Vulcan Bagger Forums - Its NOT backfiring.....especially on modern bikes

Never has been, never will be, unless you have a real serious engine problem like a burned intake valve, or somehow the valve timing or ignition timing got way off, because.

REAL "backfiring" is the air/fuel mixture igniting when an intake valve is still open and the actual combustion is spitting out backwards into the air intake. Think of the engine belching like we do when you have bad indigestion. Sometimes those belches can be "tasty" in a bad way, verp.

What we hear, nearly always is exhaust system "popping" on closed throttle decel when the bike momentum is actually pushing the engine while the throttle is closed. Not enough air/fuel at higher engine rpm on a closed throttle means it only ignites when enough mix has built up in the combustion chamber. Bikes with stock intake/exhaust do it all the time, but the much larger stock intake/exhaust is designed to dampen out the popping. But we riders can't leave stock alone if we can get more power, and especially more noise from the bike.

So,, off comes the stock exhaust and on goes a "low restriction" intake and exhaust, usually with much smaller volume mufflers (if any), and the fuel/air mix gets FURTHER leaned out. The aftermarket exhaust doesn't have the volume or chambering to dampen the popping, so now we really hear in all its shotgun noise glory. Sometimes very loud. Another way it happens is an intake leak or exhaust leak (like at the header flange) which also can cause the engine to run a bit more lean, so the popping increases and get louder. But its still NOT backfiring.

First off, it does NO damage whatsoever to the engine. It can pop like crazy, pop really load, and it causes no damage to the engine, it just makes a lot more noise. Its easy to prove its not backfiring. Next time it happens, pull in the clutch lever, and the popping stops immediately. So if you mod the intake and exhaust and want to get rid of most of the popping means you have to increase the fuel to air mixture, give the engine more fuel/air to let it actually combust normally against the breathing demand of a closed throttle against a coasting decel load. I proved this on one of my bikes. I made my own exhaust design. Works well, a bit more sound, but more popping on closed throttle decel. I was able to decrease the popping by tuning up the fuel injection to add a bit more fuel on closed throttle running.

Just one of my pet peeves when anyone talks about their modded bikes and calls it backfiring. Really irksome when a "mechanic" calls it backfiring, which it never is. Or when a person goes to the serivce manager complaining about his modded ride backfiring, and the SM calls it backfiring but has no idea what it really is and how to decrease it. Note, I didn't say "eliminate it" because with a modded intake/exhaust you will have some degree of popping.