Idle Mixture Screw
- Thread starter shew01
- Start date
On a Zenith carb (as OP has), yes. You are correct.
shew01
Well-known member
Roughly how miles will it take to “color” the plugs lean or rich? 25? 100?
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ABOUT PLUGS FOULING:
If plugs are fouled do not start replacing plugs.
1. Extended Idling (10 to 20 minutes) with no other driving will make CENTER electrodes carbon black. Outside steel ring of plugs will mostly remain carbon black.
Concern is with the center white Porcelain electrode NOT the steel part of plug.
2. A series of short drives like to store and shutting down until next day or for several hours and continuing short trips and/or idling some more will foul plugs.
3. My experience is that after plugs start to foul all that you need to DO IS TO drive the car for 10 or 12 miles "at speed" That is 6 to 8 miles at 30 to 35 mph with a stretch of 2 or 3 miles at 45 to 50 most times will clear fouling.
Longer trips will clean plugs even more.
Do not expect a correct reading of plugs after extended idling or a series of short trips.
60 years ago we called this procedure "Blowing out the carbon" and was needed with all cars of the day!
If plugs are fouled do not start replacing plugs.
1. Extended Idling (10 to 20 minutes) with no other driving will make CENTER electrodes carbon black. Outside steel ring of plugs will mostly remain carbon black.
Concern is with the center white Porcelain electrode NOT the steel part of plug.
2. A series of short drives like to store and shutting down until next day or for several hours and continuing short trips and/or idling some more will foul plugs.
3. My experience is that after plugs start to foul all that you need to DO IS TO drive the car for 10 or 12 miles "at speed" That is 6 to 8 miles at 30 to 35 mph with a stretch of 2 or 3 miles at 45 to 50 most times will clear fouling.
Longer trips will clean plugs even more.
Do not expect a correct reading of plugs after extended idling or a series of short trips.
60 years ago we called this procedure "Blowing out the carbon" and was needed with all cars of the day!
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Big hammer
Well-known member
50 + years ago when I was a teenager, family friend asked me to wash their late 60’s Thunderbird, why yes I said ! After washing their car you have to blow the water off ! Later she said the Thunderbird was running great ! I had blown red brownish out of the tailpipe, we called it “ blowing the cobs out” . 390 4 barrel !!!!
David in San Antonio
Well-known member
Also known as an Italian Tuneup. Probably because American drivers were accustomed to the torquey, low rpm engines and didn’t wind their new “foreign jobs” to higher revs. Jeez, it must be decades since I’ve heard the term “foreign job” to mean an imported car.
Ayers1
Well-known member
Back in 73 my dad bought a new Datsun pickup. After he had it for about a month it wasn't as peppy as it had been so he took it back to the dealer. The salesman took a ride with him and told him he was driving it wrong. Told him he had to drive it like a diesel to keep the carbon from building up in the engine. After that dad would wind it up between shifts and it was fine.
shew01
Well-known member
Thanks for the post. I recall stories about “blowing out the carbon” from when I was a kid, but I never really thought about that for Model As. I actually thought that might have been mostly an excuse for having a joy ride. ;-)
Here are my new plugs with about 100 miles on them. What do you think? Too lean?
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Fullraceflathead
Well-known member
That light amount of tan on the center electrode is the correct color for a good mixture.The reason you're seeing a lot of white is because you only have 100 miles on them. It appears to be burning pretty good to me. If you're worried it's a little lean open the GAV another 1/8th of a turn.
Typically if it’s running lean you will tend to have a flat spot on acceleration especially in high gear under a load.
Typically if it’s running lean you will tend to have a flat spot on acceleration especially in high gear under a load.
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Shew01:
With the vacuum gauge hooked up, you turned the idle air adjustment screw full in and the car dies (as it should). Correct?
With the wiper line hooked up, you turned the idle air adjustment screw full in and the car continues to run (as it should not). Correct?
With the vacuum gauge hooked up, you turned the idle air adjustment screw full in and the car dies (as it should). Correct?
With the wiper line hooked up, you turned the idle air adjustment screw full in and the car continues to run (as it should not). Correct?
shew01
Well-known member
I need to go back and do that. I didn’t use a torque wrench.
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shew01
Well-known member
That is true, but it should be an issue this time for me. I replaced everything from the shutoff valve to the carburetor. I added a filter on the shutoff valve as well.
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shew01
Well-known member
Shew01, thanks for posting back. I enjoy posting on these issues. It helps me retain the information I have learned on this forum and other sources:
Suggest you plug the wiper line, as suggested in one of the above posts. This will eliminate the vacuum port as an issue. The plug is a pipe thread and is available at hardware stores. Take the vacuum gauge for a match???
(Many carb issues often turn out to be electrical in nature i.e. weak spark for instance. We are assuming that your points are clean and set at ~.020 gap, and your timing is correct. Grounds are clean, bright and tight and so are all connections from the battery through the primary ignition and back to the battery + pos post. Battery is at sufficient charge and the generator / alternator is putting out ~7.2 volts at high idle)
There are several posts above that describe the procedure for setting the base idle. You can remove the malfunctioning throttle linkage at the carburetor throttle plate to set the base idle. Set the idle air adjustment screw to 1-1/2 turns open. Warm the car at idle for 15 minutes or so to reach op temperatures. Retard the spark and hook up your tach. Set the idle as low as you can, 350 - 400. Open and close the GAV and it should not have an effect. Gently turn idle air adjustment screw in all the way. The engine should stumble or may die. If it dies, reset to 1-1/2 open and restart. Test by opening to 3-1/2 - 4 turns. The engine should at least run poorer. If it does, set the adjustment in between. This likely will be close to 1-1/2 turns open, 1-1/4, 1-3/4 etc.
Advance the spark a few notches and check the tach. Hook up your throttle linkage. 500 - 550 will be alright. It is a bit high on a well sorted engine, but the stalling at a stop should be gone. If not, report back and we can proceed from there.
There is a very thorough article on the FordGarage that discusses the 'stalling at a stop' issue.
Suggest you plug the wiper line, as suggested in one of the above posts. This will eliminate the vacuum port as an issue. The plug is a pipe thread and is available at hardware stores. Take the vacuum gauge for a match???
(Many carb issues often turn out to be electrical in nature i.e. weak spark for instance. We are assuming that your points are clean and set at ~.020 gap, and your timing is correct. Grounds are clean, bright and tight and so are all connections from the battery through the primary ignition and back to the battery + pos post. Battery is at sufficient charge and the generator / alternator is putting out ~7.2 volts at high idle)
There are several posts above that describe the procedure for setting the base idle. You can remove the malfunctioning throttle linkage at the carburetor throttle plate to set the base idle. Set the idle air adjustment screw to 1-1/2 turns open. Warm the car at idle for 15 minutes or so to reach op temperatures. Retard the spark and hook up your tach. Set the idle as low as you can, 350 - 400. Open and close the GAV and it should not have an effect. Gently turn idle air adjustment screw in all the way. The engine should stumble or may die. If it dies, reset to 1-1/2 open and restart. Test by opening to 3-1/2 - 4 turns. The engine should at least run poorer. If it does, set the adjustment in between. This likely will be close to 1-1/2 turns open, 1-1/4, 1-3/4 etc.
Advance the spark a few notches and check the tach. Hook up your throttle linkage. 500 - 550 will be alright. It is a bit high on a well sorted engine, but the stalling at a stop should be gone. If not, report back and we can proceed from there.
There is a very thorough article on the FordGarage that discusses the 'stalling at a stop' issue.
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