, but it will NOT ignite if either too lean or too rich....it will only ignite if all conditions for cumbustions are met....those conditions require heat, fuel and 02.
Too lean provides heat, too rich provides fuel.....Leaks provide air....Get the right combination of each and you have pop.
First fix any possible leaks, then provide enuff fuel to keep temps down, then tweek the map to remove any REAL excess fuel.....from there live with the results all short exhaust systems produce because all 4 stroke engines have a hole in the cylinders due to cam overlap where the engine sucks and blows thru the exhaust...It blows out, then sucks back because of cam overlap when both valves are open and piston is still moving up n down.......Picture it and you will get the idea.
Stock exhaust today are very restrictive...they are restrictive in both directions, plus they do a really good job of muffling sound. You might never hear decel pop with stock exhaust.......But you will fersure hear it with aftermarket pipes.
In the normal course of events, expect SOME decel pop on long engine braking coastdowns with throttle shut tight, but never accept it when performing hard throttle shifts when you roll off the throttle to click another gear. If you get pop then, you got a real problem and need to look for leaks, excess fuel or a too hot exhaust gas temp from too lean condition.
A quick decel leaves unburned fuel in the exhaust train......Unburned fuel mixed with the right amount of air can be triggered to explode if provided enuff heat and air, right?
There can be NO exact determination of the cause until you control 2 of the three things that make decel pop happpen. You either have a leak that allows fresh 02 to get sucked into a hot/rich mix, or it is too rich to start with and ignites when engine is at high vac and air gets in thru the back end of an otherwise tight exhaust system.
A too lean condition will cause decel pop when it raises exhaust gas temps enuff to explode unburned fuel.....There is ALWAYS unburned fuel in the exhaust when engine braking, but it will NOT ignite if either too lean or too rich....it will only ignite if all conditions for cumbustions are met....those conditions require heat, fuel and 02.
Too lean provides heat, too rich provides fuel.....Leaks provide air....Get the right combination of each and you have pop.
First fix any possible leaks, then provide enuff fuel to keep temps down, then tweek the map to remove any REAL excess fuel.....from there live with the results all short exhaust systems produce because all 4 stroke engines have a hole in the cylinders due to cam overlap where the engine sucks and blows thru the exhaust...It blows out, then sucks back because of cam overlap when both valves are open and piston is still moving up n down.......Picture it and you will get the idea.
Stock exhaust today are very restrictive...they are restrictive in both directions, plus they do a really good job of muffling sound. You might never hear decel pop with stock exhaust.......But you will fersure hear it with aftermarket pipes.
In the normal course of events, expect SOME decel pop on long engine braking coastdowns with throttle shut tight, but never accept it when performing hard throttle shifts when you roll off the throttle to click another gear. If you get pop then, you got a real problem and need to look for leaks, excess fuel or a too hot exhaust gas temp from too lean condition.
The bottom line is these engines pop right out of the factory. They're air cooled engines. Air cooled engines run hot. To try to cool them down a little more fuel is added. The little extra fuel sometimes doesn't burn and ends up in the exhaust. The hot pipes create a pop/small explosion. To quench the pop fuel is added at certain RPM's that are causing the issue.
It's the nature of the beast with an air cooled engine.
Decell popping can be created by a number of factors which are causing a combustion event in the exhaust system including but not limited primarily to a rich or lean condition, an exhaust leak, or reversion in the exhaust system from a free flowing design, short design, or large baffle design. Because EFI requirements change greatly based on manifold pressure there are very different things going on when the throttle is closed during decell. There is really no right or way to start tuning out decell popping as it can happen from a rich mixture where there is simply unburned hydrocarbons in the exhaust system that ignite. Many Jap bikes have a secondary air injection system that injects a controlled air stream into the exhaust under decell to burn these gasses which causes problems when a free flowing exhaust is added and many of you remember SMOG pumps on vehicles which along with catalyst work off the same principle. Bikes will just as easily decell pop from a lean mixture where in this case the exhuast gas temp skyrockets and will super-ignite gasses in the exhaust stream that would not normally ignite. There are also issues due to exhuast leaks where reversion will essentially introduce O2 into the exhaust system and cause popping. Reversion plays a huge part of decell popping and is inherent in many exhaust designs. In most cases if you have the exhaust sealed at all joints and have a properly tuned bike you can tune most decell popping out, however some "backing down", "gurgling" or small pops would be considered normal.
__________________
Jamie Long / Fuel Moto USA
The USA's Leader V-Twin EFI & Performance www.fuelmotousa.com
Contact 920-423-3309
Email jamie@fuelmotousa.com
Jamie Long / Fuel Moto USA
The USA's Leader V-Twin EFI & Performance www.fuelmotousa.com
Contact 920-423-3309
Email jamie@fuelmotousa.com
Road Warrior
Joined: Mar 2009
Posts: 1,885
Likes: 0
From: Chicago area
very true, thx 4 resurrecting this thead.
Elite HDF Member
Joined: May 2006
Posts: 3,927
Likes: 5
From: Midwest
I have a stage 2 Big Sucker, D & D slipons, the fullsac dual conversion kit and a DFO. After I put the dual conversion kit on I had more popping than I thought was necessary. I replaced all the clamps and used copper sealant on the joints and now all is well. I just changed plugs when I got back from Sturgis and they were a nice shade of light gray.
Road Master
Joined: Jun 2006
Posts: 812
Likes: 0
From: Jaw-Jaw (Near The BIG Chicken!)
Decell popping can be created by a number of factors which are causing a combustion event in the exhaust system including but not limited primarily to a rich or lean condition, an exhaust leak, or reversion in the exhaust system from a free flowing design, short design, or large baffle design. Because EFI requirements change greatly based on manifold pressure there are very different things going on when the throttle is closed during decell. There is really no right or way to start tuning out decell popping as it can happen from a rich mixture where there is simply unburned hydrocarbons in the exhaust system that ignite. Many Jap bikes have a secondary air injection system that injects a controlled air stream into the exhaust under decell to burn these gasses which causes problems when a free flowing exhaust is added and many of you remember SMOG pumps on vehicles which along with catalyst work off the same principle. Bikes will just as easily decell pop from a lean mixture where in this case the exhuast gas temp skyrockets and will super-ignite gasses in the exhaust stream that would not normally ignite. There are also issues due to exhuast leaks where reversion will essentially introduce O2 into the exhaust system and cause popping. Reversion plays a huge part of decell popping and is inherent in many exhaust designs. In most cases if you have the exhaust sealed at all joints and have a properly tuned bike you can tune most decell popping out, however some "backing down", "gurgling" or small pops would be considered normal.
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