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  1. #1
    Senior Member LXCHEV's Avatar
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    Default SBC - Troubleshooting oily spark plugs

    Hi all,

    Would love some opinions/thoughts on what might be causing the spark plugs in my 381 SBC to be constantly covered in oil (all the way up the threads). Fairly consistent across all 8 plugs - some slightly better than others.

    This current engine turns 10 years old later this year (very low km weekend cruiser). It's always had this issue since first built. I didn't pay a lot of attention to it in the early years, but it's something that bugs me now and I'd love to diagnose the cause and then fix it. It doesn't 'foul' plugs as such. Having said that, if I do replace the plugs after say a year or more of use, it'll always feel sharper again. Car does all the right things though, makes good power on the dyno, performs well on the track. Compression test returns a consistent 190PSI.

    Some of the American forums have interesting info - with some guys saying an excessively rich carb can coat the spark plug threads with unburnt fuel (that actually looks like oil). However I'm fairly confident my current carb is now configured pretty decent (I ran a 750 Ultra HP d/p for about 8 years, before switching recently to a 650 Street HP). I'm certain it's oil.


    • There are no external oil leaks, outside the engine is bone dry and has always been bone dry.
    • It's coming through the combustion chamber - so rings or valve guides/stem seals right? Could it be intake manifold gaskets? How do I troubleshoot the cause?
    • The engine doesn't use excessive amounts of oil, but I will periodically top it up from time to time - so there is a small amount of oil consumption present.
    • I didn't run a PCV valve for 8 years, but I do now.
    • I've always run a thick oil (Castrol 25/50) and have high oil pressure (60 - 70PSI on a mechanical gauge).
    • The heads are old Pro Topline's, but were fully re-co'd at the time of engine build - new intake valve guides, honed exhaust guides. All components replaced with new items (valves, valve stem seals etc).
    • Heads were also decked significantly (0.080" to achieve 65cc chamber target).
    • My engine builder did open up my clearances slightly as I had threatened to run moderate nitrous at the time (which never eventuated) - so that's one mistake I made. I'm trying to find my notes with the exact specs, but can't find them.
    • Plugs are NGK BP6FS (1 heat-range colder than standard), with the engine making ~440 Flywheel HP. Tried NGK 5's at one stage just to see, but even after a couple of days use, could already see the oil on the threads.


    Open to any and all feedback.
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  2. #2
    Senior Member LXCHEV's Avatar
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    The 3 photos I've shared above are all from different era's/tunes by the way. The first 2 pics are typical of what I see when I pull the plugs. The 3rd photo with the red background was when I had the NGK 5's installed for a few days only.

  3. #3
    Senior Member LXCHEV's Avatar
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    Here's a shot of the Pro Topline's... (Iron Lightning) - to show what valve stem seals were used...
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  4. #4
    Senior Member BasicQ's Avatar
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    Id love to know too. Mine do exactly the same thing and I have accepted it.


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  5. #5
    Senior Member LXCHEV's Avatar
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    Really? Wow, you've already made me feel better just knowing that I'm not alone, hahaha!!

  6. #6
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    hi
    valve guide wear ,,smoke on start up ??
    Remove and inspect inlet mani and look at gaskets ,,leaking oil into ports

    Compression check ALL WAYS CRANK THE CYLINDERS THE SAME AMOUNT OF ROTATIONS
    dry differences between cylinders
    wet 3 pumps from oil can,,,if psi comes up rings are not sealing [it will come up but by how much look at diff between cyl.]

    Leak down test
    WHAT EVER U DO REGULATE FEED AIR TO 100PSI

    Use either single/dual gauge set up and zero as per instructions
    3% leak seal race motor
    5% for everybody else
    10% borderline wear very obvious

    float level // fuel psi check

    light throttle cruise mixture 60-80km/h 14;1 air fuel ratio
    power valve opening just under cruise vacuum
    power valve mixture restrictor drilled to acheive WOT mixture

  7. #7
    Senior Member LXCHEV's Avatar
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    swampy - it's hard to detect smoke on startup with mine as I run dump pipes under the rear of the car. I might re-install my tail-pipes for a while so I can monitor that more closely.

    comp testing - yep, I always crank each cylinder the same amount. All my tests have been dry, with super consistent results - have done this every couple of years. I should try a wet test though just to see.

    A leak-down test has been on my radar for a while. Assuming it's between 5% - 10% though (even if it was say 15%) - would this identify the cause of this issue or just tell me I have general wear? I assume using a stethoscope and just trying to listen to where air leaks is the go.

    Fuel - mechanical pump, self regulated at 6.5 PSI. Float levels no higher than mid-window.

    AFR gauge purchased, just need to install!

  8. #8
    Senior Member BasicQ's Avatar
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    I came back from a drive just now (tuning on a 975 Race Demon).

    For mine I have tended to agree with the fuel theory as stated above. Second pic below shows first 2/3ds of thread dry. So I am thinking what looks like oil is condensed fuel and carbon.

    [img]https://i.ibb.co/QXMs4x7/9-FD47-C14-1458-411-C-A029-F6441-F5-EEDE6.jpg[/img]

    [img]https://i.ibb.co/bQPtQ5T/0-FD6-B68-E-5893-419-B-B34-E-A37-AAF5-A4096.jpg[/img]


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  9. #9
    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by BasicQ View Post
    I’d love to know too. Mine do exactly the same thing and I have accepted it.


