Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
There are two possibility's when you have this problem. One is what is known as a rich fuel mixture. If this is the case you should have notice excessive fuel consumption, and the spark plugs should have a gas smell on them, and be black. The other possibility is pretty bad news, because it is serious problems with the engine. There is a test that check for serious engine problems it is called a leakdown test. It checks for the type of engine wear that causes the plugs to foul. You can also have an oil analysis performed that will tell you a lot about what is going on inside the engine. In the past this test was phenomenally expensive. It was only performed on industrial engines like ships, aircraft, and locomotives. But as technology has progressed the price has come down to $25. I cant overstress how much this can tell you about the condition of your inside engine, I highly recommend you have this type test performed. I will post a link HERE where you can he this test performed.
If the engine is running too rich, fuel system repairs will have to be performed. But if the engine has wear or internal breakage that the leakdown, or oil analysis detects the engine will have to be opened up for repairs. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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