How often do you guys change the spark plugs, wires and do a general tune up? Check the timing or anything else. A friend of mine just purchased a boat (97) with 250hrs on it, 454 EFI, 385hp. Had a mechanical survey done and the guy said everything was fine but he should have a tune up done.....wires, plugs, rotor cap? check the timing. The guy wants $250 to do it. Is this a every 100hrs thing or a as needed thing???

Usually do mine every other year. I average 50hr a season.

My opinion only:

To maintain optimal performance, you should change the plugs, cap and rotor once per year, but you can certainly go longer. Even a slightly corroded cap and rotor can have a significant impact on top end performance. Plug wires rarely have to be changed because the engine compartment in a boat does not get very hot. He can change the cap, rotor and plugs himself for a grand total of about $40.

I've lost three sets of Mercruiser plug wires in 400 hours. Twin BBC, well-ventilated engine compartment.

I've gotten to where anytime I detect a stumble, I get in with the motors and watch and listen for arcing. It's been the plug wires more times than not.

Ric above is right if you start with good wires like mC below him said they break alot on some boats !!
Plugs and cap and rotor 1 time a year and wires if you use say Taylor Pro series 409's once every maybe 4 years ,there are alot of good hi-po wires out now Taylor I like but their hard to make
MSD is second order from summit ,ect

Troutly you can I think order them from summit to fit if not I'll make you a custom set Labor free and send them just advise what color ,I've made 3 sets this week already ,be sure to get the pro wire