1979 fish natique.. 454 with 1410 marine carb . Pertronix dist and wires. Engine 2 years old carburetor and distributor system only 1 year old..park the boat late August and just trying to crank it this weekend and it is running rough and backfiring... I replace the spark plugs for process of elimination.. usually backfires when first giving it gas. Then again at idle I hear some popping out the exhaust..