Carter YFA Question
Senior User
Joined: Jul 2014
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you need a vacuum gauge and tune to the highest vacuum (and then ever so slightly leaner) (obviously you want the engine to be fully warmed up)
anything else is just wrong way to do it.
Senior User
Joined: Jul 2014
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it should be around
1 1/4 turns (really lean)
1 3/4 turns (REALLY RICH)
you need a vacuum gauge and tune to the highest vacuum (and then ever so slightly leaner) (obviously you want the engine to be fully warmed up)
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
Whatever the initial starting point recommendation for mixture settings on a carb - 2 and 1/4, 1 and 1/4 and such, is just a nominal setting to get in the ballpark so it will run. It will be "close" but only kinda close. A vacuum gauge and tach works slick.
Save the mixture adjustment for last, when everything else is done (timing, for example). When using the gauge you'll see the vacuum indication increase when adjusting, as well as idle speed. The idea is to adjust for the highest vacuum, going back and forth, also adjusting the idle RPM screw down/up as required to spec. Very small mixture adjustments, just a hair of a turn, makes for a big difference so it's worth taking the time to do it right. Just slightly leaner than max vacuum is optimum as Patrick mentions.
Save the mixture adjustment for last, when everything else is done (timing, for example). When using the gauge you'll see the vacuum indication increase when adjusting, as well as idle speed. The idea is to adjust for the highest vacuum, going back and forth, also adjusting the idle RPM screw down/up as required to spec. Very small mixture adjustments, just a hair of a turn, makes for a big difference so it's worth taking the time to do it right. Just slightly leaner than max vacuum is optimum as Patrick mentions.
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FTE Legend
Joined: Jul 2010
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From: Northeast, OK
While it is true that in is leaner on a YFA, that is not true of all carbs. There are many emissions carbs that have clockwise as richer on the idle air screws. And, there are carbs with left-hand threads on the idle air screws so that clockwise is really out. So blanket statements on idle air screws can be a problem.
As for adjusting the idle, Dad taught me to do it w/o a tach or a vacuum gauge. And while that was about 60 years ago it still works today. They are nice tools and I have them, but they are not necessary.
As for adjusting the idle, Dad taught me to do it w/o a tach or a vacuum gauge. And while that was about 60 years ago it still works today. They are nice tools and I have them, but they are not necessary.
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FTE Legend
Joined: Jul 2010
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From: Northeast, OK
Get it running and turn the idle air screw in a quarter of a turn at a time. Turn it in until the RPM drops. Note the position and turn it out, a quarter turn at a time, until the RPM drops. Go back in until it drops. Split the difference between the point it drops when turned in and turned out, and then open it up 1/4 turn.
The last 1/4 turn out should sometimes be in, or not at all. But Dad taught me out. The issue is that each engine is different, but in general they like to be a bit richer than perfect more than they like to be lean. However, you can play with the last quarter turn and see what works best for your engine.
The last 1/4 turn out should sometimes be in, or not at all. But Dad taught me out. The issue is that each engine is different, but in general they like to be a bit richer than perfect more than they like to be lean. However, you can play with the last quarter turn and see what works best for your engine.
Senior User
Joined: Jul 2014
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I think it is bad idea to tell people, (who don't know what they are doing), to do it without the tools that any decent mechanic will use.
A vacuum gauge costs 20 dollars from amazon. If you have a car and cannot afford that then what can I tell you?
A vacuum gauge costs 20 dollars from amazon. If you have a car and cannot afford that then what can I tell you?
Senior User
Joined: Jul 2014
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A good vacuum gauge/fuel pressure gauge can save so much time and effort when fixing an unknown problem. Especially with our older cars, a vacuum gauge is so easy and cheap...
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
A vacuum gauge may be easy and cheap, but it isn't necessary. Dad taught me how to set the idle on a carb when I was about 10 (1957 if you are wondering), long before he had a vacuum gauge, and we used that method for many years and on the cars of lots of friends and family. Successfully I might add.
The idle mix on a carb doesn't affect the air/fuel ratio at all once you get the throttle open enough to get the vehicle to 25 MPH or so, and only marginally up to that point. So assuming the engine isn't going to be sitting around idling all day the main requirement of the idle mix is that it allows the engine to run reasonably well and not die when you drop the tranny in gear or the A/C comes on. In other words, it isn't critical and most people on here should be able to tune their idle mix by ear. It may not be perfect, but it'll do.
The idle mix on a carb doesn't affect the air/fuel ratio at all once you get the throttle open enough to get the vehicle to 25 MPH or so, and only marginally up to that point. So assuming the engine isn't going to be sitting around idling all day the main requirement of the idle mix is that it allows the engine to run reasonably well and not die when you drop the tranny in gear or the A/C comes on. In other words, it isn't critical and most people on here should be able to tune their idle mix by ear. It may not be perfect, but it'll do.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
"To each his own."
Generally though have found over the years it's better to learn a given procedure "by the book" first, becoming familiarized with exactly what's going on and how it affects other things and then with experience, deciding what matters and what doesn't.
This is the best method of instruction too. (In my opinion) Set the example, and they can learn shortcuts on their own. We all do, thankfully.
Incidentally a vacuum gauge is easily one of the most useful yet inexpensive diagnostic tools for the mechanic to own. Also known as "the poor man's Sun machine." I don't know why anyone would be without one, especially on the vintage iron.
Generally though have found over the years it's better to learn a given procedure "by the book" first, becoming familiarized with exactly what's going on and how it affects other things and then with experience, deciding what matters and what doesn't.
This is the best method of instruction too. (In my opinion) Set the example, and they can learn shortcuts on their own. We all do, thankfully.
Incidentally a vacuum gauge is easily one of the most useful yet inexpensive diagnostic tools for the mechanic to own. Also known as "the poor man's Sun machine." I don't know why anyone would be without one, especially on the vintage iron.