what if spark plugs burned?

If by burned you mean the electrode has melted away it is way too lean and you have a carb/FI problem or more likley a leak in the inlet manifold around that cylinder. I would assume that it idles rough too, get a pray can of WD40 and whist the engine is running at idle spray the WD around the inlet manifold and listen for the engine beat to change, any change in note means that air is passing the manifold joint and you should strip and replace the gasket . Also check the brake servo hose for splits as this is a common cause of excess combustion air.

If it was burned in so much that is was oliy and black this means that this cylinder has blow-by of oil caused by stuck or broken piston rings, maybe even a worn bore.... far more expensive to fix !

SB
November 2006