Help with optimizing jet size on Holley 750 carb with 440 engine

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    Author Topic: Help with optimizing jet size on Holley 750 carb with 440 engine  (Read 10334 times)

    Offline NorthWestcuda

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    Help with optimizing jet size on Holley 750 carb with 440 engine
    « on: May 21, 2008 - 12:09:55 PM »
    I’m finally going to try and change my primary and secondary jets.  I have a 750 holley carb (#4779-3) sitting on a 440.  The car came with #80 size jets on the primary and #90’s in the secondaries.  The spark plugs are almost completely black.  After I let the car run in the shop for 5/10 minutes my eyes will really start to hurt\sting because the exhaust seems so strong, even with the garage doors open.

    I was thinking of starting with #70 in the primaries and move the #80’s to the secondaries.  Your thoughts?

    When I test the plugs to see if they are “creamy” colored, I’m going to run the car at about 2000rpm for 5 minutes or so and than shut it off without letting it idle and than check the plugs.  Does that sound like the correct procedure? 

    How do I check the secondaires jets to see if they are the correct size for the engine? :clueless:
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    1973 Cuda, 440 engine
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    Offline Chryco Psycho

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #1 on: May 21, 2008 - 12:45:08 PM »
    go for a drive 2- 3 miles is lots shut the car down & pull the plugs to check color , try to do a steady cruise on a highway or at 30-50 MPH , if you change more then 6 jets sizes to get the color you want you have other problems , look up the factory jet size for that carb originally & start there , you should be no more than 4 sizes up  or down to get the plug color to a tan color
     long duration cams can seem rich at idle no matter where the jetting is at so that is not really an indication , if the plugs go black at idle you may have a leak at the power valve or the power valve has a blown diaphragm
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    Offline NorthWestcuda

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #2 on: May 21, 2008 - 01:35:44 PM »
    Thanks CP, I understand the checking process behind the primary jets, but how do you check to make sure you have the correct size secondaries jets?
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    1973 Cuda, 440 engine
    1972 Chevy 4x4 PU stepside
    2003 Ford F-350 Diesel

    Offline Chryco Psycho

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #3 on: May 21, 2008 - 01:45:28 PM »
    basically the same theory floor the throttle for as long as youy can shut down immediatly & see if the plug color gets lighter or darker , the best place to do this is on a dyno
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    Offline Changin Gears

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #4 on: May 21, 2008 - 05:13:17 PM »
    Stock jets were 70 and 73.  Unless the power valves have been blocked you are way rich.
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    Offline MEK-Dangerfield

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #5 on: May 21, 2008 - 05:24:47 PM »
    It would help if we knew more about the engine. Ported heads, big cam, headers, and such. BUT those jets are HUUUUGE for a 750.  :eek7: My guess is if your engine is close to stock build, you'll need less than 70's in the primary, and I would guess at 73-75 in the secondaries.   :dunno:  OMG, your plugs must be caked with soot.   :22yikes:


      Mike
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    Mike

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    Offline NorthWestcuda

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #6 on: May 22, 2008 - 01:14:46 PM »
    Quote from: MEK-Dangerfield on May 21, 2008 - 05:24:47 PM
    It would help if we knew more about the engine. Ported heads, big cam, headers, and such. BUT those jets are HUUUUGE for a 750.  :eek7: My guess is if your engine is close to stock build, you'll need less than 70's in the primary, and I would guess at 73-75 in the secondaries.   :dunno:  OMG, your plugs must be caked with soot.   :22yikes:


      Mike

    I would say the engine is “a little” more than stock, stock heads, slightly more aggressive cam, headers, aftermarket distributor and intake, typically stuff. The person I bought it from didn’t know much about the engine and I just finished putting the body back together in March 08.

    I replaced the primary jets last night with 74’s from 80’s.  After I pulled two plugs, 1 & 2 plugs, I noticed that only the very end of the plugs, “ground electrode”, was starting to get a cream color.  The insulator “ceramic part” along with everything else is still very soot colored.  Tonight, I’ll put 70 & 74 in both fornt and back and see how they look after running it for awhile.

