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Backfires when letting off throttle

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I am hearing a back fire "pop" when letting off throttle after being under load and I am seeing a knock spike during let off. What are some areas to look at? Anything to worry about?

The AFR goes super lean when letting off which I believe is expected.

What is your ignition timing (advance) in the high RPM, throttle off (low MAP) sites? Increasing the timing (more advance) can help eliminate the pops. Also, you can try reducing the fuel to zero, and you don't need any torque at that point. Many ECUs have an "Overrun Fuel Cut-off" to completely stop fuel injection during the overrun -- this sometimes needs tuning to make sure it turns the fuel back on at a sufficient RPM that any idle control can prevent a stall, if the driver were to put the clutch in suddenly.

Ign timing in those sites are ~30 deg

Knock certainly won't be an issue under these conditions. I agree with David's points above and over run fuel cut will likely sort out your problems. This can also come from the fuel table not being optimised correctly in the over run areas as we usually can't access these on a dyno. You can see what the AFR is in these areas ion the road by careful use of the throttle to bring the load point down lower than you can see on the dyno though.

If I run less than 40* of advance on our racier engines I get intense backfires. However this is in a rotary application.

-Dave

Just as a data point, the stock ECU for a Suzuki GSXR 1000 has 80 degrees of advance at low throttle and high revs (8000+). Do not be afraid of 40 - 50 degrees of advance at very low cylinder filling (low manifold pressure, closed throttle)

Thank you all for your input! Very helpful!