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  1. #1

    Default Off Idle stumble

    My 4180C carb stumbles between Idle and 1800 Rpms. I went from stock 5.0 to a rebuilt and modified explorer block with GT40 heads, TFS1 cam, Wieand Stealth, and shorty headers. The squirters are working fine. IDK. Do I need an 8.5 PV? Any help appreciated.
    Mike
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  2. #2

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    Sorry I'm not much help with a carbd car....

  3. #3
    FEP Super Member bwguardian's Avatar
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    Is it stumbling at tip in, wide open throttle, or both?
    HAD
    '82 GT monochromatic (red)...black cloth

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  4. #4

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    It runs like a top at 2000 rpm’s. It stumbles at tip in.
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  5. #5

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    Being as you don't have a stock motor, I don't know if this bit of info is applicable to you....

    But Ford had a TSB where they replaced the accelerator pump cam on the linkage of the stock 4180 carb, due to an off-idle stumble. It made a world of difference on mine.
    1985 Mustang GT (Currently mothballed, but acquiring parts)

  6. #6

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    Gotta be careful with descriptions here. My experience with off-idle stumble was a continuous stumble that occurred with the throttle barely open. Carb was running on idle jets and transition slots. The cure for me was to increase the primary jets from 61 to 63. Hesitation is when you step on the gas and the engine falls flat on it’s face. This means an increase in the accel pump shot is needed. Accel squirter increase can cure this, assuming the accel pump or the plumbing to the squirter is all properly sealed. I changed my squirter from a 28 to a 31. Car runs well now.
    Last edited by WilliamCapri; 07-14-2022 at 01:58 PM.
    W

    As always, "It ain't what you don't know that gets you, it's what you think you know that just ain't so."

  7. #7

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    Thanks! To be a little clearer: It idles fine. When I step on the gas to pull out from a standing position, it will stumble, or barely run off idle until I blip the throttle, then it takes off slightly breaking loose. When cruising, lets say in 4th or 5th gear, anything below 1800 RPMs and it stumbles and sometimes bucks. I rebuilt this carb about 6 or 7 years ago with the JA Cook mods. Ran like a top. I had the car in the body shop for a while, and then it sat while I had the new engine built. A couple of thing happened with it as I had bad gas in it and thought it was the PV. So, I put a new stock PV in it. Still the same problems. Thanks for the replies!
    Mike
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  8. #8
    FEP Super Member bwguardian's Avatar
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    ^^^...What WilliamCapi suggests is what I did from the get go when I went through mine 10-12 years ago. I have stock bottom end with the 4180c sitting on a Ford Racing high rise intake, GT40p heads, and an E303 cam. It's always felt like an EFI system being very crisp and sensitive to throttle response. Only things I've done to the carburetor then while I rebuilt it was as he described.
    HAD
    '82 GT monochromatic (red)...black cloth

    HAVE
    '85 GT vert two tone (white on charcoal)...white leather
    '00 F350 two tone (white on silver)...gray cloth
    '00 Excursion Limited two tone (white on tan)...tan leather...wifes ride
    '08 Taurus Limited ice blue...tan leather...daughter ride
    '08 Edge Limited white sand tri-coat metallic...tan leather...other daughters ride

  9. #9

    Default

    What is your timing set at? Stock 302 was at 18° and was bucking while cruising it’s at about 16.5 now and working good,Just a thought. but the accelerator pump is another and probably the better guess.

  10. #10

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    I checked the Jets: 62. Power valve that came with the 3-1346 was a 4.5. I put a 6.5 in and it runs much better. I still might swap out the jets for 63's. Thanks guys
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  11. #11
    FEP Power Member qikgts's Avatar
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    If the car ran fine before going into the body shop then I'd be thinking that maybe the issue now is a result of being in the body shop.

    Perhaps "body shop dust" has clogged up the air bleeds. Have you cleared them out with a liberal spraying of carb cleaner and quick poke with a paper clip?
    '85 GT

  12. #12

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    No I’ll try that. Good idea
    79 Coupe, 357W balanced, AFR 185s, Vic Jr, Holley 750, C4 roller, 4.56 8.8 spool, 26 MT Drags, 11.00 @ 123
    83 Mustang GT, 88 GT engine, T5

  13. #13
    FEP Super Member gr79's Avatar
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    Could try that 8.5 pv before rejetting. Easily reverse-able mod. A 7.5 is also available.
    'Timing' term also applies to pv. May need jet change too. One thing at a time.
    Simple cleaning every few years never hurts. Can be done on car. Stumble means lean.
    Tuning to find out what each engine likes is the hard part, especially if souped up a bit.

    Just did my riding mower carb. Was stumbling then picking up. Finally is quit and no restart.
    Supply to carb ok. Would run a bit with a shot of gas direct. Aha lean or no fuel.
    Walbro carb bowl had brown powder cake buildup (first clean after 25 years). Float would not drop far enough.
    Took it off. Carb cleaner spray and rag.
    Starts much quicker, runs quieter, more power to scalp the grass at 1.5".


    https://www.holley.com/products/fuel...s/parts/125-85

  14. #14
    FEP Power Member qikgts's Avatar
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    Quote Originally Posted by 79stang351w View Post
    No I’ll try that. Good idea
    '85 GT

  15. #15
    FEP Senior Member 854vragtop's Avatar
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    When I ran my 4180C, I had to step up from the stock 62 jets to 64's to prevent the steady state cruise bucking. Everyone online attributed it to the increase in ethanol in the gas. This was way back in 2000. I always had issues with tip in hesitation with the 4180C. Starting from a dead stop, getting back on the gas after shifting. The best remedy is to do the following:

    1) With the car warm and the choke off the high idle detent, adjust the accelerator pump bolt/spring combination to take out ALL the slack. You should see fuel moving out of the pump nozzles as soon as you crack open the throttle.

