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Thread: Design and Drawings done for carb manifold finished

  1. #151
    Just Plain Nuts Boo's Avatar
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    Quote Originally Posted by stevedmc View Post
    What's ironic is Bill complained to me saying Lou was slandering a certain member of this forum.

    Ironically, Bill thinks its okay to slander Chad.
    What member of the forum did I "slander" ?

    Steve you have to start taking Bill's claims as they ready are....fake.

    And you thought Bill was a real friend....sad how the real face of people surface..
    Lou and "Boo" - The man you love to hate.

    Boo- The car you love to hate.

    ~~~~~~~~~~~~~~~~~~~
    "Boo"-VIN 5835
    'Fastest naturally aspirated PRV'
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    "Blue"-1985 Fiero GT
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  2. #152
    Senior Member 48jeep's Avatar
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    Just a quick update, a bit behind schedule because the wife's car blew a head gasket and I've been slaving over that. Will be back on schedule after that is resolved.
    Rebuilding basket cases in parking lots since 2005.

    87 Vette, 56 Metropolitan, 78 Vette, 81 DMC.

  3. #153
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    Quote Originally Posted by Lou and "Boo" View Post
    Steve I am running the same jet size you are and if you stand behind my car while it's running or even drive or ride in it with the Windows open you stink of fumes.
    Does it stink of fumes at idle? If so, then your idle mixture screws need to be adjusted. There are many guides on how to do this, but in general, you want the idle mixture to be set as lean as possible while still getting maximum engine vacuum/RPM at the specified idle RPM.

    Venturi size is not going to change the air/fuel mixture. The venturi size matters because it needs to be matched to the maximum RPM and displacement of the engine. The size of all of the venturi on a carburetor determine how much air can pass into the engine at a certain vacuum differential. At maximum RPM and Wide Open Throttle, you want the engine manifold vacuum to read below 1in because the engine is getting all the air it wants through the carburetor venturi. If the venturi are too small, not enough air will be supplied to the engine, and it wants more, so the vacuum will be higher than 1in and the engine won't make the maximum power that it should.

    If the venturi are too large, then the speed of the air passing through the venturi at part-throttle is not sufficient to atomize the fuel and the engine is not as responsive as it should/could be.

    So, both small and large venturi have their benefits. In a 4 barrel, the primary venturi butterflies are opened first, and then the secondary butterflies are opened beyond half-throttle. A spreadbore carb has the best of both worlds because it has small primaries (for good part-throttle) and very large secondaries (for ample air supply at WOT). Most 4 barrels also have a vacuum operated flap or vacuum operated secondary butterflies that only allows air to go through the secondary venturi if the engine requires that much airflow. Otherwise, at WOT at low RPMs, the engine would bog because the airspeed through the venturi would be too slow and vacuum from the engine insufficient to pull properly atomized fuel into the engine.

    A small change in venturi as discussed here is not going to be a big deal on the engines we're talking about. I've had a 650CFM 4 barrel and an 800CFM 4 barrel on the same 450hp 350cubic inch (5.7L) small block Chevy. The car made power and had good part-throttle response with both carburetors. I've also had a 650CFM and 770CFM carburetors on a 455cubic inch (7.4L) Buick engine. Both made power at WOT and had good part-throttle response. (Larger worked better on both engines for reasons not related to venturi size.... idle circuits and other differences caused the larger carbs to work better on those engines.)

    In short, if you're smelling fumes, the carb needs to be tuned. Instead of replacing the carb (and needing to tune the new one anyways), get a book or read guides on how to tune a carburetor and then do it yourself. Knowing how and why a carb works makes them much less mysterious and much easier to diagnose. Getting the idle speed and mixture set is step 1, and it sounds like that still needs to be dialed in.
    5786: DPI cams and cat-less exhaust, galvanized and powder coated manual frame for a proper 5-speed conversion, Kenwood head unit, Polk DXi speakers, and Pioneer-loaded DMC-CA sub box.

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  4. #154
    Senior Member 48jeep's Avatar
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    Just checking in again. Not dead, just had my wife's head gasket blow, my cars check engine light trip, and got laid off all in about a month. Conserving funds, have a job lined up next week I think, then will be back on the ball.
    Rebuilding basket cases in parking lots since 2005.

    87 Vette, 56 Metropolitan, 78 Vette, 81 DMC.

  5. #155
    Just Plain Nuts Boo's Avatar
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    Quote Originally Posted by delo5786 View Post
    Does it stink of fumes at idle? If so, then your idle mixture screws need to be adjusted. There are many guides on how to do this, but in general, you want the idle mixture to be set as lean as possible while still getting maximum engine vacuum/RPM at the specified idle RPM.

