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backfire while shifting?>


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My 77 FI 280z was doing this when the anti-backfire mechanism was "dialed back" too much. This is the screw adjusted valve mechanism under the throttle body that limits the rate at which the engine decelerates when you release the throttle. If it is dialed up too much then the RPMs decrease too slowly and you get no engine breaking power--very annoying/weird shifting. If it is dialed down too much (essentially off) then RPMs drop very quickly. At higher RPM upshifts, this causes the fuel-air ratio to lean out too much (1:20) causing anything from a pop to a bang as a backfire( or is it a prefire?) occurs.

I think this adjustment problem is more sensitive when you have a low back pressure exhaust.

The answer is to play with the anti-backfire deceleration control valve mechanism until you get the right balance, ie proper deceleration without the poof/pop/bang.

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