I purchased a tester from Ebay .You just touch the plug lead to see if it firing.,a bit like a pen Or just pick up new leads for nos 3 - 6 ..Other than that its the dustributor top or rotor arm .But if you have replaced them both ,then its must be the leads ..or the push on cover for the plug . I dont like the screw on type .Not sure if they are on a merc but not 100% on that