    2) Check to see that the accelerator pump cam lobe isn't worn down. Any reduction in profile height will give you less pump squirt and cause hesitation. You can find the Holley Accelerator Pump Cam Assortment Kit 20-12 just about anywhere online for about $35 bucks. I've always had the best luck with the green pump cam. You can clock it at 2 positions with with the mounting screw.

    I've said this many times on this board about switching to the Summit Racing 600 CFM vacuum secondary carb. It is by far the best carb that I've ever run on my '85 GT 5 speed. Great throttle response, great cold weather driveability with the engine cold, NEVER stumbles, easy to change jets, no leaks, Holley Quick Change Vaccum Spring kit bolts right on, made with some genuine Holley parts, comes with green pump cam installed. The only issue with installation is that I had to relocate my coil from the stock mounting position on the intake to a spot on the A/C support bracket because of the dual feed line on the R/H side of the carb.

    See my signature below for my engine set-up.
    '85 Mustang convertible GT, 5 speed, 4V
    Stock bottom end, Comp Cams XE264HR-14, GT-40P heads w/ Alex's springs, Weiand 8124 Street Warrior,
    Summit Racing 600CFM carb, 8.8 Turbo Coupe rear end w/ 3.55 gears, '94/'95 Cobra brakes, '85 Town Car M/C, '93 Cobra booster, MM Panhard Bar, MM Strut Tower Brace, MM 4 point K-member Brace

    '68 Mercury Cougar, w/ '88 5.0L, 4V
    My photo website:
    http://www.twilightphoto.com/

  16. #16
    FEP Senior Member 854vragtop's Avatar
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    I wanted to give a little more of an explanation as to why you need to adjust the accelerator pump arm spring/bolt combo with the car up to temp and with the choke off the high idle detent. If you were to take out all the slop/clearance with the choke on and the throttle plates cracked open on the high idle detent, as soon as the choke comes off the detent and the throttle plates closes down to the normal idle setting, you'll have introduced a gap in the linkage. That extra throttle movement required to rotate the pump arm and lift the pump arm delays the accelerator pump shot. Any delay and you'll have hesitation or bog.

    For those of you new to carburetors, the sole reason for the accelerator pump is to prevent bogging and hesitation when opening the throttle plates.

    Here's the reason:

    Each barrel of the carburetor has a necked down passage way called a venturi. When air flows through it at its narrowest point, it has to speed up. At the same, when the air speeds up, the pressure drops (look up Bernoulli’s principle). This is also where they've placed the booster venturi, right in the middle of the narrowest point of the main venturi. This local pressure differential between the venturi and the atmospheric pressure acting on the fuel in the float bowl(s) helps fuel to get sucked out of the booster venturi where it atomizes and mixes with the airflow. The problem with all carburetors, is that it takes a split second from the time the throttle plates open, to the time the air starts flowing through the venturis, to the time it takes the fuel to start moving through the carb internal passages, atomize and mix with the air, etc. But, your engine needs that fuel NOW to keep the mixture ratio constant and the engine running smoothly. There will always be this time lag for the fuel to get moving and mix with the air. So, the engine leans out briefly and stumbles. To make up for this delay, the accelerator pump system sprays in fuel immediately to compensate. Getting the volume of fuel from the accelerator pump just right is there reason they have all the different pump cams, squirter nozzles, and adjustments on a Holley. Holley’s, by far, have the most adjustment to dial the accelerator pump in just right.

    One of the advantages of the 4180C is the annular booster venturi in the primaries. The design of the annular booster has more holes around the perimeter and more surface area to help atomize the fuel. This also helps with crisp throttle response. Note: the Summit Racing carb has annular boosters on all 4 barrels.
    Last edited by 854vragtop; 08-14-2022 at 05:19 PM.
    '85 Mustang convertible GT, 5 speed, 4V
    Stock bottom end, Comp Cams XE264HR-14, GT-40P heads w/ Alex's springs, Weiand 8124 Street Warrior,
    Summit Racing 600CFM carb, 8.8 Turbo Coupe rear end w/ 3.55 gears, '94/'95 Cobra brakes, '85 Town Car M/C, '93 Cobra booster, MM Panhard Bar, MM Strut Tower Brace, MM 4 point K-member Brace

    '68 Mercury Cougar, w/ '88 5.0L, 4V
    My photo website:
    http://www.twilightphoto.com/

  17. #17
    FEP Super Member gr79's Avatar
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    Although my car is a carb 2.3, same thing happens from time to time.
    When stopped (If i remember) I just blip the throttle then clutch it.
    Is the only time a stumble may happen. Most of the time it rolls out smooth without any thought of bog.
    Is a 2v Holley with annular boosters and center pivot bowl. 30cc pump, 8.5 pv, close to stock size jetting.
    The manual choke allows course 'tuning' with the air/fuel mix.
    Same thing happened with the smaller stock Holley/Weber 5200 2v.
    I figure it more of engine design, mods and vacuum hose config, special tuning.
    May find out how to dial out the hesitation someday without messing up any of the other 99% of goodness.
    Cant always drive a 2.3 like a v8 rpm wise. Prob not the carb but the driver here.

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