Two things or both come to mind.

1. exhaust leak... fresh oxygen mixing with unburnt fuel.

2. Rich idle/ transition circuit...When throttle plates are slamed closed, engine braking occurs. very high intake vaccuum.lots of fuel pulled from idle circuit. not much oxygen however.


this creates a very rich afr. may not fully burn in combustion chamber. unburnt fuel enters exhaust. Too much unburnt fuel can ignite on hot parts of exhaust system. add oxygen from leak and eaiser to ignite.