From what I understand a backfire is when you get combustion happening in the intake instead of the combustion chamber as intended. I thought there was only a few reasons for this to happen. First would be the timing of the valve positions and/or spark not matching the fuel injector firing. The other would be an extreme rich condition in the manifold such as when an injector is stuck open. Add a little spark and boom!!
Maybe a complete tune-up would help?
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