When quickly decelerating or going down a hill in a lower gear (engine under load) the car backfires, pop-pop-pop. Mechanic told me car is running too rich. Dont know if this correct.
Timing was set a few months ago. Spark plugs are pretty good - center is brownish color, but the edges are black.
How much difference does it make in the way the car runs if you play with the adjustment screw on top of the FI control unit. Doesn't that screw somehow make the car run rich or lean?
Should I consider adjusting this? If so, is clockwise direction rich or lean?
Have all new vacuum hoses and are tight
Any suggestion on what else to look at?
Thanks.

ED

Is the valve lash within spec? My experience is the ECU knob does not make that big a difference.

Do you smell fuel during idle? Does it make a difference if the engine is cold or hot?

decel valve?

Don’t be afraid to turn that knob.
Mark a Mark so you know where you started.
Right is rich, left is lean.

Valves were adjusted a few months ago.
No, I don't smell any fuel
Backfire seems to happen once the car is hot.
I was thinking decel valve too. Gonna pull the small hose from the front of it and see what happens.
Will mark and adjust knob on FI control unit.

Thanks for the suggestions.

Check your exhaust port areas for leaks ; where the H/B's meet the cyl heads look for any black carbon deposits*. Also check the gaskets where the H/B's meet the muffler as well. Snap crackle pop on decel can def occur from all of these areas, if there are exhaust leaks present.
*If you need to tighten...make sure that you check with specific Tq. value, as you don't want to compromise a stud <<

Yes. Exhaust leaks cause popping on decel!

Sounds like you had work done on the car recently. Was it backfiring before, too?

backfire can be caused by an exhaust leak or a bad muffler that does not have enoough back pressure.
The control unit adjustment is for small idle mixture corrections only so will not affect the mixture during regular running.
If you suspect rich running (which CAN cause back fire) I would evacuate the air pressure sensor, check the head temp resistance when hot and check the fuel pressure. THEN check the compression and as previously stated, check the valve adjust

Check all the plugs too. One cylinder might not be doing everything it should be.

Valves adjusted by you, or by someone else? I would check valve clearance yourself. Turns out mine were adjusted too tight by a shop and that was the cause of my car backfiring/popping on decel. A good lesson that it's best to do this work yourself if you can. Running too lean will also cause backfiring. I don't believe that running too rich will do that, but I'm willing to be corrected by those of you with more experience. Don't be afraid to adjust fuel/air ratio knob. Just make sure to know where you started at so you can at least go back to where you knew it was running. If you are more adventurous you can also adjust at the AFM, but opening up an AFM is not for the faint of heart. Helps to have access to an exhaust gas analyzer. If you are here in California you could also pay to have a smog check pre-test. I know your car does not require a CA smog check, but that will let you know what your fuel mixture is under load and what adjustments you need to make. Well worth the $40 or so to get that information before you start burning valves by running too lean.

rjames,
To answer your question, yes. Car was recently in shop for new front struts (local independent Porsche shop). While there, I asked them to check the timing as I didn't think I had it just right. They supposedly did. Now it backfires under decel. Should have left it alone. Time to pull out the timing light and 10 mm wrench. Getting too old to fit down in the engine compartment.
Thanks for all replies.
ED