Remove all of the spark plugs and take a good look at them. Make sure the electrodes still have square edges, not rounded off and worn. Make sure sure the gaps are close to the Recomended spec. .035”. Make sure the porcelain is not black, if it is put a new set in. Shiney black on the porcelain is oil fouling, due to worn rings, cylinder bores, or valve guides. Flat black is fuel fouling, caused by a carburetor or choke problem.