454 MAG Quits When Running Wide Open But Starts Back Up???
 

I have a 1989 Century Ultra I with the 454 Mag (carb) and Bravo I outdrive. The motor is totally stock except for stainless marine headers and risers which were installed at the beginning of this season. I have owned the boat a few years and it has always been turn key with absolutely no issues at all. Over this last winter I put on the headers. When I went to start the boat in the spring for the first time it would turn over but wouldn't fire. After some investigating I pushed the reset button (breaker) on the left lower side of the motor (looking backwards) near the fuel filter and it started right up? What exactly is that? I thought maybe from reconnecting the battery it had tripped it or something. I only mention this in case it might be somehow related to my current problem. My current problem is several times now if I run the boat wide open it will stall. The first time it happened it seemed like I just turned off the key it quit instantly but started right back up. I took it easy on it for a minute then opened it back up again and it stalled again once wide open. It started right back up and I ran it back close to the launch and ran it wide open a few more times and it was fine no stalling. The next day out I ran it all day with no issues although never wide open. On the way back to the launch I opened it up and again it stalled only this time it sputtered like it ran out of gas. It didn't start right back up either this time it took a little while cranking it over. Once running I took off easy and when I opened it up a few minutes later it stalled again. Again I got it running went back near the ramp and ran it wide open again and it was fine, no stalling. Any ideas? The first day it seemed like ignition because of the quick stall and then starting right back up but the second day it seemed like it may have run out of fuel. I was thinking a coil or maybe the screen where the fuel lines goes into the carb. is plugged? Thanks

Well, the great thing about these older motors is that they are simple, simple, simple. There are only a number of components that could fail.
It would be easier to debug if you know if it was a fuel or spark issue. Judging by your last comment, seems like it may be a fuel issue. Easiest think to do is check all the filters... the copper filter in the carb bowl, and the water seperator filter should be checked. But my best guess is in the carb itself. It sounds ilke ... posssibly... the fuel pump can keep the bowls full at mid throttle, but not at WOT. At WOT your motor is sucking the carb bowls dry, and waiting for the fuel pump to refill. I would bank on a bad fuel pump, or clogged filters.

Check the main wiring harness plug on the motor for corrsion, Vibration might be disconnecting it.

I had same problem on a fuel injected 454mag. We satisfied ourselves there was spark, then bypassed the fuel system completely. No dice. It turned out to be the wiring harness.

I have also experienced this exact condition.

I would also suggest the wiring harness.

Disconnect, inspect, each connector. You will probably find that at least one has corrosion and/or pitting, indicating poor connection. Spreading the pins a bit can keep them making contact.

If during inspection you notice corrosion under the insulation, replace the harness. Its less expensive and less trouble than you think. Mercury can provide the exact harness if you give them your serial number.

screen at the carb has caused this for us before white particles act like a check valve when the fuel is at a high flow rate.
My 2 cents

Bottom line, you need to narrow it down to either spark, or fuel to really pin point the issue in the first try.

Any luck on this? What was the result?

Thanks for the replies. I did inspect the wiring harness and wiggle everything around while it was running and didn't notice the motor cutting out although this was at an idle naturally. I was going to pull the line off the carb. and check the screen next.

Maybe a long shot, but check the check valve in the top of the fuel tank. Usually just pop it out and throw it away.

I would say a wiring harness issues would be a good idea for random intermitten issues, but this seems to be a consistent issue when the engine is at WOT. Sounds like a fuel starvation issue. Go with checking filters, including the water separator filter. Also, check the float bowls fuel level. Depending on what carbs you're running, pull the site plug out. Then fire it up, fuel should be at the site plug. Use a rag to soak up the dripping fuel.

Also... very common... the secondaries often don't have a filter. Check it.. It looks just like the primaries, but there's often no filter in there. What happens next, if there's an issue with dirt, it just plugs up the block.. typically the needle and seat or the jets.

It would be ideal to pull off the arrestor when WOT, if the engine quits... pump the throttle to see if any fuel is being pumped in... of course this is very dangerous, because if it's an electrical issue, and it decides to fire when you're looking in the carb, it will shoot flames and burn your eyebrows off.
lol
So if you decide to do that test... be very careful not to look straight down the barrell... so to speak.