I have a 1976 9.9 merc. I picked it up at a garage sale $40.00. The endgine is like new according to the repairmen. I had to replace the coils but am having trouble fouling out the plugs. I have dumped the gas mixed it again this time with synthetic oil. It runs great for awh9le then fouls the plugs. Do they make hotter plugs or do I have another problem???