Can anyone offer some ideas about weird air/fuel mixture readings? I am trying to tune my mild 350 engine using a air/fuel gauge and vacuum gauge. The engine runs very smoothly at idle, at cruising speed, and during acceleration. I am attempting to tune it for better performance and economy. Recently I added an Autometer digital Air/Fuel gauge with a heated narrow-band oxygen sensor mounted in one header collector. This A/F gauge is mounted next to a vacuum gauge so I can see them while driving.

A picture of the gauges at steady-state cruise:
http://i58.photobucket.com/albums/g244/f1970chevelle/Chevelle/vacuumA-Fgauges.jpg

The behavior I don't understand is why the A/F mixture reads richer during deceleration than it does at cruise. The A/F gauge shows near stoich (ideal) mixture at idle (18 in vacuum) and during cruise (16 in vacuum), but when I let off the gas pedal, the gauge goes a little lean for about a second, then it reads very rich until I step on the gas again. Vacuum holds steady near 25 during deceleration. I would think the gauge should read lean when decelerating. A rich reading means to me that little oxygen or too much fuel is in the exhaust. The engine doesn't idle down very fast when I lift my foot.

Does anyone have any thoughts why the gauge would read rich when it should be lean?

This is the setup in the car:
small block 350
Edelbrock 1406 Performer carburetor, set with stock springs, rods, and jets (economy setting)
carb is tuned for 600 RPM idle and ideal stoich mixture
Edelbrock 2101 Performer intake
stock iron heads
mild RV hydraulic cam
Hooker 1 5/8" full-length headers
dual exhaust
Mallory Unilite distributor and Accel coil
vacuum advance is hooked up
12 degrees initial timing, lighter springs installed for quicker mechanical advance, stock total advance
fuel pressure regulator is set for 5 PSI
new PCV valve and air filter
fuel economy is consistent at 14.5 MPG

I would appreciate any advice. Thank you.

Unburned fuel in the exhaust when decelerating....another example, ever heard a pop pop pop when decelerating on an open or more open type exhaust? Same thing, the unburned fuel is being ignited finally at the collector or area of the exhaust tubing where more oxygen is present. Since the vaccuum of the engine is still pulling in the air/fuel into the chamber at RPM, it acts somewhat the same except on deceleration, the compression of the pistons is used to slow down the vehicle instead of speed it up, therefore, the fuel is not totally burned due to the decreasing air flow qualities. I'd say don't worry about the deceleration....just put it in neutral or disengage the clutch if you're running a manual tranny. Who's going to tune their engine while slowing down anyway?

I've heard race cars burning fuel in their exhaust. I think that happens when the combustion chamber does not contain the entire explosion, since a lean mixture burns slower and the gasses are forced out into the exhaust before all the fuel is burned.

My understanding of the oxygen sensor is that is reads only oxygen, not remaining fuel. When fuel is burning in the exhaust, oxygen must be present for combustion to happen, so the sensor should read lean.

What I am trying to do is reduce the amount of wasted fuel while I'm driving. When the throttle closes, air volume is reduced and fuel supply should be reduced as well. I wonder if the vacuum advance, at 25" vacuum, is throwing off the timing enough to change the oxygen reading.

I have a pair of O2 sensors mounted in my 454 with a Holley 750 DP and see the same behavior.

Way rich on the narrow-band a/f gauge doesn't really mean way rich. It just means richer than about 14.5 (or so) to one. It could still be in the 14.0 to 14.4 range and not have any adverse impact on fuel mileage. Problem with a narrow-band sensor is you can't tell if it's only a little rich or a lot rich. Only solution to this is a wide band sensor setup or a trip to the local chassis dyno shop to see what it's really doing when decellerating. Either option will cost you way more than the fuel you'll burn in the long run due to a rich decel a/f ratio.

It's probably the fuel level rising inside the emulsifier tubes during decel's higher vacuum, so you probably could fine tune your carb's idle circuit and jetting to minimize this. But personally, I wouldn't worry too much about it since you're only decellerating about 10% of the time you're driving anyway.

JMO.