Setting the air/fuel mixture

[Racemaster]

Ok, fighting with air/fuel mixtures. I have the meter for setting the matching air flow but getting the air/fuel mixture just right is a big difficult.
I had planned on doing it with exhaust temps, using a laser temp gun on the exhaust manifold. That works ok for the first few minutes but once that cast iron exhaust manifold gets warm it doesn't change temps much.
Let's hear what other people are doing to get the air/fuel mixtures matched (2 carb 110hp engine).

[bbodie52]

Here is some discussion on the subject. It seems to say 1½ turns from bottoming the idle mixture screw on each carburetor. Beyond that, either Fahgettaboudit, or play with it until you get tired, and then return to the 1½ turns from bottom setting and Fahgettaboudit.


http://corvaircenter.com/phorum/read.php?1,155467

Here is the 1965 Shop Manual version...

[66CorsaConv]

Until I replaced the throttle shafts and plugged up allot of vacuum leaka and throttle blade bypass leaks, I could not get the mixture screws to do much of anything

I agree with Brad on the 1 1/2 turns.

Now for a Ford trick out of the mid early 70s. Lean Drop Method. Get a propane torch, plain hand held kind. Ford had us back out the mixture screws until rpm would no longer go up. Then gradually turn them in to lean it out.

When you pass a NON burning propane torch over the carburetor... & if the RPM picks up, It is lean.

The Ford method called for a 50 RPM increase in idle speed with a waft of propane.

I have used the propane ever since. But I set for NO rpm increase. I do use propane to find the slightly lean side of adjustment.

[Jerry Whitt]

Several other methods are available.

1. Adjust the idle mix screws until the highest idle speed is reached.

2. Since the engine is an air pump, measure its function. Use 2 vacuum gauges. One hooked to intake manifold vacuum drivers side, other to passenger side. Adjust the idle mix screws until the highest vacuum reading is reached on each side. You will then have to adjust idle speed and balance.

3. Take a 6 pack to your local smog shop. Explain what you want. He can put a probe into the exhaust, and if you have dual exhaust, even better. He can measure the
hydrocarbons, carbon monoxide, oxygen and oxides of nitrogen. Certain numbers are appropriate and the smog tech will explain.

4. Next is the "full body test". In this test, lean against the car. Use your ears to sense lower or higher idle speed as mixture screws are adjusted. Use your body "feel" for the smoothness. The better the feel and sound, usually a better mixture.

[miniman82]

Wideband.

[Racemaster]

I've set carbs by "feel" for years, no exact science there. I used the propane torch method years ago on a straight six engine but forgot about that idea. Maybe I will play with that again. I wish I had somebody locally with a sniffer (smog machine) to actually let me set the mixtures the same at least. I've got mine set by ear and by using a vac gauge on the carbs but I still feel like the driver side is off based on the sound of the exhaust. This task will become even more fun when I set up my 4 carb engine in one of my Rampsides. :-)

[azdave]

This task will become even more fun when I set up my 4 carb engine in one of my Rampsides. :-)

Normal 4-carb setups only add two secondaries that don't have idle speed, idle mix or chokes to set up so what is harder about that?
Granted you have two more carbs to maintain and all but the actual part of dialing them in is nearly identical.

[Racemaster]

You are correct, they don't have air/fuel mixture screws. I forgot about that.