CIS Question - Backfire on Overrun
 

I have a pesistent backfire on overrun. It seems to me that the problem lies with a defective auxilliary air device.

I isolated the device and found no change in the problem. However, if I give the slightest touch to the accelerator pedal on overrun the backfire dissapears. This tells me that it would seem that the auxilliary air device is not doing its job.

My testing is not consistent with what the manual says. The manual says that with the device is isolated then "if a change in rpm is noted, the device is leaking. If the valve leaks considerably, it will not be possible to adjust idle speed. Replace the auxilliary air device." I get no change in revs, but then I am not able to finely set the idle speed.

If the device is not defective then I must have a leak in the airbox but I doubt this. I have a pop-off valve and have renewed all that vacuum hoses.

Is my diagnosis correct?

Does anyone know the part number for the auxilliary air device. This part is not the same as the auxilliary air regulator which was fitted from 1974 on. I cannot find it in the PP catalogue but suspect that I will if I have the part #.

Any help will be appreciated.

Are you getting a backfie in the intake system or from the exhaust?
If the exhaust is popping on lift-off, you may just have an exhaust leak. My flanges on my 911SC with SSI heat exchangers are slightly warped allowing an exhaust leak at the collector with the muffler and that causes a pop or backfire in the exhaust when I lift off after blipping the throttle. It is a loud bang if open throttle on the track and then lift to shift. Kind of embarassing sometimes.
If no exhaust leaks, then you may have issues in the intake.

The backfire is out of the exhaust. I have not thought about a leak in the exhaust system. I will check.

Thanks.