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Carb settings / what am I doing wrong?

Discussion in '1965-1970 Shelby Mustang GT350 & GT500' started by skidado, Sep 18, 2006.

  1. skidado

    skidado Well-Known Member

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    '67 gt350 4-speed, rebuilt the engine to stock spec, and using original 735cfm vac sec carb and manifold.

    When I built the carb, I set the floats using a gauge I had made some years earlier, but the settings I wrote on the gauge have rubbed off, so I don't know the actual numbers.

    I have re-used the original jets (worked fine before - not damaged or worn) and replaced the power valve with the one in the rebuild kit.

    Car now runs very rich. On idle, I can screw the idle adjustment screws all the way in to the stops (both sides) and it still idles! :eek:

    Does anyone have the standard set-up for this carb? (jet sizes, power valve number, float level setting)?

    This is driving me mad. I can't find any vacuum leaks, but the engine 'hunts' lilke it's very lean or sucking air. But when I pull the plugs, they're black!

    Any ideas anyone?

    Thanks

    David
     
  2. GT350N4

    GT350N4 Active Member

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    Well, I'll take a shot at your problem since no one else has. First, your car should have a 715 cfm carb. Are you sure it is a 735? Either way, something you may want to check is your secondary butterfly seat adjustment. If you pull the carb off and flip it over there should be a set screw on the side that sets the amount that the secondary butterfly's are open while the throttle is in the idle position. If I remember correctly, this adjustment is part of your idle circuit and may be the reason for your richness problem. Good luck and let me know what you find out. B)
     
  3. KMCBOSS

    KMCBOSS Active Member

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    First of all turn both idle mixture screws all the way clockwise then back off one and a half turns. This is the starting point for the idle ixture screw adjustment. Next, disconnect the spark plug wire connecting the coil and distributor and set it aside. Put the car in neutral or park, remove the sight plug from the primary bowl of the carb and see if gas comes out (it should just trickle out). If float bowl level is good crank engine with remote starter and observe gas just trickles out - make an adjutment to the float level here if needed. Check the secondary bowl the same way. After carb float level adjustment is made, hook up vacuum gage to a vacuum port on the intake manifold. Start the car and allow to warm up. Next adjust idle mixture screws on carb in small increments - driver side then passenger side while watching vacuum gauge. adjust screws until you have the highest level of vacuum (should be about 14 inches of mercury). You may have the bump the idle speed setting to get the most accurate reading -just adjust the speed back when your done. Hope this helps - Ed
     
  4. philpughe

    philpughe Member

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    i had this and the main cause of the leak was from the pivot rod where the butterflys are attached. the pivot arm goes into the sides of the carb to do the pivoting nad has small white plastic sleeves. this area wears down only slightly but enough to let air in and also difficult to detect. bought a new carb in the end to fix this, although mine is not a shelby, so not the 715 prices.

    on another note, has my cousin spoken to you yet about your tail-lights? either he or I were going to come and take a look at yours as there might be some bits missing off his that we may need to locate.
     
  5. skidado

    skidado Well-Known Member

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    Yes, my mistake, of course it is a 715cfm, not a 735 as I stated in the original post.

    I cannot set the float levels as you suggest KMCBOSS, as this is the original shelby fitted carb that has LeMans bowls with no sight plugs. The only way to adjust the float levels is to remove the bowls and bend the tabs to achieve the right float measurement - hence my question about whether anyone knows what this measurement should be.

    Since I can screw both idle screws in to the stops and the engine still runs, it must be getting fuel from somewhere - primary jets, secondary jets, or power valve. Is there anywhere else it might come from?

    I will have a look at the secondary butterfly settings. I don't recall changing this in any way when I stripped and cleaned the carb prior to refitting, but it's such a long time ago now that I might have done and forgotten.

    Float level settings would be much appreciated if anyone has them.

    Thanks
     
  6. BillH

    BillH Well-Known Member

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    I have the carb specs from an old Holley master catalog at home which I will post later.
    Getting to your problem:
    The only place fuel should be entering the engine is through the primary idle circuit. There is an idle circuit in the secondary side, but it is not adjustable and is only there to circulate fuel so it will not get stagnant if the secondaries are not opened periodically. (Apparently, the Holley engineers don’t know Shelby guys very well ;>) Anyway, if you can screw in the idle needles and the engine keeps running, there is fuel getting in somewhere where it shouldn’t. Most of the time, the internal leak is around the primary metering body gasket. Normally, the solution is to tighten the bowl screws properly. The Holley spec is 40 to 55 INCH pounds. Use an inch pound torque wrench and go over them until they don't get any tighter. You will be amazed how tight 55 inch pounds really is.
    Another thing to check is the surface where the bowls bolt to. If this is warped, you will never get it to seal. When I rebuild a carb, (I have done HUNDEREDS of them) I always run a flat file over this surface. I have seen them so uneven that they require a lot of time to get flat by draw filing.
    Float levels are important because they time the idle and main circuits. Fuel dripping out of the booster venturi when the engine is idling is a dead giveaway that the float level is too high. Flat spots in acceleration is an indication that it is too low. Worn throttle shafts will make it very hard for the engine to idle at the same speed consistently. The throttle shafts can be re-bushed. Use Holley parts, a 1” 12 point socket for the power valve, be careful with the little raised sealing ridges on the power valve body.
    Good Luck
     
  7. skidado

    skidado Well-Known Member

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    Many thanks for this Bill - I will check for these things at the weekend. The standard specs would be very useful if you find them.
     
