General Category > Quadrajet Carb Talk and Tips
Off idle lean condition, suggestions?
novadude:
There is absolutely no concern with being lean at part throttle / light load if it is not causing drivability issues. Zero engine damage will occur.
In fact, every text book about engines written in the "pre-emissions" era suggested a target A/F of 16-17:1 for best economy if the engine will tolerate it without misfire/ surging.
Personally, I've dialed on my own Q-jet to go down the highway at 15.5-16:1 (level load, steady cruise). The only time you need to worry about "too lean" is under heavy load. Mine will trend even leaner (high-16s) as you lean into the throttle, until you get the point where the power piston spring overcomes vacuum and lifts the primary rods. In my case, it took some experimenting with PP springs to get there.
Lean keeps the oil and spark plugs cleaner, and also has the benefit of keeping more gas money in your wallet. LOL
Cadman-iac:
Thank you gentlemen,
I appreciate your support. Novadude, thanks for the explanation about the acceptable range for the AFR during certain conditions. I had noticed that it would lean out as the throttle tipped in and then would richen up as the power piston raised.
What is really worrying is when the AFR drops beyond what the gage can register at 18.0 and I can tell it's starting to misfire from being too lean at that point.
I've been trying find the specific parameters where this occurs as it's not often, but I have seen it a few times.
It's mainly when I'm accelerating slowly, but I've got to do it a few times to make sure I can duplicate it and explain exactly what the conditions are.
When I first posted this question I had not noticed this particular situation, I thought I only had an issue with the initial startup with the choke on.
But at least I now know that a 17.0 with a light throttle is nothing to worry about. Thank you for that.
Rick
Cliff Ruggles:
In any and all cases tune for results vs what a gauge is telling you. At very light engine load nothing at all wrong with being lean provided there is no power loss, engine miss-fire, going "flat" on power production, etc.
I'd be interested to know what the distributor is doing thru all this and how it is set-up. It's a big player in tuning, especially at light engine load as it uses a vacuum advance to add timing to effectively burn a lean mixture....
Cadman-iac:
Cliff,
I've got the distributor set up for a maximum of 41 degrees advance, 10° initial, 16° mechanical, and 15° vacuum.
The mechanical is all in at 3000 rpm, and I'm using manifold vacuum to the advance unit. It's an HEI unit.
I just ran a 200-mile round trip to Tucson and back yesterday, and it seemed to be fine except for a slight fuel starvation issue due to the stuff I have in the supply line. I've got a shut-off solenoid and an electric pump about midway between the tank and the mechanical pump. When I noticed the problem, I turned on the electric pump, and that solved it.
I think I'm going to relocate the solenoid to above the mechanical pump, not my preferred location due to it being seen easily, but better than letting it cause a problem. (Down here on the border; it's necessary if I want to keep it in my possession).
I did have a heat issue coming back because I was running the air conditioning, but it never overheated, just increased by about 15-20 degrees. I'm running a 180 stat, and the gages registered the normal temperature for when it had a 195 stat, so I'm attributing it to the extra load and the steady rise in elevation between Tucson and here.
The engine has just over 1350 miles on it now, still using a slight amount of oil, I believe, due to the Moly rings. I pulled a couple of plugs just to check and they looked great, a slight brown coloring in spots, which I believe is normal.
I did notice that while cruising at 75, at times the AFR would run upwards to 16.0, but if I gave it a little throttle it dropped back down into the richer zones. I'm getting used to seeing the gage fluctuate a lot and I'm paying attention to the circumstances at any given time.
Pulling, accelerating, the AFR is always in the 10.5 to 14.0 range, depending on how many toes I've got shoved into the carburetor.
On the distributor specs, is what I've provided enough, or do you need more specific details? If so, please let me know and I'll find out and post it here for you.
Thanks for your help and expertise I really appreciate it.
Rick
77cruiser:
You need about 10 more degrees. Put the initial up to about 15.
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