exhaust backfire

How about a leaky header collector gasket on the passenger side. Allowing fresh air to get sucked into the passenger side exhaust pipe.

If it is running lean like RRR said, the addition of more air could make it pop on the way out to the back.

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Also is the muffler on the passenger side buldging out from the backfire pressure?

Bad backfires in the exhaust system can split the mufflers wide open at the seams.