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  1. #1
    Member
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    Jul 2006
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    Corvette(s)
    1963 Convertible, 1969 Lemans Blue Coupe, 1970 LS5

    Default Fouling out plugs

    I seem to have a strange issue with my 70 ls5. I have been going through plugs after it sits overnight. This thing has always run a little fat and have not had a problem until recently. I thought it was the the carb so I purchased a new Holley 750 4 corner and I'm having the same problem. I also checked spark, fuel pump and all the other obvious things. Could my problem be related to a vac. leak in the car??? If it were a vac leak wouldn't the car run lean?? I have had a couple of mechanics look at it and we did jet the carb down. Still the same problems. After going through this for the past month am I missing something simple? Could it be a vac leak?? It seems to point to a fuel problem and everything that could be replaced was. Any direction would help.

  2. #2
    Member zachh's Avatar
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    Aug 2005
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    Corvette(s)
    1979 black 'n' silver L82 1978 Trans Am Y88

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    Do you put premium in it? Mine used to foul plugs alot. It sounds like your firing up to rich. It could be a carb adjustment, or your rings might be wairing out. Start with the cheapest options first. Try enlarging your spark plud gaps a little. That took care of my problems. Once the care is running, any unwanted oil in the cylinder will burn off. Ya just need the cylinder to be firing.
    Good Luck
    zachh

  3. #3
    GerryLP's Avatar
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    Corvette(s)
    1981 HD Suspension; ZN1 Option

    Default

    Quote Originally Posted by djk1566
    I seem to have a strange issue with my 70 ls5....
    djk,

    The boil-over usually is caused from the heat of the engine and misadjusted float levels. Perhaps you had that problem with the old carb?

    When the fuel floats are set too high and the heat from the engine cooks-over the fuel, it spills (almost as if boiling-over) and it tends to flood the manifold.

    The float level on the Holley 750's should be to the bottom of the tank's fuel level screws. Some Holleys have a see-through plug at this location. Moroso also sells these see-through plugs. WHen you lightly shake the car, the level of the fuel in the carb tanks should be visible. If when removing the sloted plugs the fuel spills-out, the you should adjust the float by holding the sloted screw AT THE TOP of the carb tank with a screwdriver, and rotating the jam nut counterclockwise to lower the level of the fuel in the carb tank. When turning about 2 flats of the jamnut from a reference point, this should lower the level perhaps a 1/16".

    Once the levels are correct, and if you can still smell (and sometimes hear) the fuel boiling, then perhaps you can add an insulator to the base of the carb to shield some of the heat from the engine.

    Check also your power valve. If you have experienced backfires, and the carb was missing power valve protection, then your power valve could be blown, and it is delivering lots of fuel down the intake.

    GerryLP

  4. #4

    Join Date
    Jan 2002
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    Washington, Michigan
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    Corvette(s)
    '67 Marina Blue Convertible

    Default

    What heat range plugs are you using?

  5. #5
    Registered User
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    Troy, Missouri
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    58
    Corvette(s)
    Plum Crazy Purple 1974 coupe

    Default

    Any sign of oil fouling? My '74 had that condition some time back. Oil from the valve train was seeping past failed valve stem seals once the engine was shut off. I suppose this could worsen any fuel fouling.

  6. #6
    Member
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    Aug 2006
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    Corvette(s)
    1970 BRONZE COUPE/WHITE 69 CONV

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    Quote Originally Posted by djk1566 View Post
    I seem to have a strange issue with my 70 ls5. I have been going through plugs after it sits overnight. This thing has always run a little fat and have not had a problem until recently. I thought it was the the carb so I purchased a new Holley 750 4 corner and I'm having the same problem. I also checked spark, fuel pump and all the other obvious things. Could my problem be related to a vac. leak in the car??? If it were a vac leak wouldn't the car run lean?? I have had a couple of mechanics look at it and we did jet the carb down. Still the same problems. After going through this for the past month am I missing something simple? Could it be a vac leak?? It seems to point to a fuel problem and everything that could be replaced was. Any direction would help.
    IF YOUR MOTOR IS STOCK YOU SHOULD HAVE 18 TO 20 INCHES OF VACUUM AT 1000 RPM. USUALLY IF YOU ARE FOULING PLUGS IN A SHORT PERIOD OF TIME IT IS EITHER OIL FOULING, OR A WEAK IGNITION. DO YOU GET ANY SMOKE WHEN THE CAR IS FIRST FIRED UP AFTER SITTING FOR A WHILE? IF SO THE VALVE SEALS COULD BE WORN. GOOD LUCK. HAPPY MOTORING RUSS

  7. #7
    Member
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    Wasilla, Alaska, U.S.A.
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    595
    Corvette(s)
    '77 L-82, black on black. Full mod 406 small block

    Default

    you said the engine has always run rich. so i would go ahead and do a compression test. you may have washed out your rings' seal on the cylinder walls and are getting oil from that. this will also make your compression drop and increase sooting in the combustion chamber at low rpm. the other checks mentioned are also a good idea.

