Everything on Decel pop
Pops, snaps, farts, and burbles on the overrun. Technically called after-fire. These are normal to bikes with open end-cap style mufflers. Not harmful to the engine, exhaust, or tune.
Loud cracks and bangs that sound like a gunshot. These are not normal and need investigation. Check exhaust for air leaks, cracks, and failed joints. Also inspect valve lash, fuel tune, and ignition system.
Riders who converted their bike to closed-course use may notice decel noise that wasn't there before. The pop was always happening — the restrictive stock end cap was muffling it. Open-flow exhausts (Pro Moto Billet end cap, FMF, Yoshimura, Pro Circuit, etc.) have:
- +Less muffling effect overall
- +Lower gas velocity at the outlet
- +A larger opening that allows fresh air to revert back into the muffler chamber due to temperature variation and intake air reversion
That fresh air mixes with superheated exhaust gases and causes the snaps, pops, and burbling. Normal and not harmful. Maintain muffler packing at the recommended 150–200 hour interval.
In engineering, there's no free lunch. When you move the needle of performance and engine cooling away from the stock configuration, some decel noise is part of that trade-off.
If your muffler came with a quiet insert, use it. This reduces the end tip diameter and dampens the air reversion happening at the outlet. The effect is especially noticeable on the Pro Moto Billet end cap with the Ultra Silent insert.
- ▲Loudest: Pro Circuit, FMF 4.1 — minimal internal baffling
- ▼Quietest: OEM muffler + Pro Moto Billet end cap + Ultra Silent insert, FMF Q4
Video Discussion
Harley-focused but the mechanical explanation applies directly to single-cylinder four-strokes
Watch on YouTube ↗