5.0L Mercruiser Ran Great at Dock, Bogs on Water

[Ragnarokii]

Hello All,

I have a 1998 Mercruiser with Alpha drive on a 21' Maxum. This season it's started having some problems.

- After de-winterizing it ran fine on muffs. Dropped it in the water, and my wife ran it up to our dock (about a seven mile run). She stopped for about 20 minutes while she waited for me to pick her up, and the boat refused to start. It would turn over, but not catch. We got a friendly boater to pull us the 100 yards to our dock.

- Spent the morning working on the boat. I started with a new fuel separator (didn't replace it when I did the de-winterize), and what came out looked like water, not gas. The battery was on the weak side (five years old), so I replaced it. I can see jets of fuel in the carb when we pump the throttle. The boat fired up just like it should. It would run up to 4k with no hesitation, and idled just fine. Nice and smooth. Let it get up to operating temperature, everything sounds fine.

- Family and friends show up, we shove off, and there is immediately a problem. The boat bogs, and refuses to go beyond idle even when the throttle is pushed forward. It vibrates like crazy, and is making knocking and pinging sounds. I tried this several times, let it sit in idle for a moment, and then the engine died. It would start up again, but same problem. So we decided to let the kids swim etc., for a while. When it was time to go home, repeated attempts to start resulted in the engine turning over, but not starting. It just wouldn't catch. So, second tow home for the season.

All of which has left me a little perplexed. If it runs great with no load, but refuses to perform when under load...could/would that be a problem with the Shift Interrupt Switch?

At one point I pulled a plug and checked it for spark...it was a bright day today, so it was a little tough to see, but the spark looked a bit weak. Could it be that the heating and cooling of the coil is causing it to die?

Since it ran great with no load, I'm inclined to think it's electrical, not fuel. But I am absolutely open to suggestions.

This is a Thunderbird V ignition.

Any help would be greatly appreciated!

Rags

 

[lr591]

You might have already fixed it since the date shows 2009, but I had similar problem great on muffs but not under load in water, I found that If I disconect the shift interupt switch every thing good except for shifting from forward to reverse , it requires a little extra force. But thats not normal, if need to adjust the little arm on that switch and im also going to check upper and lower shift cable.
I have a maxum w a merc 4.3lx. So try disconecting that switch, but dont leave that way, its suppose to temporarily stall the engine shifting forward and reverse.

 

[Fun Times]

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