I have never tried this but I have seen guys set the idle jet mixture screws with a vacuum gauge. I have always set them by ear. Which is the best way?
I always use a vacuum gauge to set idle/air mixture, not just idle. Once I adjust idle air mix to the highest vacuum reading then I use my tach to set idle speed. The two aren't the same thing.
I really haven't paid attention to how I do it recently. I think I set them both to zero turns, then out 1.5 turns, then see how it runs. I might move them in or out (both the same number of turns) if it's not idling well, and see if it makes a difference in idle RPM. If so, I set them so they are out about 1/4 turn past the point where it has the highest idle rpm. Then reset idle rpm to what it wants to be for that car. It's been decades since I tried to use a vacuum gauge for setting idle. When I had a 2 gas exhaust analyzer, I used it often to set idle. I'd usually set it for lowest HC, then perhaps half a turn rich from there. Again, it depends on what the engine wants, if it would have any hesitation issues or want to die, I'd richen as needed.
Best is an exhaust analyzer. Barring that, a vacuum gauge and/or a tach is a good tool to develop the 'ear'. Vacuum = highest, smoothest reading RPM = highest RPM tending toward lean. If you tend rich, it will be a bit harder to start warm. Jim like it a bit richer than I do. Probably too many years of me trying to get everyone's junk to pass testing. Most single carbs should be around 3/4 to 1-1/2 turns out from lightly seated if throttle opening and float levels are right. Multi carbs tend to be a bit tighter in my experience. I haven't played with 4 corner carbs.
It would be an interesting test to setup the car using a vacuum gauge for best idle and so on, then hookup a wideband to see how they compare…..or don’t compare.
you'd probably find that it idles best at 12-13 afr, but less if it has more cam, more if it has less cam. Each engine has it's own sweet spot.
Vacuum gauge method works, but I like to use a tach and adjust for highest idle speed, then lean it out to achieve ~20 rpm drop in idle speed. And going over the process a couple of times helps to achieve smoothest transition from idle to off idle.
Anyone who did emissions testing back before closed loop was on all the cars did just this. You would adjust idle mixture to get the CO (rich-lean) in specs while also making sure the HC (misfire) didn't climb. Cams would make it a real balancing act. Anything that was less than ideal (worn valvetrain, vacuum leaks, weak ignition) would have to be tracked down. A vacuum gauge would be used to help diagnose valvetrain and would often still be connected when doing final adjustment. The old gas analizers didn't read NOx, just CO, HC C02 and 02. If the 02 was high, you had exhaust leaks or the probe wasn't far up enough.
I named my bathroom "Jim" so if anyone asks, I can honestly say I go to the "Jim" several times a day. If you name your vac gauge "Ear", you can honestly state that you set your idle by ear. I hope this response was helpful as I've grown to realize not all my advice is appreciated or viewed as being useful.
I was raised on a farm, Dad had 1930's farmall tractors, so I was always playing with the main jet in the carburetor. Plus the timing on the mag was adjustable from the seat. No gauges, all by "ear"
Thank you all for your help. The information available from all of you has been a big help to me. Great bunch of people
I just use the widebands' one in each side. It gets me very close and then just finish it off by drivability, because 1 sensor sampling 4 cylinders. At idle I only have 6" of vacuum. Not only can I set idle but also very light throttle, cruise and wot. Can also see accelerator pump and nozzle changes.
LOL. Most people don't realize that the name Jack is usually a nickname for John. I always went by Jack because John reminded me of a toilet.
An air/fuel mixture that's lean at idle isn't a problem, and usually keeps a car from having a smelly exhaust. Once off idle different circuits change the air/fuel mixture, so a lean issue isn't a problem unless it was off idle, not at idle. No reason to have a rich idle mixture.
Verify & Trust you gauge / gauge's , I have many gauges mechanical and Electric, When I am testing ,most of my gauges are quality Commercial grade . When using Mechanical I use large Face / Display. None dual purpose Vac for vac , Pressure for pressure. One of my Vac gauge is 6 or 8 inch face the increments are broke down to 10 hg per 1 hg of Vac. Not in my hand , it might be 20 per 1 hg? I tend to Over think
I like to set the carb by ear and then check with an AFR gauge, but I like to drive with an AFR gauge and a vacuum gauge. I rarely miss it by ear, but that comes from years of jetting at the race track.
