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?