    Venturi size is not going to change the air/fuel mixture. The venturi size matters because it needs to be matched to the maximum RPM and displacement of the engine. The size of all of the venturi on a carburetor determine how much air can pass into the engine at a certain vacuum differential. At maximum RPM and Wide Open Throttle, you want the engine manifold vacuum to read below 1in because the engine is getting all the air it wants through the carburetor venturi. If the venturi are too small, not enough air will be supplied to the engine, and it wants more, so the vacuum will be higher than 1in and the engine won't make the maximum power that it should.

    If the venturi are too large, then the speed of the air passing through the venturi at part-throttle is not sufficient to atomize the fuel and the engine is not as responsive as it should/could be.

    So, both small and large venturi have their benefits. In a 4 barrel, the primary venturi butterflies are opened first, and then the secondary butterflies are opened beyond half-throttle. A spreadbore carb has the best of both worlds because it has small primaries (for good part-throttle) and very large secondaries (for ample air supply at WOT). Most 4 barrels also have a vacuum operated flap or vacuum operated secondary butterflies that only allows air to go through the secondary venturi if the engine requires that much airflow. Otherwise, at WOT at low RPMs, the engine would bog because the airspeed through the venturi would be too slow and vacuum from the engine insufficient to pull properly atomized fuel into the engine.

    A small change in venturi as discussed here is not going to be a big deal on the engines we're talking about. I've had a 650CFM 4 barrel and an 800CFM 4 barrel on the same 450hp 350cubic inch (5.7L) small block Chevy. The car made power and had good part-throttle response with both carburetors. I've also had a 650CFM and 770CFM carburetors on a 455cubic inch (7.4L) Buick engine. Both made power at WOT and had good part-throttle response. (Larger worked better on both engines for reasons not related to venturi size.... idle circuits and other differences caused the larger carbs to work better on those engines.)

    In short, if you're smelling fumes, the carb needs to be tuned. Instead of replacing the carb (and needing to tune the new one anyways), get a book or read guides on how to tune a carburetor and then do it yourself. Knowing how and why a carb works makes them much less mysterious and much easier to diagnose. Getting the idle speed and mixture set is step 1, and it sounds like that still needs to be dialed in.
    Thank you for the detailed response. I drove today and my clothes stunk of fumes
    Lou and "Boo" - The man you love to hate.

    Boo- The car you love to hate.

    ~~~~~~~~~~~~~~~~~~~
    "Boo"-VIN 5835
    'Fastest naturally aspirated PRV'
    ~~~~~~~~~~~~~~~~~~~~
    "Blue"-1985 Fiero GT
    3800sc swap
    ~~~~~~~~~~~~~~~~~~~~
    "Spalding"-1973 MG Midget
    All original

  6. #156
    Quote Originally Posted by 48jeep View Post
    Just checking in again. Not dead, just had my wife's head gasket blow, my cars check engine light trip, and got laid off all in about a month. Conserving funds, have a job lined up next week I think, then will be back on the ball.
    How are things going these days, sir?
    Bungee Cup Holder Guru

  7. #157
    Uncensored Hypocrite stevedmc's Avatar
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    Quote Originally Posted by Duplicate Account View Post
    How are things going these days, sir?
    Ditto. Any updates?
    Rest assured, we have a backup of Farrar's car blog and it will be restored in the near future. (Steve Rice - March 2016)
    Rest assured, we have a backup of Shep's posts and all of them will be restored in the near future. (Steve Rice - March 2017)

  8. #158
    As seen on TV! Dracula's Avatar
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    Quote Originally Posted by stevedmc View Post
    Ditto. Any updates?
    Steve's Peugeot manifold is in my hands and is, at present, being adapted for a 2100 carb. As for the fabricated manifolds; despite what some people of questionable mental health may profess, I have nothing to do with this project beyond loaning templates and manifolds.
    -Chad K.
    Previous owner of VINS: 6982, 1601, 1265, and 1269

    Dammit, Jim. I'm an undertaker, not a mechanic.

  9. #159
    Uncensored Hypocrite stevedmc's Avatar
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    Quote Originally Posted by Dracula View Post
    Steve's Peugeot manifold is in my hands and is, at present, being adapted for a 2100 carb. As for the fabricated manifolds; despite what some people of questionable mental health may profess, I have nothing to do with this project beyond loaning templates and manifolds.
    Yeah. Any of uf with a brain know that Phil is the guy behind the new ones.

    Hope all is well for him though. I forgot he got laid off.

    Sent from my SM-G900V using Tapatalk because I'm too lazy to turn off the mobile signature thingie on my phone.
    Rest assured, we have a backup of Farrar's car blog and it will be restored in the near future. (Steve Rice - March 2016)
    Rest assured, we have a backup of Shep's posts and all of them will be restored in the near future. (Steve Rice - March 2017)

  10. #160
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    Any update on this project? Inquiring minds (read "I") want to know :-)

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