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Thread: 350 SBC 6-71 Blower Carburetor Adjustments
          
   
   

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  1. #1
    amgnvan's Avatar
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    350 SBC 6-71 Blower Carburetor Adjustments

     



    Hi,

    I have a 350 SBC with a Weiand 6-71 supercharger that I am trying to get fine tuned.

    Firstly, some specs about the motor:

    -Stock short block (I know, will be replaced this winter)

    -AFR 195 heads
    https://www.summitracing.com/parts/AFR-1036

    -COMP XE-284H camshaft
    12-250-3 - XTREME Energy

    -Brawler 650 supercharger carburetors
    https://rdracinginc.com/product.sc?p...0&categoryId=2


    I have an AFR gauge installed, and it was reading very rich (i.e.: saturated at 10.0) under load. I have adjusted the carburetor main and secondary jets from 68 / 76 to 64 / 72. For the most part, this looks to have leaned out the motor correctly. all eight carburetor idle mixture screws are set at 1 full turn out.

    Where I am now having an issue is with the following symptoms:

    -hunting idle. The truck seems to hunt between 700-1000RPM now, which did not happen before when I had the carburetors in their stock configuration. Does this have to do with how I have set the idle mixture screws? I wouldn't think this would be affected by the re-jetting.

    -slight stumble between idle and off idle. It almost feels like a misfire, but I have confirmed there is no ignition issues. What could be the problem here?

    I am fairly new to carburated motors, but am keen to learn how to tune these things.

    Thanks everyone, any help is greatly appreciated.

    -Alex

    Some pictures:








  2. #2
    34_40's Avatar
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    Hi Alex. Beautiful truck you've got there. You had limited experience with carbs so you jumped in with a supercharger and duels!! That is how you get'er done! LOL..

    A couple questions for you. First you said, "I have an AFR gauge installed, and it was reading very rich (i.e.: saturated at 10.0) under load. I have adjusted the carburetor main and secondary jets from 68 / 76 to 64 / 72. For the most part, this looks to have leaned out the motor correctly. all eight carburetor idle mixture screws are set at 1 full turn out." What does "leaned the motor out correctly" mean? And at what rpm?
    The mixture screws are for operation at idle. So we're talking about 2 different tasks. Also how is your AFR installed?
    Has the blower maker offered any help or guidance? Or have you even asked them? And I'll end with this last question, at what elevation are we talking about? Sea Level?

    Sorry for all the questions, but it's info we'll need to know.
    Driver50x likes this.

  3. #3
    firebird77clone's Avatar
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    I'll throw in a couple more queries: inquiring minds want to know.

    Timing: base, total, and curve springs. Is your vacuum ported or manifold. Hell, there I go making assumptions; merely because you are running carbs need not negate the possibility of a high end multi capacitive discharge system.
    .
    Education is expensive. Keep that in mind, and you'll never be terribly upset when a project goes awry.
    EG

  4. #4
    jerry clayton's Avatar
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    take that screen out of the scope---------

    Lock the curve in the distributor and set timing at 32-36 degrees-you may need a retard switch for starting

    open idle mixture screws to 1 3/4 turns and adjust from there
    NTFDAY likes this.
    By popular opinions-just a grumpy old man key board bully--But really, if you are going to ask for help on an internet site, at least answer questions about what you are asking about-----

  5. #5
    amgnvan's Avatar
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    Quote Originally Posted by 34_40 View Post
    Hi Alex. Beautiful truck you've got there. You had limited experience with carbs so you jumped in with a supercharger and duels!! That is how you get'er done! LOL..

    A couple questions for you. First you said, "I have an AFR gauge installed, and it was reading very rich (i.e.: saturated at 10.0) under load. I have adjusted the carburetor main and secondary jets from 68 / 76 to 64 / 72. For the most part, this looks to have leaned out the motor correctly. all eight carburetor idle mixture screws are set at 1 full turn out." What does "leaned the motor out correctly" mean? And at what rpm?
    The mixture screws are for operation at idle. So we're talking about 2 different tasks. Also how is your AFR installed?
    Has the blower maker offered any help or guidance? Or have you even asked them? And I'll end with this last question, at what elevation are we talking about? Sea Level?

    Sorry for all the questions, but it's info we'll need to know.

    Thanks! I do love it, it's been a hobby now for a while. I had a single carburetor on it for a while with an Edelbrock RPM Air Gap intake, but she wasn't making enough power! This truck is a learning project, and let me tell you I've learned lots so far! I'm not new to motors (I have had different fuel injected V8's), but definitely new to carburetors. I was thinking about swapping to a Holley EFI setup, but it wouldn't be true to the era!

    "how do you know the motor leaned out correctly".
    After re-jetting the carburetors, under load the AFR is now around 12.5:1 (but I have not tested it at full throttle yet). As for RPM, the motor shifts at 5200; I have not seen that RPM yet, maybe 4000RPM or so (the trans shifts too low I know, but that's another thing to work out later).

    "The mixture screws are for operation at idle. So we're talking about 2 different tasks. Also how is your AFR installed?
    Yes I agree. The AFR is installed in the passenger side (I drilled out the wrong header) header, just past the collector in the 1 5/8" long tube header. See picture below (not allowed to make fun of the welds).

    "Has the blower maker offered any help or guidance? Or have you even asked them?"
    The blower is a Weiand 6-71; I have not yet asked them for any guidance.

    "And I'll end with this last question, at what elevation are we talking about? Sea Level? "
    Yep, sea level.

