Mercruiser 7.4 Liter Slight Miss

[rpraye]

I am running a 1990 Mercruiser 7.4 liter engine, with Thunderbolt Ignition system, in my boat. Engine has about 350 hours on it and is completely stock. The engine has seemed to developed a slight miss. Not all the time, but once in awhile at cruising speed. I have added two gallons of methyl hydrate (a gallon per fill up) and just put on a new quicksilver fuel filter. Engine still missing. I am using Champion spark plugs. I am aware that Merc's prefer AC Delco, but this is the second season for the plugs. Wondering if this is something simple before I start buying parts I don't need. Thanks

 

[Bondo]

Re: Mercruiser 7.4 Liter Slight Miss

I am running a 1990 Mercruiser 7.4 liter engine, with Thunderbolt Ignition system, in my boat. Engine has about 350 hours on it and is completely stock. The engine has seemed to developed a slight miss. Not all the time, but once in awhile at cruising speed. I have added two gallons of methyl hydrate (a gallon per fill up) and just put on a new quicksilver fuel filter. Engine still missing. I am using Champion spark plugs. I am aware that Merc's prefer AC Delco, but this is the second season for the plugs. Wondering if this is something simple before I start buying parts I don't need. Thanks

Ayuh,.... Welcome Aboard,... Seein's ya just changed the fuel filter, what was In the old 1,..??

I'd start with puttin' AC plugs in it, 'n atleast a thorough Inspection of the cap, rotor, 'n wires,...