Timing Effect on Oxygen Content
filed in 1966 Mustang
Question: Got a quick query. How much effect does timing have on oxygen content? I’m trying to dial in an engine and am using an O2 sensor. It’s a small block Chevy V8 and the sensor reads rich, which is very much what I’d expected re: rich smell, fouled plugs, atrocious mileage. The engine’s new and the carb’s used but in excellent (visual) condition. The carb’s a 650cfm double pump Holley that’s in very new condition (used but still in original box) but got it from my machinist who drag races. The jets indicate stock but who knows if somebody drilled them. The distributor’s a Mallory Comp 9000 and at this point who knows what the spark curve’s set to. So, as I try to dial in the spark curve and fuel, I’m wondering, “Can the timing curve significantly change O2? Am I trying to dial in the wrong system first?”
Answer: Oxygen levels in combustion gases are as much a function of combustion efficiency as they are applied fuel mixture. I use an LM-1 to tune my car. Lower numbers indicate a richer mixture, and high numbers mean a lean mixture. The magic 14.7:1 will provide a good balance of power and economy for a near stock engine. If you’re using a radical cam, big heads, etc, you’ll need to run a little richer for smooth operation. Best power production is generally 13-13.5:1 or so. I’ve set mine up to have a mixture of about 13.8-14.0 at light throttle and cruise, and around 13.0-13.2 under power.
Here are two things I’ve noticed: 1. Yes, timing curve does affect sensor readings. 2. If the mixture is very rich, the reading actually goes up – looking lean. If you add fuel, the readings drop (as you would expect). If you add even more fuel, the numbers start coming back up. Why does that happen? I don’t know, but it does. If you’re indicating a near normal reading (13:1 or higher), but it still smells rich and fouls the plugs, use a smaller jet. If the carb was used on any sort of a “drag” motor… then it probably has the wrong power valve in it (or the power valve is damaged); and the power valve is opening during part throttle cruise. There’s a description of how the power valve works here: http://www.chevyhiperformance.com/techarticles/74058_carburetor_valve…
“See if this description sounds familiar: Your Holley-carbureted street car idles terribly, has taken to fouling spark plugs every week or so, the gas mileage absolutely sucks, and there’s an ominous black cloud swirling from your exhaust.”
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