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Backfire on decel
  #1  
Old 05-31-2011, 08:27 AM
98Redlne 98Redlne is offline
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Default Backfire on decel

So since the beginning of this season I have had this annoying backfire on decel. I figured that after the winter my carbs needed either some running time (haven't had much of that) or the shotgun/peashooter (haven't had much time for that either).

Well, yesterday I finally got some down time and while out cleaning the bike and the cars I noticed that when I started the bike just to warm it up a bit (was wearing shorts and sandals at the time) I could feel a puff of exhaust on the inside of my ankle. Ah Ha! Now I know why I have the decel backfire.

Tightened up all of my header bolts (yes they were all loose, in spite of assembly using blue locktie) fired the engine back up now she growls as she should on decel but no more stinkin' backfire

So my question is, why does an exhaust leak cause the backfire?

I understand that it is due to air getting sucked in and causing the unburnt fuel in the exhaust to detonate in the header, however I can't seem to get my head around how the air gets in. It seems to me that the exhaust header should be at atmospheric pressure when a cylinder is not firing into it and slightly above when one is. In order to draw air in you would need negative pressure. So how do you get negative pressure in the header?
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Re: Backfire on decel
  #2  
Old 05-31-2011, 09:42 AM
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SpecOps13 SpecOps13 is offline
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Default Re: Backfire on decel

???? Butterflies are closed which creates a vacuum to the cylinder. The deceleration enricher is trying to keep the mixture to a point that there's no back fire but your pulling air at the exhaust port and it's detonating right there..

I don't really know if that makes sense but that's my best guess...

If nothing else, this'll keep your thread active for a more expert answer...
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Re: Backfire on decel
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Old 05-31-2011, 10:10 AM
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Thevmaxrider Thevmaxrider is offline
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Default Re: Backfire on decel

To really understand what is happening you have to think of your exhaust gas pulses like waves. As a pulse, or wave, of exhaust gas flows through the header at a certain velocity it actually creates an area of low pressure behind it (remember you exhaust valve closes so you have a close ended system for a split second until it opens again). This low pressure area will pull air into the header wherever there is a leak for a split second just after the positive pressure pulse passes by. The amount of air it pulls in is just enough to promote combustion, the unburnt gasses are hot enough to ignite but in an oxygen deficient atmosphere until the outside air enters, and bang you get ignition and a backfire.
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Re: Backfire on decel
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Old 05-31-2011, 10:57 AM
98Redlne 98Redlne is offline
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Default Re: Backfire on decel

Ok that makes sense to me.

I would also think that as the gas gets further down the pipe, it cools and contracts adding to this low pressure area.
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Re: Backfire on decel
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Old 05-31-2011, 11:16 AM
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Thevmaxrider Thevmaxrider is offline
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Default Re: Backfire on decel

Actually its cooling does quite the opposite. That is why you see gents use header tape to maintain the exhaust gas temp. As the gasses cool they loose their velocity and the scavenging effect (the low pressure behind the pulse) which creates more power by evacuating exhaust gasses from the combustion chamber by "pulling" the gas out with the low pressure. If your gasses cool and slow down the pressure differential will actually decrease hence a lowering of the exhaust scavenging. By keeping the heat up in the headers, also accomplished with ceramic coating by the way, you improve your exhaust performance. Have been talking a lot with Lang Hindle about exhausts, he wants to build me a custom stepped exhaust to maximize Toxic's performance, and learning a ton!!!!
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Re: Backfire on decel
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Old 05-31-2011, 02:17 PM
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SpecOps13 SpecOps13 is offline
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Default Re: Backfire on decel

More Better Answer Jim, Thanx....
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Re: Backfire on decel
  #7  
Old 06-01-2011, 10:19 AM
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Ebein_486 Ebein_486 is offline
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Default Re: Backfire on decel

Thanks for the info! Ive been getting the occasional pop on decel as well. Will double check all the connections on my Kerker 4-1. Was thinking
the switch to the comp baffel was causing me to run a bit too rich.

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