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  1. #1

    Default Unusual Spark Plug Fouling

    When checking on the old spark plugs removed from engine (1983 5.0) I discovered that spark plugs 1,2,5,6 are really sooty while 3,4,7,8 are a little too clean. Looked around internet and couldn't find much. Have any of yall ran into this or have a possible explination?

    The engine is mostly stock except from carburetor (holley, 80457-13 written on choke) up and no egr spacer and valve.

    Spark plugs 2(sooty) and 7(clean) shown

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  2. #2
    FEP Power Member Broncojunkie's Avatar
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    The sooty plugs could just be due to ignition problems, such as bad plug wires.
    79 Pace Car - 331, t5
    79 Pace Car- 302, 4 spd
    79 Cobra - working on 351w, t5
    82 Capri- working on 302, t5
    82gt - working on 408w, c4

  3. #3
    FEP Power Member richpet's Avatar
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    That would be my guess, too. Also make SURE the wires are FULLY seated on plug and dizzy.
    Worst case - do a compression test if wires are not the issue. If those cylinders are not sealing as well (valves/rings) then it may not be fully burning either.
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! .040 over 302, Edel E-street heads... Currently building a 347 because, why not?

    "Never underestimate the power of stupid people in large groups"

  4. #4

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    Along with the sparkplugs i changed them with new wires. I'll update this thread when I inspect the spark plugs again. I'm still baffled and the stark contrast and the particular set of cylinders afected.

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  5. #5
    FEP Senior Member
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    sounds like intake gasket leaks or base gasket under carb. The front plugs rich, rear plugs lean. If you have the stock carb, could be idle jets need adjusted.

  6. #6

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    This is amazing thanks for sharing this post I have become a fan of your post now. This post is so interesting and informative.

  7. #7

    Default

    Is the intake dual plane?
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear, dura grip (both Yukon)
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  8. #8

    Default

    The intake is the stock aluminum dual plane. I have checked for manifold and carb base leaks before and found none.

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  9. #9

    Default

    I assume the planes don't map to your groups of foul plugs?
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    K&N filter in a stock dual snorkel, GT40 heads, Edelbrock 3721 intake, MSD 8456 Dist., MSD 8227 coil
    Comp cams XE254H, hypereutectic pistons
    Hooker Super Comp Shorty Equal Length Headers, catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear, dura grip (both Yukon)
    Ford Performance Springs, Firehawk A/S 225/55r16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 245/40r17 and 255/40r17 on OE cobra r wheels (race)
    AOD rebuilt with a 6 clutch direct drum, Koline steels stacked with 8 clutches, Kevlar band, superior shift kit, new torque converter. --Everything else stock and fully functional.

  10. #10

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    That is true. The difference is front half back half not left bank right bank.

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  11. #11
    FEP Super Member erratic50's Avatar
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    wonder if there's a fuel atomization problem with the carb spoiling the party

    I would make sure everything is completely clean.

    Also that there isn't a vacuum leak where the carb seals to the intake.

    Introduce a leak and all bets are off.

    Seems to me I fought this once because of a brake booster. The stupid thing still worked OK, but it leaked enough to cause problems.

  12. #12

    Default

    Hi there, I have similar issue, front 4 plugs clean rear 4 sooty! Did you resolve your problem and how ? Thanks Bob

  13. #13

    Default

    You say you have the stock factory 5.0 intake manifold.
    You mention that you’re not using the EGR cast iron carb spacer.
    Unless you’re using a special block off plate, there can be an exhaust leak at the carb base.
    The factory manifold has a weird design that uses the EGR spacer to completely seal the intake system.
    This happened to me when I tried to remove it and use a standard holly carb spacer.
    This could be a source of your problem.

    I would also hook a vacuum gauge up and see what the engine is doing.
    The Vacuum readings will help eliminate other problems such as bad valves etc.
    As previously stated, you might “just” need a mixture adjustment, an a vacuum gauge is the best tool for the job,
    Scott
    84 GT 5.0L, T5, 8.8T-Lock, 5lug with 93 Cobra Brakes,
    MM RCL's,CC plates, Koni Reds, 17" Tri-Stars...CoilOvers next.

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