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

[Gint]

BTW, I bought the AEM wideband AFR gauge setup from Chris. Nice unit, great deal.

AEM AFR gauge classified ad on 914world

[Mark Henry]

Post all the specs on the carbs and engine

carb size
Main Venturis
Main Jets
Idle Jets
Emulsion Tubes
Air Correctors - Main circuit
air correctors - Progression Circuit

[michael7810]

I have the same problem. From a small load to pedal to the metal the AF is 12.5-13.5. When at cruise with no hills it's good above 3k rpm but up to 3K the AF is 9.9-10.5. When I give it just a little more throttle it leans out to 13-ish. My numbers (by memory as I'm on travel)
60Idle; 135 main; 220 Air, 40 IDF with 32mm vents. Can't remember the other numbers. The emusion tube is "stock" according to the Weber book but I can't remember the number.

[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....

[LotusJoe]

You might check out the information on this site.

http://www.daytona-sensors.com/tech_tuning.html

According to the WEGO page looks like your not that far off.

[Gint]

Post all the specs on the carbs and engine

carb size
Main Venturis
Main Jets
Idle Jets
Emulsion Tubes
Air Correctors - Main circuit
air correctors - Progression Circuit

I added this to the first post.

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

[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.

[Gint]

When at cruise with no hills it's good above 3k rpm but up to 3K the AF is 9.9-10.5. When I give it just a little more throttle it leans out to 13-ish.

I think that's primarily because as you give it more throttle it gets more air as well. I would think that would be expected. Amateur swag at best though. Again, I was just surprised to see such a fat cruise.

[McMark]

If your venturis are too small, you'll get into the main jet earlier due to the higher vacuum in the throat. This might be happening.

[Trekkor]

I have the same a/f gauge.

13.1 at WOT under load up hill on track.

I didn't pay attention to anything else...


KT

[Gint]

I got some 32's from ACE. They'll go in after I get the carbs rebuilt and baselined again.

[ww914]

Mike

I don't know if this will help you or not, but after installing my Innovate MTXL A/F gauge, I have been playing with different configurations. I am running a McMark 2270 with 40 DRLAs. This is what I ended up with and it is running very strong:

34 venturis
145 main jets
75 idle jets
50 pump jets
180 air corrections jets
9164,2 emulsion tubes

Here's a shot of one run:


Good luck, Warren

[bulitt]

Also, the lower the rpm the more retarded the ignition is.
At high rpm's your ignition is firing well before tdc resulting in a better burn.
As your ignition retards the spark fires closer to tdc so you may not get a complete burn and some fuel may get pushed into the exhaust resulting in a richer reading.

[Gint]

Thanks Warren! That's cool.

Yeah, it occurs to me that on the trailing throttle at cruise might be rich with carbs because you're still on the gas, but the throttle plates are closing off air intake. I'm curious what FI does in the same situation. Say D or L jet just to keep it simple.

I'm looking forward to rebuilding and then re-jetting the carbs now that I have a baseline. It's more fun when you can actually see data and compare.

[72hardtop]

Your idle jet is to big. Drop to a 50 idle and adjust LBI to 13.0

At part throttle light load condition your on the progression (idle circuit).

On a 2.0L you should be:

28mm vent
50 idle jet
120 main (maybe 125)
180 air
F11 tube

[stugray]

Here is an interesting way to view data:

http://datazap.me/u/stugray/914?log=0&...;zoom=1716-4499

That should show my AFR for a ~10 minute drive around.
Sometimes you have to click and drag on the graph to get it to regenerate & show the lines
If you click on the EGTs you can see them too, but then it autoscales so you cant makeout the AFRs.

They seemed to be in the 11-13 range most of the time.

I'll get my carb parameters tomorrow.
They are dual 40 IDFs on a 2056 with a webcam 86b & ~9.5:1 CR.