Ask The Editors: Smoking from the Spark Plug

Where there’s smoke, there’s oil.

Great start on your investigation – the three ingredients for combustion are of course fuel, spark and air. It sounds like you’re getting all of these and that’s why it does occasionally start for you. That smoke coming from the spark plug hole of the cylinder is the culprit for the rest of your troubles.

Burning oil is what causes smoke and since this smoke is escaping from the top of the cylinder, it’s a safe bet you have engine oil entering the ignition chamber. Next time you try to get the quad started and this is happening, remove the plug and take a look at the electrode / strap. It will likely appear greasy or wet with oil.

Once the plug gets oiled up, it stops producing reliable spark and, as you are noticing, starting it can be impossible or, when it does fire, keeping it running correctly isn’t in the cards either.

So the question then becomes why is this happening? There are a few potential causes. We would start with the most likely – a worn oil ring on the piston will allow engine oil that originates in your ATV’s casing, to escape past piston directly into the combustion chamber. If this is the case a top end rebuild is in order. Could you get away with just replacing the ring? Possibly but if it’s that severely worn, so is everything else and if you already have it apart, this is the time to drop in a new piston, rings and fresh gaskets. Keep in mind the piston itself could also be the culprit – all it takes is enough wear to allow oil to sneak past.

Less likely is that the cylinder wall is either warped or cracked. You can’t be sure until you remove the top end, inspect the piston and rings for wear. A micrometer will let you know if the cylinder is warped. Check the walls for imperfections, dents, cracks etc.

Another possibility is the valve seals. Valves open and close with each cycle to allow fuel and air into the chamber. If these seals wear out, oil necessary to lubricate the assembly will be free to enter the cylinder.

Lastly bad gaskets can allow oil to enter in extreme cases. The head gasket sits between the head and block and its purpose is to seal off the combustion chamber while keeping the oil and coolant (on liquid cooled models) from mixing. The reason we put this one last is when the head gasket fails, there are usually telltale signs that you haven’t mentioned – among these white exhaust smoke as coolant is burned with the fuel and milky engine oil as, again, coolant will have mixed with the oil supply. In the case of an air-cooled unit like the 125-UT, you’d still be getting excessive smoke from the exhaust when it did fire up.

We’d start with a fresh top end and go from there.

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