    Last edited by amgnvan; 12-04-2018 at 09:48 AM.

  6. #6
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    Quote Originally Posted by firebird77clone View Post
    I'll throw in a couple more queries: inquiring minds want to know.

    Timing: base, total, and curve springs. Is your vacuum ported or manifold. Hell, there I go making assumptions; merely because you are running carbs need not negate the possibility of a high end multi capacitive discharge system.
    This is where I do not have the answers, and will need to check on the weekend. I'm fairly certain I do not have enough total timing (or initial, for that matter). Right now I have set the motor at 10* initial, and am not sure what springs are in the distributor (have not checked total timing yet). I bought this distributor:

    https://www.summitracing.com/int/par...0205/overview/

    "Is your vacuum ported or manifold"

    Sorry, are you able to clarify your question?

  7. #7
    amgnvan's Avatar
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    Quote Originally Posted by jerry clayton View Post
    take that screen out of the scope---------

    Lock the curve in the distributor and set timing at 32-36 degrees-you may need a retard switch for starting

    open idle mixture screws to 1 3/4 turns and adjust from there
    I can try this on the weekend but I don't have a switch on hand,...so it might be tough. is 1 3/4 turns too much? I currently have them set to 1 turn and it idles very rich.

  8. #8
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    Manifold vacuum is from the base of the carb or directly off the intake manifold. Ported means it's taken off the cab, above the butterflies, so you only get vacuum above idle.
    .
    Education is expensive. Keep that in mind, and you'll never be terribly upset when a project goes awry.
    EG

  9. #9
    amgnvan's Avatar
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    Quote Originally Posted by firebird77clone View Post
    Manifold vacuum is from the base of the carb or directly off the intake manifold. Ported means it's taken off the cab, above the butterflies, so you only get vacuum above idle.
    Ah, right. All of the vacuum lines are hooked up below the carburetors, but above the supercharger. The carburetors are boost referenced, to a connection below the supercharger.

  10. #10
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    All great comments and suggestions above.
    You really need to develop a "tune-up", even plot it out in a graph if you can, you must know what the AFR is across the rpm range, And I'm not talking about 4000 rpm or 7000 rpm.. but your "normal" range. You really need to establish a baseline and then test and tune the problems out. I'm not overly familiar with your setup (cam / heads etc.) but I'm not a fan of "extreme" components generally. But that's just me.

    I'd try re-adjusting the needles as Jerry suggested above. Perhaps just do the fronts and see how it reacts. Also move the timing up some and see how it reacts to that. But note the changes and the results - if it reacts badly then you can save yourself a step instead of repeating the mistake.

    Also, do you have a target AFR in mind? I ask because I'm curious if you'll settle for something close or are you trying to reach a goal.

  11. #11
    amgnvan's Avatar
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    Quote Originally Posted by 34_40 View Post
    All great comments and suggestions above.
    You really need to develop a "tune-up", even plot it out in a graph if you can, you must know what the AFR is across the rpm range, And I'm not talking about 4000 rpm or 7000 rpm.. but your "normal" range. You really need to establish a baseline and then test and tune the problems out. I'm not overly familiar with your setup (cam / heads etc.) but I'm not a fan of "extreme" components generally. But that's just me.

    I'd try re-adjusting the needles as Jerry suggested above. Perhaps just do the fronts and see how it reacts. Also move the timing up some and see how it reacts to that. But note the changes and the results - if it reacts badly then you can save yourself a step instead of repeating the mistake.

    Also, do you have a target AFR in mind? I ask because I'm curious if you'll settle for something close or are you trying to reach a goal.
    Thanks for the input! I'm not too sure about the cam choice, I bought it before I knew I was installing a supercharger; it's possible I will change it out in for a different grind.

    I'll take some more detailed notes on the weekend (if the weather isn't too awful out), that's good advice.

    As far as AFR, I'd like to see around 13.5:1. It's running so rich right now that it's an absolute pig on fuel (I never thought a 350 with a 6-71 would be good, but this is just atrocious); I also believe it should operate better around this AFR.

  12. #12
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    Whats the blower overdrive ratio?? Engine compression ratio??

    You have to get absolute control of your timing-------the only way to be 100% positive is to lock out the advance in the dist by wiring the advance at the spring hooks or in some dist by using a plug to block the slot-no vacume hooked up-------you will eventually need an MSD retard for the start cycle and some of there boxes have this built in------set the static timing in the mid 20s to low30 degrees------

    Also, what are you running for fuel??????Fuel pump?? fuel pressure to carb??????

    I'll wait for answers---------
    By popular opinions-just a grumpy old man key board bully--But really, if you are going to ask for help on an internet site, at least answer questions about what you are asking about-----

  13. #13
    amgnvan's Avatar
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    The blower pulley setup is a 38 upper / 34 lower; this is an 11% under drive as indicated by the Holley chart. The engine compression ratio is 9.0:1.

    Are you saying to lock out the timing? I have been told this will result in poor driveability under normal driving conditions,...is this true?

    For fuel I am running the factory mechanical fuel pump; I do not yet have a mechanical fuel pressure gauge, but I am looking to install one this weekend (don't know exact fuel pressure).

  14. #14
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    Here's a better picture of the motor, if it's worth anything. The only thing changed in this pic is the exhaust is now 1-5/8" headers with 3" pipe.


  15. #15
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    I'm not so much a fan of IPA, but it makes an interesting change of pace on occasion.
    .
    Education is expensive. Keep that in mind, and you'll never be terribly upset when a project goes awry.
    EG

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