Powerquest_Baby!!

So, I finally take my boat out last weekend. Ran it all day Saturday and all day Sunday and she runs perfectly--not one issue until the very end of the day on Sunday when I was running at 3500 rpm...She all of a sudden looses 500 rpm for about a second or two...It went back to 3500 but 20 seconds later it happens again...Ten seconds later again. So, I pull the throttle back to 3000 rpm and drive her to the marina (about 5 minutes without a problem).
Im trying to determine possible culprits. Coil? Fuel pump? Fuel filter? The engine is a 1988 Mark IV 454 mag carb which is 100% stock and original (550 hours) so we dont have to worry about ECU, vapor lock or injectors. Engine and drive oil look fine...I hate to start throwing parts at her but want to explore options. Ideas?

If I do decide to replace the coil does it matter which one? I was just on summit racing and there are 10+ choices. Some with more resistance, higher voltage etc. Any advantages or disadvantages?

MILD THUNDER

iTrader: (3)

stock coil will work, so will a msd. I had good luck with the MSD high vibration coils, had them for years.

When was the last time the cap, rotor, plugs and plug wires, fuel filter were changed? I'd start with those items, as its routine maintainence anyway.

Problems like this are hard to diagnose over the internet. We can give suggestions, but it's no replacement for actually being there and seeing it acting up. I'd start with tune up parts, and if no luck, figure a way to plumb a fuel psi guage in, and have someone watch it. If fuel psi drops off when it acts up, i'd look for $hit in the fuel tank, fuel lines eaten up from ethanol, etc.

sprink58

Sounds like ignition. Don't start throwing parts at the problem until you think it through. You didn't say but is the ignition Thunderbolt? If so could be a module headed south. Coils breakdown when they get hot but don't put an automotive coil in the mix or you'll be adding misery to the mix.

Get a new coil that matches what you have and take it with you the next time you head out. If this condition repeats immediately change the coil on the spot...then you'll know. I know it's a PITA to do this while you're out enjoying yourself but if you set your rig up to make the change quickly you will be able to do it in no time.

If you can get the specs on the coil you have you can test it with a VOM to compare but it's difficult to re-create "in the water" conditions on the bench.

Good luck with this.

Griff

Star with the basics. Cap, rotor, plugs and fuel filter.

CRG

1. new plugs,cap rotor, wires. still same issue.
2. lost spark one night leaving a bar
3. new pickup installed, had spark for 3 mins (smoked it)
4. pulled the dist, put a HEI unit in, set the timing done!
5. had marina look at both boxes on the dist. one had a short

Powerquest_Baby!!

Yesterday I replaced the coil, fuel filter, cap and rotor. Everyone Ive talked to seems to think I had water in my tank since this was the first time out this year.
Crossing my fingers that it was something simple...

NightHawk

Had the same symptoms with my 454 at one time. It was one broken plug (ac MR43T) arcing intermittently on the inside.

Your symptoms sound much more like ignition rather than fuel.

Powerquest_Baby!!

Well, I replaced the coil, cap, rotor and fuel filter and the problem is gone. Im really thinking it was water in the fuel but since I replaced 4 things at once I cant say really determine what the issue was...Main thing is she's running perfectly.

10M

Looks like you figured it out, but I'll add my version as an FYI. Had the same problem at the end of last year. It was due for cap/rotor/plugs/wires, and I've had corroded cap cause similar issues in the past, so I got the stuff for that. Started the boat before doing the work and it ran fine, the problem was intermittent, so I did the tune-up anyway. Went to start it after changing everything out, and it wouldnt start at all. Was just running fine 45 minutes earlier. Got out my trusty diagnostic fuel squirt bottle and sprayed into the carb...would run on spray but not on the fuel pump. To make a long story short, traced the wire for the fuel pump and one of the spade connectors was broken off the terminal on the back of the motor, right next to all the stuff I was just messing with. Connection must have been on its way out, then I probably bumped it and finished it off while changing the ignition parts. So you never know. When the problem was happening I never would have guessed that wire connection in a million years. And if it decided to fall off while out on the water it would have been a problem.