Seafoam is another product you can use to clean up valves. Probably the easiest way to do it, if you buy the type that hooks up by the throttle body.Humidity, heat, fuel, it could be a bunch of things. Spraying distilled water in the lower intake runners will help reduce carbon build-up on the valves and cylinder face by steam cleaning them. It will also reduce the charge temp helping prevent detonation.
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