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Spark Plug Lead Fouling


onthegas
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Over the weekend I competed in a vintage trials and a poker run on my M98 “Sherpina”. Both had some high-speed open field sections where I had the bike WFO in 5th gear. Twice the bike began to miss and sputter at full throttle. Changing the spark plug fixed the issue. Looking for an answer I discovered lead-fouling of spark plugs. This is more common in aviation, but it seems to make sense. Back in May I purchased some leaded race fuel, Trick 110, which I mixed 50/50 with non-ethanol premium (92 RON+MON/2), for running in my vintage motocross bike which needs the higher octane. Since I didn’t want the fuel to go to waste, I have been using it in other bikes. Two of the bikes my son rides/races have developed intermittent spark issues. He’s only 13 and hasn’t been able to fully explain what’s happening other than the motor dies. After my experience this past I believe the issue lies with the fuel.

Has anyone else had a similar experience?

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Points, condenser and coil were replaced ~10 years ago. Bike is only ridden 2 to 3 times a year. My sons Honda has an ignition that’s 2 years old and his TY80 had the points and condenser replaced 3 years ago, probably the original coil. The trouble on all 3 bikes started after using leaded race gas and after running at full throttle. The spark plug has some deposits (I’ll post a photo later). Each time changing the plug solved the problem short-term.

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It may not be lead fouling. It may be the change in fuel type causing existing carbon deposits to be released and which then stick to the plug. When we (in Australia) changed to unleaded "pump" petrol in 1986 from leaded petrol, fouled plugs on two strokes were suddenly very common where they hadn't been a problem previously. After a short time of running on the new fuel, the frequency of plug fouling went back to normal. You could see carbon deposits on the plug nose that looked nothing like a deposit that had formed gradually. Some but not all were like whiskers. I will be interested to see your photos.

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Not the best photos. The first two are of a plug with ~2 hours on it. The second two are of a plug that has been in the engine for roughy three years. Both plugs are NGK BP5ES. The bike started mis-firing when riding WFO. It would still idle, but would mis-fire under load (mainly my fat a***). Carbs are clean on all three bikes. 

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Best check carb jet sizes again, those pictures look as though the plug has been extremely hot, hotter than the grade of plug used. The gap will have an influence on spark power. If you carry a plug put it in a container that will protect it from the other stuff you carry. Your petro-chemistry is questionable in that the additives might not be suitable mixed together. If that fuel worked in your M/X bike ok then it might be the fuel has gone stale past or it could be the amount of oil you are adding is lowering the octane rating too much.

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It's definitely something related to the fuel. It only happens when riding WFO pulling a load. I had my Go-Pro going when my bike started misfiring the second time. Showed it to my youngest boy and he said that's exactly what his bikes were doing. I can rule out the pre-mix as his MX bike is a 4-stroke. It's strange that my older son and I haven't experienced this in our vintage MX bikes, but those bikes aren't ridden WFO for long distances.

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