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Spark plugs... some chocolate some white

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Quack_Addict, Apr 15, 2002.

  1. Quack_Addict

    Quack_Addict 1/2 ton status

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    I changed out the plugs in my `84 K20 yesterday. Plugs were R44TS; 3 plugs came out with chocolate colored tips, 3 came out gray/white, 1 came out white and the last one came out with crud on it almost like it was running very rich on that cylinder.

    The white plug came out of #5 and the cruddy plug from #6.

    Here's the engine... 350 bored 30 over, flat top pistons, 76cc heads with large valves. Edelbrock 650 cfm carb on an Edelbrock Performer intake. Headers & 2.5" duals. Maybe 3-4k miles since I rebuilt it. This is the third time I've replaced the plugs since the rebuild (first two sets were R45's, last set was R44's, put R45's back in yesterday)

    Basically the truck runs excellent with fresh plugs... then after a little while (500 miles or so) performance goes down hill and power surges during acceleration.

    Does this sound like a carb problem... like a rejetting issue? The plugs tell me it's running lean on some jugs fine on others and I don't know what all that crud came from on the other plug.

    Thanks for any help.
     
  2. Confederate9

    Confederate9 1/2 ton status

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    i dont have an answer but a question, if i changed my spark plugs, what should i expect to notice performance wise.
     
  3. TopOff

    TopOff 1/2 ton status

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    Check for leaks in the intake manifold on #5. Also, are you running the EGR?

    Check that for proper operation (or ditch it until smog time).

    Good Luck.
    TO
     
  4. Quack_Addict

    Quack_Addict 1/2 ton status

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    Nope, no EGR.

    Confederate9 - Assuming you haven't changed your plugs in a while, you could pick up some ponies you haven't seen in a while. Spark plug electrodes erode after a while, faster if you run a higher voltage coil like some of the aftermarket units. New plugs = smoother idle, maybe a little extra fuel economy (which you probably won't notice), better acceleration, etc. Also, if you haven't changed your cap & rotor in a while, that's a good thing to do too. Plug wires might be good to upgrade if they're old, frayed, or cross-firing. Distributors like to accumulate a build-up of crud where the spark fires and you should, at minimum, clean that off with sand paper if you're not going to replace the cap & rotor.

    I'm going to spray ether around the intake with the engine running tonight to check for an intake leak. How do you check for an intake leak inside the engine, ie. a leak that draws from the valley?

    Occasionally, when shifting gears on the SM465, I get a puff of smoke when the clutch engages out both pipes. Not sure if that's got anything to do with the issue, but it only lasts for a second and only when shifting gears.
     
  5. Blazer1970

    Blazer1970 1/2 ton status

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    An intake leak at the bottom of the port that sucks air from the valley will normally oil foul the spark plug.
     

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