  8. #8
    Member 71-84 IMSA Widebody's Avatar
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    71-84 IMSA Widebody

    Default

    Repeat from John Z, what plugs are you running? Have you switched to diff plugs lately?
    Come see me "On the Race Track" at the National Corvette Museum through May 07
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  9. #9
    Member
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    Corvette(s)
    1963 Convertible, 1969 Lemans Blue Coupe, 1970 LS5

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    Quote Originally Posted by AKRAY4PLAY View Post
    you said the engine has always run rich. so i would go ahead and do a compression test. you may have washed out your rings' seal on the cylinder walls and are getting oil from that. this will also make your compression drop and increase sooting in the combustion chamber at low rpm. the other checks mentioned are also a good idea.
    I did a compression test already and this thing is tight. Everything is where it should be. This motor was just rebuilt about 3k ago. This thing does not smoke at all and I have tried a variation of different plugs with not luck.

    What about points?? I took the points out and put is a pertronics unit?? I think thats how you spell it. Any how I'm running a msd 6. Should I go back to points with this configuration??

    Thanks for your help.

  10. #10

    Join Date
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    Corvette(s)
    '67 Marina Blue Convertible

    Default

    Again, what type/heat range of plugs are you using? Is that a secret?
    John
    '67 Convertible

  11. #11
    Member
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    Corvette(s)
    1963 Convertible, 1969 Lemans Blue Coupe, 1970 LS5

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    Quote Originally Posted by JohnZ View Post
    Again, what type/heat range of plugs are you using? Is that a secret?
    Started with Accel 576s. That was what was recommended by a local speed shop. I then jumped up 2 heat ranges with NGK's but what number I don't recall and I'm not next to the car to look....to many numbers in my head as it is. After 2 the increase still fouled out all 8 plugs...

  12. #12
    Member 71-84 IMSA Widebody's Avatar
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    71-84 IMSA Widebody

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    PLUGS, "could" be the whole problem, many times speed shops/racepeople recommend plugs way too cold for regular use, a too cold plug will foul out with one to a couple uses if you idle the car for even 1 minute. You also have to be careful on plug "heat ranges" between different maufacturers. From one a 5 is a very cold plug from another a 5 is a veryhot plug. I run a cold plug if all out racing Champion 4 or 5, for street show I run a Champion RC9YC or 12. On the other hand it may not be plugs at all, but its an easy one to eliminate.

    Please check this also, I was "told" that an increase number on NKG's is colder, if so, wrong direction. Verify if you can with your mechanic.
    Come see me "On the Race Track" at the National Corvette Museum through May 07
    http://community.webshots.com/user/david788 or www.fquick.com/IMSAWIDEBODY
    BESTS: 218mph 9.58@148

    Wisdom is in the eye of the beholder:
    "To do is to be." -Nietzsche
    "To be is to do." -Kant
    "Do Be Do Be Do," -Sinatra

  13. #13
    born2saw
    Guest

    Default

    Make sure you check your PCV valve, it is a common overlooked part responsible for plug fouling. Quick, easy, & cheap to replace. If your engine was recently rebuilt, make sure they replaced it with the correct one for your engine.

  14. #14
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    May 2004
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    TOLEDO,OHIO
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    Corvette(s)
    1968 BLACK ON BLACK COUPE

    Default

    CAN YOU TELL IF THE PLUGS ARE FOULING OUT WHEN YOUR CRUISING OR IDLING? IF IT'S IDLING AND FOULING THE PLUGS YOU MAY HAVE TO LEAN DOWN THE IDLE CIRCUIT AS THE JETS DO NOT AFFECT THIS CIRCUIT. I HAVE A BIG BLOCK THAT WOULD FOUL OUT NUMBER 3 COMPLETELY WHILE THE OTHER 7 RAN EXTREMELY RICH. I INSTALLED A MSD BOX AND LEANED THE IDLE CIRCUIT AND NEVER HAD ANOTHER PROBLEM AGAIN.

  15. #15
    Member
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    Quote Originally Posted by FLATOUT427 View Post
    CAN YOU TELL IF THE PLUGS ARE FOULING OUT WHEN YOUR CRUISING OR IDLING? IF IT'S IDLING AND FOULING THE PLUGS YOU MAY HAVE TO LEAN DOWN THE IDLE CIRCUIT AS THE JETS DO NOT AFFECT THIS CIRCUIT. I HAVE A BIG BLOCK THAT WOULD FOUL OUT NUMBER 3 COMPLETELY WHILE THE OTHER 7 RAN EXTREMELY RICH. I INSTALLED A MSD BOX AND LEANED THE IDLE CIRCUIT AND NEVER HAD ANOTHER PROBLEM AGAIN.
    The PCV valve was replaced and is correct so I know that is out. It is fouling out only at idle so I'll try and lean down the idle circuit tonight to see if this works. I really didn't even think about that. Thanks

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