TECH TIP - Dirty Spark Plugs (2023)

by Mike C.-

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Oh yeah, the title says it all. You tuned and tuned the engine, you tuned the carburetor until you couldn't tune and tune anymore. ANDstillYou take out the spark plugs and they go black like the ace of spades. What gives? Where should I turn now? Well my dear folks at VW, I have been given the difficult and somewhat scientific task of explaining what causes sooty spark plugs and what can be done about it. I understand many of you have had or still have this problem and just hit a dead end on Sooty Street. I hope I was able to help you find a solution to your problem...

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TECH TIP - Dirty Spark Plugs (1)

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"Soot" is defined as a black, somewhat powdery material (carbon deposits) covering the center and side electrodes and center porcelain of the spark plug. In high enough concentration, soot can cause power loss or misfire due to a weak or improperly jumped spark. What I mean wrong by that is this: the sparks fly along the path of least resistance. If that lowest resistance is somewhere other than the center electrode, the spark will find a spot where less carbon has been deposited and jump there. This causes the flame to spread differently and not always push the piston down before it loses strength. In other words, lost power! I will go through the causes one by one. There could be more, but these are the most common and are the most likely to cause the soot problem.
  1. First check the condition of the air cleaner (oil bath element for models up to mid '72 and pleated paper element for later models). When the filter is dirty it can act like a restrictor and draw in more fuel as it restricts the airflow to the carburetor. You will not get a solution to the problem unless you remove them first. Also, if you unnecessarily restrict airflow through the intake system, you will lose power.
  2. Stuck or poorly adjusted choke mechanism or choke not coming out. A stuck choke consumes more fuel after starting the engine than is required for cold engine operation. This causes excess fuel to stick to the spark plugs and creates soot as the fuel burns. If the choke does not come out, check the choke removal diaphragm and make sure it is not punctured or hardened and cracked. Make sure the choke is set according to the VW workshop manual. Also replace the heated choke element if you are not sure how old it is.
  3. Just driving your VW around the block to the shop and back will not fully warm up the engine. Also, constantly turning the engine on and off just to get the car moving will definitely damage the spark plugs. This makes the problem worse by preventing the choke from fully disengaging and burning off excess fuel. Allowing the engine to fully warm up will keep the internals much cleaner, as well as the spark plugs. At least once a week, take the VW out onto a freeway or highway and drive it longer distances for half an hour to an hour. Don't we like to drive in our VWs anyway?
  • TECH TIP - Dirty Spark Plugs (2)
  • The carburetor is not adjusted correctly. Solex carburetors are finicky beasts in the sense that there is a specific way to adjust them. Whatever the manual says, follow it. If the fuel mixture is incorrect at idle (small screw), you may be leaking more fuel than is needed to idle the engine. If the float level is too high, excess fuel will leak undispensed from the carburetor into the engine, causing a rich condition that cannot be corrected with the normal mixture screw adjustment procedure. Try to get the timing right before making any adjustments to the carbs as timing will affect the final mix as I will explain later.
  • A loose carburetor or warped carburetor flange or intake manifold flanges and bad gaskets will render adjustments useless and the fuel mixture deformed, so fix it.
  • A clogged heat bridge in stock intake manifolds can cause the mixture to go bad. The reason? It is necessary to heat a mixture of air and fuel so that it completely vaporizes so that it does not accumulate at the bottom of the intake manifold. Once the fuel has accumulated, it rushes to the cylinders, then to the spark plugs, and then ... you get the point! Old collector heat risers are likely to become clogged with carbon buildup over the years. Refurbished manifolds can be purchased and are often the only solution to this common problem.
  • The engine may not reach the correct operating temperature. I know many of you have already removed the thermostat and cover flaps from your engines. That might be fine in the summer, but if you drive your VW a lot, the engine won't warm up as quickly in the winter. The engine will last longer if it warms up faster and more completely at any time of the year. I can say with confidence that if they fail, these thermostats will fail completely, so the engine will not overheat. Do yourself a favor and leave the thermostat system on, or turn it back on when it's not on the engine.
  • The ignition system could be weak or something is preventing all of the spark energy from reaching the spark plugs. Remove a spark plug with the wire still attached, touch the spark plug (hold it by the wire) to the engine or body, and have a friend crank the engine with the idle solenoid disconnected. The resulting spark you see should be blue in color, not white or yellow. Replace the ignition coil if the spark is not blue. When the spark plug wire insulation is old and cracked, the spark energy escapes through the spark plug. You usually feel this as a backfire and can see it in the dark at night. Whenever possible, replace all leads with genuine Bosch spark plug leads as these are some of the highest quality replacement leads available for your VW air-cooled engine. Make sure the tips and condenser are in good condition and the tips are seated properly. If the ends seem burned and punctured and have seen better days, replace them. Electronic ignitions (which replace spark plugs) either work perfectly or not at all, so this in itself is not usually a problem related to fouled spark plugs. Again, make sure the start time is set correctly. The longer the cranking time is delayed, the more idle fuel is required, contributing to the soot plug problem.
  • Make sure the heat range of the installed spark plugs is correct. If it's a very "cold" heat area, the spark plug won't get hot enough to effectively burn off the deposits. I found the Bosch W8AC to have a very good heat range for both stock and smooth road applications, but experiment if you have a race engine or hot road to determine the correct heat range. The correct color for the spark plug porcelain on the center electrode should be a very light beige near the tip that turns white with maintenance after the engine has run normally for a few days or weeks. Check that the spark plugs are fully seated and tight. I know it sounds simple, but it does happen and can cause other potentially serious problems in addition to soot plugs.
  • There may be an exhaust restriction. While it's pretty hard to imagine such a restrictive VW exhaust system, look for piping that's bent or crushed beyond normal design limits. If a sufficient amount of exhaust gas returns to the cylinder, it will surely foul the spark plugs, causing a lean condition and loss of power.
  • TECH TIP - Dirty Spark Plugs (3)
  • Running a street or racing engine with a "big" camshaft could be the cause of fouled spark plugs. A camshaft with too much overlap (intake and exhaust open at the same time) will result in strange mixture flows at lower engine speeds, especially if the cam is designed to be efficient at 5000 or 6000 rpm versus 3000 rpm or higher for a good road setting is. It would be very difficult at best to tune the carburetors to eliminate the soot problem with a hot, street camshaft, although I'm sure it has been done. If you're installing a camshaft that's hotter than stock, be sure to install it four degrees ahead of the crankshaft and make sure it fits the direction you want to go. It's always best to get a little less camera than you think you might need, unless you absolutely plan on driving your VW solo at a racetrack and towing it home.
  • Finally you can make onefewdifference whether you use regular or premium petrol. Because? Regular 87 octane gasoline actually has a higher heat content per gallon than premium 93. That means 87 actually burns hotter and potentially a little cleaner than 93. That means if you can drive 87 there's no risk of detonation or ping. If your engine is well tuned and you are maintaining compression at or near 7.5 to 8:1, you can probably run 87 octane fuel. Higher on compression and you'll probably need 93 octane. Experiment with it and only use what you need. If your '87 engine seems to be running fine, use it. It's cheaper anyway.
  • I hope all of these tips get you one step closer to removing soot from your spark plugs. There's a lot to think about, but once you find the solution, it will be easier to keep track of from there. If anyone has anything else to say about fouled spark plugs, please post it on the Midsouthvw website forum or let me know. All comments and suggestions are of course welcome.

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