o2 sensors to dial in carbs

[ggenovez]

Anybody ever used an O2 sensor to dial in their carbs? I havd a dual zenith setup and a working o2 sensor I replaced prematurely from my other vehicle.

I was wondering what the temperature at the pipe is and if anybody has done this before?

Thx

[david s poole]

best to use a heated one so as to get temp up to 300c but great idea will work well.

[riethoven]

Ther used to be a setup called "Colortune" for syncing multiple carbs. They were big with the Italian car guys with all those Weber sidedraft and downdraft carbs.

[Mike D]

Go here http://www.bob2000.com/carb.htm . Down about 3/4s of the page is an easy to read chart. The entire article is good technical background for any type of carburetion.

Sync'ing by fuel mixture can not be done without sync'ing by flow first. You are going to be going back and forth adjusting the flow, then the mixture, then the flow and on and on. When you get it just right, that baby will scream!

Make sure your linkages are all good to go. No worn bushings, bent levers or the like. I've been through this with a pair of Solex's on an old 220. It can drive you nuts if all your syncing is undone by a bad bushing which allows the rear carb lever to slip at full throttle. Personal experience!

[ctaylor738]

In theory, it's possible. The CO spec for post-1970 US cars is 1.0-1.5% which is at the high-end where a sensor will read. But wouldn't you need two sensors, one in each down pipe?

My concern is that with well-worn carbs, you would have trouble getting the engine to idle at 1.5% CO and the sensors would be maxed out and not tell you anything.

[Mike D]

Yes, you do need an O2 sensor in each down pipe. Install them as close to the exhaust manifold as possible . If you have to install them more than 12" away use a three wire heated O2 sensor.

The idle Co2 reading is moot. When you exceed about 1500 rpms the secondary or cruise cycle takes over. A high (rich) reading at idle which is what you will have is replaced/cleansed by the incoming rush of air and the O2 sensor reads the temperature now being created in the exhaust.

You should not try to set the idle using the O2 sensor. Use vacuum gauges and set for best idle. It will probably read about 900 to 1000 millivolts on the O2(rich) but it will lean out when you change circuits. This is why American Benz's use a vacuum retard at idle. The late spark cleanses the exhaust and allows it to meet EPA specs.

Use the O2 and flow meters to adjust at 1500, 2500 and 3500 rpms. Run the car for a few seconds at each speed to stabilize the readings.These are your low speed, town cruise and highway cruise speeds. You probably should include 4500 RPM but running the engine at 3500 is plenty scary. Go for the best average between the levels. You will never probably get them exact but you can get them close.

You'll need two multimeters, a tach, two vacuum gauges and preferably two flow meters. Alternate the meters and gauges to average out their tolerances.

[ctaylor738]

But what are you going to do to "tune" the mixture at the higher RPM? Change jets, drill out orfices? The only mixture adjustment I am aware of on the Zenith is at idle. If you know of another, please enlighten us!

My thinking was that the best you could do would be to fine-tune the synchronization and and the idle mixture.

[Matt L]

Several companies sell sensors and dash gauges to indicate the mixture while running. Summit sells a few brands. You can use a high-impedence voltmeter instead of their expensive gauge as well.

I agree that they're not as useful as a vacuum gauge at idle, but at high speed they tell a better story. And yes, you'd drill or change the jets to adjust the high-speed circuits.

[ggenovez]

Well,

I was hoping to be able to weld a bung on an extender piece and attach it to the end of the tailpipe.

Either use a regular (propane torch to heat as nessesairy) or heated O2 sensor. Didn't the dealership do something like this to dial in the carbs?

[Mike D]

You are right on the sync'ing and lack of adjustments Chuck. That is why I emphasized the flow meters and the use of the O2 sensors. I also stressed "average".

The Zeniths are "fixed orifice" carbs and as such the next step in adjustment once you get the sync as close as possible would be to change jetting or drilling out the existing jets. This is not that unusual of a step. Those carbs were designed for even cylinders and now 100K+ miles later the cylinders have worn different patterns.

I am presently going through this very process fine tuning a Rochester Quadrajet for my '73 280 M110. I do have the advantage of the excellent metering rod system which Rochester uses.

Yes Ron, I AM taking pictures and will post them at the end of the process.

The O2 sensor on the end of the exhaust is a waste of time. You aren't sync'ing carbs,you are reading total accumulative exhaust at that point.