Drilling throttle blades

Discussion in 'High Performance Modifications' started by Badkarma, Jul 17, 2006.

  1. Badkarma

    Badkarma Veteran Member Lifetime Gold Member

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    After Patrick and I going around trying to figure out how to lean out the idle of my new pro systems carb he said to drill the rear throttle Blades. I'm really nervous about doing this for one, that I have never taken a holly apart that much and two I don't want to mess it up. Could any one offer me some step by step help on how I can do this? *nervous sweat*
     
  2. TooLateVTEC

    TooLateVTEC Veteran Member

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    Does your carb have 4 corner idle circuts on it?
     
  3. Urban Samurai

    Urban Samurai Veteran Member

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    Primary idle circuit modifications

    When setting idle both primary and secondary buterflies should be open an equal amount, however, your transfer slot should not have more than .060 showing.

    If due to a high overlap cam you need to open the the throttle plate too much the first modification is to drill 3/32-in. holes in all four throttle plates. This will increase air flow at idle while allowing the throttle plates to remain in the proper position on the transfer slot.

    This is per the "Super Tuning and Modifying Holley Carburetors" book by Dave Emanuel.

    If you mess up im sure a little jb weld would close up the holes pretty easy.
     
  4. DB Z28

    DB Z28 Veteran Member

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    a other trick is to adjust the rear blades open with out drilling the blades at all. small set screw adjusts the rear
     
  5. Badkarma

    Badkarma Veteran Member Lifetime Gold Member

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    The carb dose have 4 corner idle, Its a holley HP body. With front and rear transfere slots "squared up" equal amouts the car would not idle. Hence the builder recomending the drilling. I do trust Patricks judgement, its just that this is unknown territory for me. Funny thing is, this is a rather tame cam, being a hydro roller in an other wise stock GM crate motor. It pulls 16 inches of vacume at idle, so I am pretty sure the overlap is minimal. Inlarging the airbleeds to .078 at from stock did help so maybe this is a step in the right direction. I'm guessing that the blades need to be removed while doing this?, if so how do I do it?
     
  6. Urban Samurai

    Urban Samurai Veteran Member

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    wooo, slow down! you have 16 inches of vacuum at idle?
    Does not sound to me like you need to drill anything. Have you checked your timing and then have you checked for a vacuum leake. Could even be a leak in the valley.

    IMHO it sounds like you have a problem be it in the carb or intake.
     
  7. Badkarma

    Badkarma Veteran Member Lifetime Gold Member

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    been over it all. timing is 18 intial, 18 mechanical with no vacume advance. intake is buttoned up tight. let me just post what Patrick and I have been talking about so everyone dosnt think I'm jumping the gun here
    * first email asked for tuning help*
    Hi Jason,

    Obviously be sure the float levels are not over 25
    percent up into the glass and the fuel pressure does
    not exceed 8 psi.

    1. To properly adjust the idle mixture screws, you use
    a track temp infra red gun and adjust the mixture
    screws as such. Start it up and using the track temp
    gun, get the headers so they are all within 80 degrees
    of each other (If you don't have a track temp gun,
    then adjust it like paragraph 2).

    2. Then turn the idle mixture screws in until the
    engine hesitates on a quick jab of the throttle. Then
    turn the mixture screws out until the hesitation goes
    away. That is as lean as it will idle without
    hesitating.

    3. HOWEVER, If you find it loads up after setting the
    mixture screws as described, then more initial timing
    may be needed, a higher idle rpm or hotter spark
    plugs. Also, possibly just a specific corner is too
    rich and that corner may need less fuel than another.

    If after doing these adjustments and being sure the
    throttle shafts are open similar amounts, you can
    drill out/enlarge the idle air bleeds holes (located
    in the outer four corners of the carburetor in the
    venturi area) the larger they are, the leaner the idle
    will be. Move in increments of between .006-.010 from
    current.

    Thanks Jason.