  8. DH 48

    DH 48 Member

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    Start the engine up with a vacuum gauge on it and see what the vacuum reading is. Then disassemble the front float bowl and take the power valve out and throw it away! Get a proper Holley power valve with the correct gasket, NOT A STEEL,BRASS OR ALUMINUM WASHER. Get a power valve with a number 2 to 4 digits below your vacuum reading. Example; if your vacuum reading is 10in, get a power valve around 6.5 or 7.5. Reassemble the carb and set the floats to the windows in the float bowls, then set the idle screws. Set the idle screws to about 1 1/4 turns out to start. Then adjust the idle screws in or out to get the highest vacuum reading.
     
  9. BillH

    BillH Well-Known Member

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    Holley List 3259 AAS
    Jets: Primary: 67
    Secondary: 78
    Power Valve: 85
    This is the early 65 Cobra (Of which they used very few of these carbs on) and the '65 GT-350

    List 3259-1AAS
    Jets: Primary: 68
    Secondary: 78
    Power Valve: 85
    Later design, used on '66 and '67 GT-350's.
    These carbs are not to be confused with what they used on the 427. That carb was a 3255, 780 CFM.

    Float Level: (Both)
    Primary 3/8"
    Secondary 1/2"

    I wouldn't worry about the power valve. If your engine has the stock camshaft this carburetor will work fine set up with stock specs. The float levels are kind of a pain to set properly. The trick is to slide in your gauge until it just clears the float without un-seating the needle valve. Take your time and you should be fine. Is your carb the correct one for the car (Not a service replacement?)
     
  10. skidado

    skidado Well-Known Member

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    Thanks for the specs Bill. I will go back to basics and check everything against these.

    I'm pretty sure the carb is original shelby fit (it has a Shelby part number). I will check this at the weekend.

    Thanks again.

    David
     
  11. eljimb0

    eljimb0 Well-Known Member

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    When I finished my car (clone) I took it to a Dyno guy and spent half a day fooling with jets ect. It was great fun. It is such a thrill to hear a motor that you put together with your own hands.. Ported windsor C9 heads(by me), Rods from 3 different motors ect, screaming at 7000 rpm... (nothing sounds like a 289 :thumbsup: :thumbsup:)
    anyhow... I didn't know what the factory spec jets were supposed to be so we went back and forth with them untill the motor did it's best (209 rwh..IIRC?).. the jets we came up with were spec jets...
    go figure...
    jimbo
     
  12. Texas Swede

    Texas Swede Well-Known Member

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    Your problem could be with the float adjutments.
    It shall be 3/8" from the top of the bowl on the primary side
    and 1/2" on the secondary. Turn the bowl over and meausure
    between the hanging float and the bottom of the bowl,
    i.e. the top but turned over. If you send me a private email,
    I will respond with Shelby's Sevice Circular No. 9 for the Holley 3259
    attached. It shows exactly how to adjust the floats.
    It was good idea as well to check the vacuum and compare with
    the power valve. The lack of reaction on the idle screws indicates
    that the power valve is open all the time and it's either broken
    or the idle vacuum is lower than the opening inches of mercury,
    if you have an 85=8.5 inches of mercury and your idle vacuum is
    below that you should reduce the number on your power valve.
    As also said above, it could be the secoday flap adjustment as well.
    Good luck
    Texas Swede
     
  13. skidado

    skidado Well-Known Member

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    Thanks T-S, I'll send you a PM.
     
  14. skidado

    skidado Well-Known Member

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    Fantastic! Tex, the TSB you sent had all the right info.

    I checked the jets, and they are correct, but I couldn't find any numbers on the power valve, so I left that alone.

    Then I checked the float levels - they were WAY out (much too low)! :doh: I reset them as advised, and the car started first time and ran sweetly (once I'd done the idle adjustments).

    I haven't yet figured out how raising the floats resulted in the engine running leaner, but hey, it worked for me!

    Many thanks everyone for your input - much appreciated! :thumbup:

    David
     
  15. John B

    John B New Member

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    I now this is an old posting but wondered if anyone has a copy of the Shelby's Service Circular No. 9 for the Holley 3259?
     
  16. skidado

    skidado Well-Known Member

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    john - not much traffic on this site any more. Try the same question on saac.com/forum and I’m sure you’ll get a response.
     

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