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  1. #1

    Default adjusting carb with vacuum gauge

    which vacuum source would i hook a vacuum gauge up to to adjust my carb? my carb is a 1946 holley

  2. #2

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    If you are looking to tune your idle mix, I have always heard of attaching it to any intake vacuum source. High vacuum at idle indicates lean, low vacuum indicates rich (or a vacuum leak- watch out for those!). A wandering needle at idle indicates a varying fuel mix and signals a carburetor problem. A vibrating needle indicates worn valve guides, leaky valves, or an ignition problem. A steady drop of the needle indicates a problem isolated to 1 cylinder. Intermittent flicks of the vacuum needle indicate a sticking valve or ignition miss. As for tuning the carb, you can always do it by ear and feel. Screw the needle in and listen for idle gain. If there is idle gain, you were to rich. Listen for the idle to start to drop off or ping or feel for misfires. That indicates you are now in "too lean" territory. If there is no idle gain from screwing in the needle and the idle seems to get worse, back the needle out slowly and listen for idle gain. Once the idle gets to it's best quality and begins to get rough, you are getting into "too rich" territory. paly with the idle screw until you find a spot where the idle is the best under all weather conditions. What may be best for today, won't for this winter, but there is middle ground. Ford used what is known as the "propane gain" technique to tune your carb. The technician would use a propane injector into the air cleaner and watch the idle rpms and quality to adjust the idle mix. What I described above is basically the same thing, without propane.
    86 Capri GS 2.3L
    99 Escort LX sedan 2.0L
    11 Ranger XL 2.3L




    11 Years of Foureyedpride.com

  3. #3

    Default

    the needle is steady. it does't wiggle or anything. idle does raise when i screw it in. and it chugs when i back it out. would the idle speed be in park or in drive?

  4. #4

    Default

    Since you're working with an automatic, the idle rpm should be adjusted with the engine warm, all accessories (especially the A/C if equipped) and the transmission in drive. Your emissions label should show what RPM to set it at. Also, make sure your timing is set correctly. Advanced timing raises rpm. Retarded timing lowers RPM. Base idle should be checked with the engine warm, vacuum advance line plugged, and in drive at idle. Sounds like you're close to getting everything tuned right. If the idle raises when you screw it in, it means you are slightly rich, which isn't bad because you want to be right in that range. Just so long as when you screw it out a little it doesn't start to run rough. Then you're too rich if that happens.
    86 Capri GS 2.3L
    99 Escort LX sedan 2.0L
    11 Ranger XL 2.3L




    11 Years of Foureyedpride.com

  5. #5

    Default

    i can screw it out about a turn and a half before it gets rough. and it runs good it idles about 1000 in park and about 700 in drive. and i don't have a/c.

  6. #6

    Default

    Sounds good to me. As long as you're happy with it and it doesn't stall or run rough, I'd say you're fine. Not having A/C removes some of the complexity of setting your idle speed. On A/C equipped vehicles, a "vacuum operated throttle modulator" is added. Basically, what it does is boost the idle when the A/C is turned on. Without having A/C, you don't have that part and don't need to worry about getting your idle speed messed up by it. When you're out driving around just make sure you don't hear pinging. If you do, make sure your EGR system is working and your timing is set correctly. If you're not experiencing any ping, I would say that you have everything tuned out just right.
    86 Capri GS 2.3L
    99 Escort LX sedan 2.0L
    11 Ranger XL 2.3L




    11 Years of Foureyedpride.com

  7. #7

    Default

    ok i will put this knowledge to work in the morning if i have any problem i will ask. thanks for the info.

  8. #8

    Default

    No problem. Basically what it all comes down to with carburetors is striking that balance. Carburetors, unlike fuel injection, has no way to compensate for changing weather conditions (it does to a point, but not much), engine conditions, and driving habits. Basically, what you need to do is just get it to where it runs and drives well for you. If I were to tune my carb to the same environmental conditions as you, here in PA, my car wouldn't run well. Of course, having all the settings correct (idle, timing, carburetor stuff) is a must, making small adjustements here and there to better suit your climate and driving habits is fine.
    86 Capri GS 2.3L
    99 Escort LX sedan 2.0L
    11 Ranger XL 2.3L




    11 Years of Foureyedpride.com

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