Go with one of the cheaper things to fix first, the spark plugs. Take them out and look for heavy corrosion, cracks, build up, etc. If the plugs are coated with heavy black soot then the car is running rich and getting more fuel than air. If they are melted, mis-shapen, or look as though they have been burnt badly then the car may be running lean and not getting enough fuel. (it's better to be a little rich) Once you have determinded it to be either, you can take appropriate measures. If the car has many miles I would replace the plugs any way. Other than that it could be something much more expensive like a loss of compression. There are quite a few things that could do this really, just hope it's something cheaper haha

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