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Thread: What causes a black Spark Plug

  1. #1
    Aprilia Forum Moderator High Gain Tuning's Avatar
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    What causes a black Spark Plug

    I answered a post regarding this and went into a little more depth than I intended...

    This comes up a lot so I am giving it a thread of its own so it can be found easier at a later time via a search.

    Here's a little course on what makes a plug dark-black

    When air and fuel burns completely and efficiently, the results are CO2 (Carbon Dioxide) and H2O (Water). Nothing more, nothing less.... neither of these has color to it. It's not much different that a fire log that has burned completely and efficiently.

    If you have a fireplace or enjoy camping and have seen the results of a good burn, the results are just a fraction of what you started with...white ash left at the bottom of the pit.

    I am sure you have also pulled black bits of log out of a pit just like you have pulled spark plugs from your engine that are black on the ignition end. The black is carbon. When carbon is produced so is Carbon Monoxide which can be lethal.

    30 PPM Permissible Averaged over 8 hours
    200 PPM Maximum for acute exposure
    800 PPM Lethal < 2 hour exposure

    Carbon Monoxide is caused by partially burning fuel. In its smallest form, a fuel molecule can be compared to a fire log. If you put 1 fire log in a pit and let it burn efficiently with enough oxygen to support combustion it would leave a white ash on the outside as it burns to the core. If it continued to burn efficiently it would leave nothing but a very small amount of white ash.

    If you start to throw more and more logs (fuel molecules) in that fire pit you start to smother the efficiently burning log and the oxygen it was getting to burn efficiently is diminished. The fire will start to burn cooler and less efficient. The flame will lose its brightness and start to get more orange and the results will be the formation of black carbon.

    A Carbon Monoxide Molecule is a partially burnt molecule of fuel. If you continue to smother your log (fuel) with more logs (fuel) the flame will go completely out and leave you with Carbon Monoxide and Carbon. We cannot see a Carbon Monoxide Molecule because they are too small, colorless and order less but we do see their residuals as the black outer carbon shell they leave.

    Take your log out of the pit that was burning well but later smothered out and cut it in 2. What you will see is un-burnt log (fuel) in the center and a black outer shell (carbon). A partially burnt log is like a partially burnt fuel molecule.

    The black on your plug is the outer carbon shell. If your mixture has never burnt well, you can wipe off the black carbon and find nothing but more black carbon. If your mixture burns well at high speeds but not at idle you can pull a plug after a long ride that has a short time to idle and find carbon that can be wiped off to reveal a white residue and a clean electrode.

    Remember, a completely burnt log (fuel) leaves very little of what you started with... white ash (CO2 and H2O). A partially burnt log leaves most of the log with a thick black outer layer. This is why a good burning mixture leaves very little to nothing on the plugs electrode when a poorly burning mixture quickly starts to build up a black thick layer of carbon.

    So put more simply, the black is carbon caused by the lack of oxygen to burn the fuel completely.

    Although that may now seem simple, the cause(s) may not be. Causes can be:

    Correct amount of fuel but incorrect amount of oxygen
    Correct amount of oxygen but too much fuel
    Lack of compression to support complete combustion
    Poor spark to start a good combustion
    Cold engine temps that don't support proper vaporization

    Each of these can have a host of issues that cause them to occur that would require another page of explaining.

  2. #2
    apriliaforum Member joseverdin10's Avatar
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    I vote this be added to the faq.
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  3. #3
    apriliaforum expert PallOlavsson's Avatar
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    FAQ for sure, this is so seldomly explained.
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    apriliaforum Junkie SR SBK 2003's Avatar
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    thank you

  5. #5
    Moderator (ADSUM) Grenadiers's Avatar
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    Well, lets hope that the motion is carried!!!

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  6. #6
    apriliaforum expert TiminIndy's Avatar
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    What about idling?

    On my old Ditech, I always let it warm up 3-5 minutes until the temp needle started to rise. I know that after 13,000 miles, it had a pretty good buildup of carbon in the cylinder, although normally my plugs looked pretty good. I suppose the cylinder buildup after that many miles is pretty normal, and I never used any fuel additives back then (never had slop or sputtering on that scoot either).

    Someone posted on here once that the idling and the warming up period was when the most amount of carbon occurred and recommended a minimal warm up period, so with my new `07 Morini (for the last 2 years), I have always started it and only wait about 30-45 seconds and take off (and let it warm up as I head down the street).

  7. #7
    Aprilia Forum Moderator High Gain Tuning's Avatar
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    All engines run a richer mixture at start up to enhance combustion. The DiTech less than any other fuel system because the fuel is injected right into the combustion chamber. Carbureted and EFI engines need a much richer mixture at start up because rather than the fuel atomizing further after leaving the fuel delivery source it condenses due to the cold objects it comes into contact with, runners, block, crank etc.

    Fuel is heavy compared to air. It does not take turns at high speeds. The first cold object it hits will cause it to condense. After those objects reach a temp capable of vaporizing fuel, then they will help the fuel to vaporize further.

    If you do nothing but very short trips, you may never get the engine and plug warm enough to burn the carbon off but in most cases, what collects during the cold start enrichment is easily burnt off during the warm drive cycle.

  8. #8
    apriliaforum expert TiminIndy's Avatar
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    Whenever weather permits, I commute 11.5 mi each way to work - I suppose that would burn off anything I had gathered at warm up.

    I was going to buy some Yamalube Ring Free (per your suggestion), since my BG44 is all gone now. Is this good for regular maintenance (a little bit every other tank)? If so, how much/how often would you use to prevent carbon build up in the cylinder?

  9. #9
    Aprilia Forum Moderator High Gain Tuning's Avatar
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    I would use ring free in any 2T that is used during cold weather.... very good additive for fuel injected 2T snowmobiles. If your starting in 70F + weather you might want to do an initial dose then put it away until the Fall.

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