First days with an AEM AFR guage installed, Hmmmm.... rich at cruise? Options

[Gint]

Running pretty good. It could be leaner, but it's not horrible for a pair of carbs that sat for well over a year and a half without a rebuild. I'm getting anywhere between 12 and 13.7 to 1 when I'm getting on it. Goes a lean for a second during transition from idle to mains. I'll work that out eventually. About what I expected. Not perfect, but that was the point of installing the gauge and getting a baseline before I touched anything. The gauge does tend to fluctuate quite a bit. Even at steady state throttle/speed/grade/etc...

What's weird to my mind is that it goes stupid rich at cruise. I expected it to lean out some. Trailing throttle cruise sometimes goes as low as 10.5 briefly, usually around 11:1 though. It will go up sometimes as high as 12 at slight throttle cruise, but I expected that to be leaner as well.

Thoughts?

2.0 with about 10K on a local rebuild, 10 years ago. It's the 4 from Scott's Martini car. Stock build, pistons, cam, everything stock based on the receipt from the build. Dual Weber 40IDF's that sat for almost two years before this engine fired again this last time. I haven't rebuilt them yet. I have new jets and 32mm vents from ACE. I want to baseline what I have now, rebuild, adjust, baseline and then re-jet and check it out again. I want to see the effects of the changes as I do them.

Carbs now:
60 idles
115 mains
F11
2.0 AC
28mm Vent

[wndsrfr]

What's weird to my mind is that it goes stupid rich at cruise. I expected it to lean out some. Trailing throttle cruise sometimes goes as low as 10.5 briefly, usually around 11:1 though. It will go up sometimes as high as 12 at slight throttle cruise, but I expected that to be leaner as well.

Thoughts?

Bear in mind that at "cruise" with speed being less than about 60-65mph your throttle plates are barely cracked beyond idle. I found that out on my car since I'm on TP sensing for the SDS injection. My idle TP reading is 5 and WOT is 50. My car "cruises" at TP 5 on flat roads anywhere up to about 60mph. At TP 6 it's slowly accelerating, & gets up to about 66-70 before it stabilizes with enough road/wind resistance to balance the power. What this means is that these cars are so easily driven that normal cruising is just barely open from idle on the throttle plates.

That said, there's so little air being drawn through the carbs that the AFRs are going to fluctuate a lot--for instance with trailing throttle you should expect to go rich as the air is shut off. Others with carbs and functioning AFR readouts will chime in with better experience regarding balance of idle circuit vs mains, but I'm pretty sure that your most meaningful information is from the WOT pulls watching the AFR's there. It's when the engine is loaded hard that you get the worst detonation if you go lean....

[Gint]

What's weird to my mind is that it goes stupid rich at cruise. I expected it to lean out some. Trailing throttle cruise sometimes goes as low as 10.5 briefly, usually around 11:1 though. It will go up sometimes as high as 12 at slight throttle cruise, but I expected that to be leaner as well.

Thoughts?

Bear in mind that at "cruise" with speed being less than about 60-65mph your throttle plates are barely cracked beyond idle. I found that out on my car since I'm on TP sensing for the SDS injection. My idle TP reading is 5 and WOT is 50. My car "cruises" at TP 5 on flat roads anywhere up to about 60mph. At TP 6 it's slowly accelerating, & gets up to about 66-70 before it stabilizes with enough road/wind resistance to balance the power. What this means is that these cars are so easily driven that normal cruising is just barely open from idle on the throttle plates.

That said, there's so little air being drawn through the carbs that the AFRs are going to fluctuate a lot--for instance with trailing throttle you should expect to go rich as the air is shut off. Others with carbs and functioning AFR readouts will chime in with better experience regarding balance of idle circuit vs mains, but I'm pretty sure that your most meaningful information is from the WOT pulls watching the AFR's there. It's when the engine is loaded hard that you get the worst detonation if you go lean....

What's interesting is that it doesn't idle real fat. 14.5 500 rpm cold, 13.4 warmed up idle at 1k rpm.

When I said trailing throttle I meant just off throttle cruise, not foot off the throttle entirely.

Everything I read indicated the car should go towards lean during cruise. I was surprised when it went rich. That's really the question here. The other factors I expect to change a fair bit once I've rebuilt the carbs. And then when I re-jet.