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Read This Mysterious Plug

Discussion in 'The Garage' started by 78Suburban, Apr 5, 2006.

  1. 78Suburban

    78Suburban 1/2 ton status

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    Here's the story.. I've had all sorts of fun performace problems, including missing under throttle, fouling plugs ect. I took off the q jet in favor of an edelbrock. I took off the edelbrock and rebuilt it a week ago. I fired it up this weekend and let it idle, it was idling ok. It finally cut out. I think it might have run out of gas. While it was running I noticed a vaccume leak, and found it was around the carb.


    I borrowed a vaccume guage so I can tune my idle mix screws precisely and find out what is wrong with this motor.
    Today I realized that the vaccume leak had happened after I rebuilt the carb, because alot of the little star screws on top of the carb were very loose. I tightened them all up. Problem solved, right?
    I just threw 2 gallons of premium in the truck and sat there with the key turning over for a LONG time.. giving the starter a few rests of course. The clear fuel filter was full of fuel for quite some time. It won't even try to fire at all right now. So I decided to pull a random plug, half expecting it to be fouled with rich fuel deposits. Well, the plug is bone dry, not really wet with gas, dunno why... but here's another odd thing. It looks like it has been fouled at somepoint with black carbon deposits (man I've gotten used to cleaning those with a wire brush). But it also has a white spot on the electrode, as if its run lean.. its really throwing me for a loop.

    [​IMG]

    [​IMG]

    [​IMG]

    Anyone got any ideas?
     
  2. 1977k5

    1977k5 3/4 ton status Vendor

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    Looks pretty rich to me, I always look at the porcelain to judge lean/rich (pretty dark in your case). It could be rich due to the choke, I would not say that plug is fouled. Carbed motors are easy: If you have fuel and spark it will start. Check spark at each plug and then make sure the carb squirts fuel from the accelerator pump when you move the throttle.
     
  3. 78Suburban

    78Suburban 1/2 ton status

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    well, I did run it a while with the choke on warming it up after I put the carb on. I've had plugs be solid black, but the little white spot is really throwing me for a loop. Would a vaccume leak make it run super lean like that? Maybe before I had found the vaccume leak (from not tightening down the screws on the carb enough after I rebuilt it) it caused it to run lean enough to burn off some carbon and make it a little white? I don't understand why I can't get it to fire now... maybe the carb is actually tuned wicked lean, but I could have had a sunk float before the rebuild causing it to dump tons of fuel into the motor? I just can't figure it out.. I've only got one day to borrow this vaccume guage, so I would really like to make her crank up so I can get those idle mix screws tuned perfectly. :crazy:
     
  4. 1977k5

    1977k5 3/4 ton status Vendor

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    The only time I could imagine the tip of the electrode being black is if the plug is fouled and not firing. The porcelain tells you how the mixture is burning (somewhat rich in your pics, due to the choke probably). If you cannot get the engine to fire you are not getting either spark or fuel: period. If it starts and doesn't run well then you can worry about whether it is lean or rich.

    As far as adjusting the idle screws, I have always done it this way:

    With the truck fully warmed up: Screw the adjusting screws all the way in (not tight, they break easily) and then back them out a little. Start the truck and set the idle where you want it. Unscrew the idle mixture screws very slowly until the idle increases slightly. Reset the idle. Repeat until unscrewing the mixture screws makes the idle stay the same or drop. Make sure the idle is where you want it and you are done.
     
  5. muscle

    muscle 1/2 ton status

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    looks rich to me.. not lean.
     
  6. bigbadchev84

    bigbadchev84 1/2 ton status

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    when was the last time you put new, not cleaned plugs in? Plugs are relatively cheap, might not hurt to toss some new ones in there, you may have a bad one, ive had them bad out of the box before too.
     
  7. 78Suburban

    78Suburban 1/2 ton status

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    I've gone through two or three sets, and they all end up turning very, very black.. the white on the tip of that electrode is what was throwing me. Is it concievable to run rich if you have a vaccume leak?
     
  8. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    Well to tell what the problem is spray a shot of staring fluid down the carb and if it starts or fires at least then you know its fuel if not then its ignition.

    Also be sure that all your vaccum lines are connected and in the right places. Watch out for the vaccum ports on the bottom of the carb coming from the base plate. I missed one when I put my carb back on and it would fire but not idle.

    Also are you sure you installed everything back in the carb the way its supposed to go?
     
  9. 1-ton

    1-ton 1/2 ton status Premium Member

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    I think that the plug looks like the fuel mixture is OK to me...not perfect, but good enough not to cause a major problem. The plug looks more to me like it is not burning all the fuel completly during the combustion stroke, which can be caused by either incorrect valve train or ignition timing issues.
     

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