Backfiring in the exhaust system of your motorcycle is created by the same three necessary elements inside the engine; oxygen, fuel and ignition. Since the heat of the exhaust will supply the ignition, let’s discuss the other two. The backfiring on acceleration is a serious engine related (cam timing, bad valve seat seal, etc.) problem and can not be fixed with air fuel mixture changes. Backfiring on deceleration is much more common and often occurs when exhaust and fuel mixture changes are made. This is what we are going to discuss. All fuel injection systems turn off the fuel during deceleration when the throttle is fully closed, until a certain r.p.m. is reached and then turned back on the balance of the way down to idle. This is a most important indicator of what is happening. If a motorcycle backfires all the way down through the r.p.m. range of deceleration, then obviously the injectors are not turning off as designed, and this tells you there is some issue with the fuel injection system. There are numerous possibilities, including but not limited to; improper TPS setting, bad cold start and/or temp sensors, throttle cable adjustment, etc. Please go to your dealership or shop manual and fix this problem or problems before proceeding. Remember that your bike may well have had this pre-existing problem; adding the less restrictive exhaust or tuning fuel may just make it more noticeable.
Now, if backfiring is only present in the lower r.p.m., usually the lower third of the full r.p.m. range, that is confirming that the injectors are coming back on and now causing the igniting of that fuel. But, there has to be oxygen (fresh air) present to create that backfire. There are numerous ways oxygen can end up in the exhaust system, especially if you are riding a large V-twin motorcycle. On deceleration, with the throttle plate closed, the pistons are still pulling a large vacuum that is not being fed by the throttle opening, so any tiny opening at gaskets, seals, joints, slip fits, must be suspect. And just because exhaust isn’t coming out of a connection, doesn’t mean air can’t be drawn back through under high vacuum. So first, what have you changed on your motorcycle? This must be the first place you look. Also , the exhaust system itself maybe contributing to allow fresh air back up into the exhaust. Very short and/or no baffles can allow exhaust to rush out but fresh air to rush back in, along the sides of the inside of the pipes. This is known as fresh air inversion, and is common with very short exhaust pipes, large diameter straight pipes and/or no baffles. Also one into two rear head pipe design on a stock Harley dresser allows fresh air to be drawn up through the left hand muffler.
The intake side also needs scrutiny, even if there has been no modifications done. Fresh air can be introduced by intake manifold leaks, or in many Japanese bikes, exhaust gas emission systems. Harleys have a rubber intake gasket that can be overheated and damaged and the slightest movement while changing an aircleaner system can open up a leak. The Japanese twins use reed valves or mechanical one way valves that can get stuck, allowing fresh air to get past and into the exhaust. Some aftermarket aircleaner assemblies don’t deal effectively with converting the original EGA hoses and valving.
Based on the above data, there are some simple tests you can perform to help you solve your backfire issues:
A step by step logical approach should help you solve and eliminate backfiring.