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    ... ditto

    on my Holden 304/308 carbed race engines .. happens with NGK, Bosch or AC Platinum plugs
    (and I swap plugs fairly regularly too)

    never really worried me.. nor does it seem to affect performance (seat of the pants comment)
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  10. #10
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    hi
    LX
    When using a leakdown testfirst u have done a dry compression test then compared to wet test .This might/should give u an idea of the weakest cylinder. This will give u an idea b4 doing the leakdown. When doing the leakdown in a ""GOOD"" engine its 99% time ring wear thats the culprit . U will rings leakage hear the air rushing out the oil filler //exhaust valve leaking = exh noise and inlet valve leaking = carb leaking

    Summary ...dry /wet test diagnoses rings or valves ...Leakdown is for mainly ring/ bore wear..Over 5% = caution ,, over 10% rebuild

    Had a decent low compression giving 130psi even [ as expected ] but leak down was 10% ,hone markings were 3/4 missing .
    This engine had carby metering issues and acceleration problems

    Many OEM engine warranties must have leakdown test done
    Last edited by swampy; 16-04-2021 at 05:57 PM.

  11. #11
    Do you ever leave? EH179's Avatar
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    Quote Originally Posted by LXCHEV View Post
    swampy - it's hard to detect smoke on startup with mine as I run dump pipes under the rear of the car. I might re-install my tail-pipes for a while so I can monitor that more closely.

    comp testing - yep, I always crank each cylinder the same amount. All my tests have been dry, with super consistent results - have done this every couple of years. I should try a wet test though just to see.

    A leak-down test has been on my radar for a while. Assuming it's between 5% - 10% though (even if it was say 15%) - would this identify the cause of this issue or just tell me I have general wear? I assume using a stethoscope and just trying to listen to where air leaks is the go.

    Fuel - mechanical pump, self regulated at 6.5 PSI. Float levels no higher than mid-window.

    AFR gauge purchased, just need to install!
    Try dropping the level down to 25-30% of the window. Mid window is a bit high for a Holley.

    Who knows what they are putting in the fuels these days.

    You have tried increasing initial timing to see if that helps?

  12. #12
    Do you ever leave? EH179's Avatar
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    Quote Originally Posted by BasicQ View Post
    I came back from a drive just now (tuning on a 975 Race Demon).

    For mine I have tended to agree with the fuel theory as stated above. Second pic below shows first 2/3ds of thread dry. So I am thinking what looks like oil is condensed fuel and carbon.

    [img]https://i.ibb.co/QXMs4x7/9-FD47-C14-1458-411-C-A029-F6441-F5-EEDE6.jpg[/img]

    [img]https://i.ibb.co/bQPtQ5T/0-FD6-B68-E-5893-419-B-B34-E-A37-AAF5-A4096.jpg[/img]


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    The carbon on base ring would suggest rich condition and the chamfering/erosion on centre electrode edge suggests too hot from timing.

  13. #13
    Senior Member LXCHEV's Avatar
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    Lowering the float levels is certainly worth trying - I might start there and see if it makes any difference.

    Timing annoys me. My current ICE setup means I'm restricted to 20 initial, to keep my total at 32. My car feels great with more initial - hence my interest in the vac advance from other recent threads. I'd love to throw 30 initial at it and see what effect it might have.

    Surely there's a solution.

  14. #14
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    LX,
    Why don't you get a GM HEI for it? Put your own curve in it so you can run MVA. $80 brand new from Road Star in Sydney. I would be happy to set up the dist for no charge. I provided the procedure the other day in the other thread for determining the initial timing + amount added at idle with MVA. If you do that, then we have the numbers to set up the dizzy.
    You can get RS to send the dizzy to me & after I have done the mods, I can COD it to you with Aust Post. PM with Ph# if interested.

    As to your plugs. NGK recommend a 5 heat range, which for your combo you should be using. A common mistake is to go to colder plugs because the engine is 'hotter'. Colder plugs are really only reqd with very high CRs, not normally aspirated, continuous WOT operation such as track/rally driving.

    As to the plug conditions. Agree with others about the threads oiling up, have no explanation other than the engine in that area the head never gets hot enough to burn off the oil.
    I think the black electrodes could be combined fuel+ oil fouling. Are the v/guides worn? Rocker tip geometry ok? I am not a fan of the opt quoted 'smallest width, centered roller tip mark' on the valve tip. What this means is that at max lift, the roller is NOT centered on the valve tip & is pushing the valve sideways into the guide, right when max spring pressure is at work & therefore the side force on the guide is at a maximum. There is always going to be some side force on the guide from the valve stem as the valve goes through it's lift cycle. But I believe there will be less guide wear if the roller is centered on the valve tip at max lift [ & therefore max spring tension ] rather than centered at, say, 1/3 valve lift.

  15. #15
    Senior Member LXCHEV's Avatar
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    Very kind of you Geoff. I’m pretty keen to try a GM HEI with MVA - experimenting is the only way to learn right.

    Have you used those Road Star dizzies for a decent length of time to verify their build quality? They’re amazingly cheap. I had one for a Holden 304 that came with a project car. I decided to sell it and had the original Bosch unit re-co’d instead. Scorcher ignition at the time mentioned that the Chinese ones are decent enough, but can have lower quality/cheaper hall effect sensors (pretty much their only weak point). Do you trust them to last?

    Thanks for the other feedback too - plenty more to look into. I should put my NGK 5’s back in for 6 months and see how they go. I’ll have to take a closer look at my rocker geometry too for interests sake, but pretty confident my engine builder was pretty fussy with all that kind of stuff.

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