Why Do Skylines Backfire?

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It's running rich.. that's what they do - especially when it's using the O2 sensor to control it's A/F ratios.. put it on a dyno and watch them drop..

You'll find sometimes after having been going hard up a hill it'll dump fuel when you completely back off the throttle and sounds like someones making pop-corn in your exhaust.. that's the way it is..

Having a stock BOV won't totally resolve the issue, mine still does it.. but you've goto be doing some quick shifts usually for it to happen..