    > Hello again,
    >
    > I ran into something interesting today while
    > adjusting the idle mixture
    > screws, while working on one at a time I could turn
    > one all the way in and all
    > the motor would do is speed up. It didn't start to
    > hesitate like you would
    > expect. It did this to each screw in turn, even with
    > all for set at one turn out
    > the car was still rich. I suspected that maybe the
    > floats snuck up on me but
    > this was not the case, they were still set right at
    > the bottom of the sight
    > hole. I'm currently running 18 degrees of intial
    > timing with 18 of mechanical
    > for a total of 36, not using any vacuum advance.
    > Spark plugs are NGK TR 5's
    > lit off with and MSD ignition. What do you think
    > boss? Should I jet it down? *
    > if so how much* or should I drill some of the extra
    > air bleeds I got with my
    > carb * again by how much* Other than the rich
    > condition that car runs great,
    > no hesitation and runs like a rapped ape. Just
    > incase you want this for
    > reference here is my carb info
    >
    Hi Jason,

    The larger the outer four corner air bleeds are the
    leaner the idle will become. I would drill them to
    .078

    minimal effect installing .078 air bleeds, still get
    > watery eyes. idle still
    > only speeds up when screws bottomed out. Dropped it
    > down to 72's on the main
    > primary and was still rich, but a lot slower on
    > acceleration, switched back
    > to out of the box settings.
    >
    > Jason Lemm

    Yessh, that has a lean burn idle circuit in it too.
    How much of the transfer slot is showing on the base
    plate? Zero or less than square?


    > Sorry for the delay Patrick, its been one of those
    > weeks. Pulled the carb
    > off today to check the transfer slots. they were
    > past square, more rectangle,
    > so I went ahead and squared them up. After doing
    > that the car wouldn't idle
    > on its own with out using the idle speed screw to
    > raise the idle. I was under
    > the general impression that the exposed portion on
    > the transfer slots should
    > be square so that's how I tried setting it. So what
    > else would you recommend
    > to cure the fat idle?
    >
    > thanks man

    On that carb you can drill another set of holes in the
    REAR throttle blades to balance the air flow for a low
    vacuum engine. This will get the blades to a more
    closed position. Is there enough base timing in it
    for the camshaft size?

    As typically low ignition timing will cause low vacuum
    and the need for more open throttle blades.

    Thanks.

    timing is 18 initial with another 18 mechanical. Its
    > pulls at least 16 on
    > the vacuum gauge at idle. Drilling the rear throttle
    > blades seems a little
    > extreme to me atleast, speciallu on such a tame
    > motor. Is there anything else I
    > can do? I'm going to yank the PCV valve for a day or
    > two, maybe its pulling
    > more than it needs to and trying to burn it.


    Wow that is odd. 16 inches at idle is quite good.
    The large throttle opening required is certainly a
    mystery as your base timing seems good as well.

    However, you are correct in that the transfer slots
    should be square or undersquare and the blades need to
    be in a more clsoed position, so if it requires more
    throttle opening to idle at the selected rpm then
    there needs to be additional bypasses in the blades.

    Thanks. Keep me posted.

    there ya have it folks. Patrick has been very helpfull in walking me through this to say the least. Its just that steps that are being talked about have me a little nervous. Sorry for the long post, just wanted to bring you all up too speed.
     
  8. Urban Samurai

    Urban Samurai Veteran Member

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    when you set your idle mixture screws use a vacuum guage. turn all the screws in gently then turn them out 1 1/2 turn. with the car at norm operating temp adjust one screw in at a time about a 1/4 turn keeping an eye on the vacuum guage. Turn them in until you have your highest vacuum (it will also raise the idle) and then adjust your idle speed down to what you would like it to idle at. Once idle is back down do the same process until small adjustments make no difference. All your idle mixture screws should be at about the same adjustment.

    You were adjusting until you crack the throttle and dont get a hesitation. This is not the idle circut. That is your next adjustment after the idle circut. That will include your shooters and cam for secondaries (or spring if it is vacuum secondaries).

    Air bleads really should not have had to been messed with. I would almost go back to orig. You also need to make sure that you have the correct power valve(s) in your carb for how much vacuum you have. You will want them to open at about 14.5 inches of vacuum.

    What is your ignition curve like? At what rpm do you get full advance? I like to have it at full advance by 3000 rpm. Then take your car to 3000 rpm and set your timing to what you want at full advance.
     
    Last edited: Jul 18, 2006
  9. Badkarma

    Badkarma Veteran Member Lifetime Gold Member

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    Thanks Urban Thats how I was setting the carb,like you mentioned I just included the emails for refrence. Can anyone Else help me out here?
     
    Last edited: Jul 18, 2006
  10. John Wright

    John Wright Veteran Member Lifetime Gold Member

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    Urban, I don't want to put words in his mouth, but I think the hesitation he is referring to is when you run the mixture screws in too much and the engine stumbles, not neccesarily a blipping the throttle hesitation, because like you said that is another setting altogether.
     

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