Why I added the "lean roll" part of the setting to my post. Once you get the highest rpm/vacuum, then lean it out for ~20 rpm drop in idle. It reduces the gaseous smell at idle, and improves driveability, helps give a nice crisp response.
The smelly exhaust comes from HC, not CO. CO doesn't smell at all. CO is higher with a richer mixture. HC is the result of misfire, sometimes from lean mixture, sometimes from other things (like lots of overlap on the cam). HC usually goes up with CO when it gets really rich. The trick is finding the HC "dip" and getting on the right side of it to keep the engine running good off idle. Transitions don't happen suddenly, the idle mixture has a lot to do with how it transitions to off idle. If you can get it to run nice and not stumble with a slight lean drop, then great, run it that way. if you can't, then you might find that a bit richer covers up the problem without having to try to modify the carb in other ways. Using a stock OEM carb on an engine with a cam, that can be a problem. With aftermarket carbs, it usually is not.
Keep in mind that changing the idle speed will change the needed mixture settings . Setting with a vacuum gauge works best when left a bit rich , IME
I like aftermarket Quick Fuel or Holley carbs with all the bells and whistles on them for a hotrod. It allows me to change everything to eventually get the carb to idle well, accelerate well, and cruise well. For my daily driver '69 Suburban I use a plain old Holley 1850 600cfm vac. secondary carb as it came from Holley. Just set the idle mixture and forgot it for a decade now. The Quick Fuel on my '39 Chev has every option a Holley style carb can have, and I've tuned every aspect of that carb to make it start perfectly, and accelerate great, plus cruise down the highway smooth, and still get great mileage for a pretty high performance SBC.
Back when I worked at the Honda motorcycle shop (um 50+ years ago), we used 4 vacuum gauges to synchronize the 4 carbs on the 4-cylinder bikes. It was real fun on the first year 750 model as they used 4 throttle cables. As the cables stretched differently, always had a bike to re-do. Engineering fix the second year took care of it.
I guess I just gotta be different! First question to customer "is the engine stock"? If yes, then: (1) determine engine condition (1a) fresh, less than 1000 miles (1b) good running engine (1c) not so good running engine (low compression, oil burner, etc.) (2) determine carburetor "idle adjustment range" from manufacturer's literature (2a) most carburetors made prior to 1968 will have a range of 0 to 1 1/2 turns (there are exceptions) (2b) most OT carburetors 1968 and newer will have a range of 1 1/2 to 3 1/2 turns (think about what federal law took effect for the 1968 model year). (3) divide the idle adjustment range in thirds Example: if the range is 0 ~ 1 1/2 turns: then 1/3 is 1/2 turn 2/3 is 1 turn 3/3 is 1 1/2 turns Set INITIAL idle based on range and engine condition: Fresh engine 2/3 Good running engine 1/3 Worn out engine 3/3 So for our example: Fresh engine - 1 turn Once engine is broken in - 1/2 turn Worn out engine 1 1/2 turns If engine is not stock, then the above procedure will need modification. One caviat: if you are going to be twisting the screws, know what you are doing. Virtually ALL carburetors made in the USA after 1950 will adjust in (clockwise) to lean, out (counter clockwise) to richen. But many carburetor made prior to 1950 have an idle circuit that meters air ONLY, so this type of carburetor will be in (clockwise) to richen, out (counter clockwise) to lean. Jon
I believe the idle adjustment screws on Holley 4V carbs on Mousetangs 1983 to 85 are also "counter clockwise to lean" due to emissions regulations forcing Holley to make them air circuits vs. fuel circuits.
Turn the screws in small increments and give it a few seconds before making another adjustment, it takes a couple seconds to work it's way through the carb. The vacuum gauge is a good tool to use, then you can use your ears and hands to dial it in further, listen to the sound of the engine and feel the smoothness.
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