    Question #1, since the car has been running on 80 & 90’s before I bought it, will it take a longer time to get all the soot out of the engine & exhaust?

    Question#2, is there a way to “visually look” at the power value and see if its blown or leaking? 

    Question #3, I’m only measuring around 10-11 inch’s of vacuum at ideal, does that sound typical? The engine runs better at around 8-10 BTDC timing.

    Thanks guys,
    Jim




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    1973 Cuda, 440 engine
    1972 Chevy 4x4 PU stepside
    2003 Ford F-350 Diesel

    Offline Chryco Psycho

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #7 on: May 22, 2008 - 01:29:50 PM »
     Hey Jim
    it will take time to blow out thew soot if it ever does but the plugs should go clean in 2-3 miles as long as they are still firing  I would buy a couple new plugs to test with
     you need to suck on the power valve from the Nut side to see if it holds vacuum also when removing the metering block if it is wet in the cavity in the carb body fuel is getting in there , it should be dry so either the power valve or gasket is bad
    10-11 " is normal with a cam in the engine , what vacuum does it make idling in gear if it is an auto tranny , the power valve Must be rated at least 1" lower then the reading in gear so if it makes 6" in gear you need a 4.5" power valve or it will run too rich @ idle
     I would  bump the timing up into the 16* range at idle this shoudl increase idle quality & vacuum
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    Offline NorthWestcuda

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #8 on: May 22, 2008 - 01:58:04 PM »
    Thanks, CP

    I'll check those things out tonight.  The wife gets mad at me when I work "my girlfriend AKA the cuda" during the day!!!!

    Jim
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    1973 Cuda, 440 engine
    1972 Chevy 4x4 PU stepside
    2003 Ford F-350 Diesel

    Offline Chryco Psycho

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #9 on: May 22, 2008 - 02:01:29 PM »
    You have to spend quality time with the Gf / Mistress  :2thumbs:
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    Offline NoMope Greg

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #10 on: May 23, 2008 - 02:02:54 PM »
    Quote from: NorthWestcuda on May 22, 2008 - 01:58:04 PM
    I'll check those things out tonight.  The wife gets mad at me when I work "my girlfriend AKA the cuda" during the day!!!!


    Oh, you have one of those too?   :pullinghair:
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    Greg
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    Offline NorthWestcuda

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #11 on: May 27, 2008 - 11:58:03 AM »
    Hey guys,
         I've been reading in this board lately abotu total timing.  How do you find total timing if you don't have a "extended timing tape" on the dampner or timing marks extended out on your dampner?

    Jim
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    1973 Cuda, 440 engine
    1972 Chevy 4x4 PU stepside
    2003 Ford F-350 Diesel

    Offline dwbiggs

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #12 on: May 27, 2008 - 04:41:54 PM »
    Dial back timing light?  Or you could manually mark the balancer....do the math.  Of course you would have set the mechanical advance in the distributor first...then add whatever intial you need...ex: 15 deg initial with 20 deg in the dizzy.
    « Last Edit: May 27, 2008 - 04:43:32 PM by dwbiggs »
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    1974 Challenger Rallye - 440+.040, 0 decked, 10.5:1, Ross Pistons, Hughes 3844 cam (238/244, .536/.540), Eddy RPM manifold, Eddy aluminum heads, Holley 750, TTI headers, .96" T-bars, IAS shocks, 1 1/8" front sway bar, 3/4" rear sway bar, XHD 6 leaf springs, Firm Feel stage 2 steering box.

    Offline Chryco Psycho

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    Re: Help with optimizing jet size on Holley 750 carb with 440 engine
    « Reply #13 on: May 28, 2008 - 01:37:12 AM »
    2 3/8" clockwise on the Damper from 0  is 36* , cut a slot or a mark on the damper & see where it lines up on the scale on the timing cover
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