Engine bogs down at WOT

I've used this site a few times for help and almost always get someone who posts the fix for it so why stop now right? I've got a 98 Searay with a 5.7 Mercruiser in it. We haven't used it much the last few months but we do hook the hose to it and run it for a few minutes a couple times a month. Three weeks ago we took it out and it started right up and would go almost full throttle(RPMs were just above 3,000) and then it would try to go wide open sometimes but just for a second or two. So I syphoned the gas out and put fresh fuel with about half a can of seafoam. I took it out today and it did the same thing at first but it got worse and worse and eventually wouldn't even go 1/4 throttle without shutting off. It idles great and it can go into gear but not much above an idle. On the way back to the dock I noticed the battery volts were just above 12(usually they're about 14). I got back to the house and checked the screen in the fuel line, they looked good, the carb looked pretty clean but I just looked from the top I didn't take it apart. I looked at the alternator and saw the positive wire was pretty looks and looked burnt, so I fixed that and put the wire back on tight. I have not taken it back out yet but my big question is, could that alternator wire being loose and causing the battery not to charge properly cause it to not run WOT? I would think if it was fuel related it would at least run the same if not better, but it getting worse has me stumped. I have not checked the plugs because they are a pain to get to but that is my next step and then a full carb rebuild if that doesnt work. Any suggestions? I havent checked a lot yet, but before I go changing a bunch of parts I figured I'd ask the experts. Thanks in advance, you all have been a great help on my many projects thus far.

Start by checking the contents of the fuel filter assuming you have a canister style one, check your ignition components like cap and rotor and wires, what do the spark plugs look like and do you have 12 v at positive side of coil.

What did the gas look like that you siphoned out? If it was mucky you have to clean whole fuel system and carb not just replace the gas.

Dieter said:

Start by checking the contents of the fuel filter assuming you have a canister style one, check your ignition components like cap and rotor and wires, what do the spark plugs look like and do you have 12 v at positive side of coil.

What did the gas look like that you siphoned out? If it was mucky you have to clean whole fuel system and carb not just replace the gas.

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Ayuh,..... Ditto,.....

I have not checked the plugs yet. They gas was yellowish, but not mucky. We keep fuel stabilizer in the fuel and run it every week or two so the fuel does just sit in the carb. I have checked the fuel filters and they looked okay. I will check cap and rotor today, the wires look good though. Although I didn't notice the alternator problem until the end it seems like it went along with the problem getting worse(When the battery gauge read 13 it would go about 1/2 throttle, when it went down to about 12 it would barely go above idle). I'll check the plugs and cap and rotor. It's going to be raining here for the next few days so I probably wont get a chance to take it out again for about a week, but I'll keep you updated on the problem. Thank you.

idle circuit has nothing to do with mains. very possible the main jet is fouled. however for fuel problems you must start at the tank and work toward carb not other way. stabilizers do nothing.

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InstaGator said:

.................. I've got a 98 Searay with a 5.7 Mercruiser in it. We haven't used it much the last few months but we do hook the hose to it and run it for a few minutes a couple times a month. Three weeks ago we took it out and it started right up and would go almost full throttle(RPMs were just above 3,000) and then it would try to go wide open sometimes but just for a second or two. So I syphoned the gas out and put fresh fuel with about half a can of seafoam. I took it out today and it did the same thing at first but it got worse and worse and eventually wouldn't even go 1/4 throttle without shutting off. It idles great and it can go into gear but not much above an idle. On the way back to the dock I noticed the battery volts were just above 12(usually they're about 14).
A good healthy battery at rest (for 6 hours or so) should show a voltage reading in the neighborhood of 12.4 volts.
When being alternator charged, you may see voltage as high as 14 or 14+, but that should be the limit, and it should drop off some in short order.

I got back to the house and checked the screen in the fuel line, they looked good,
When you say "screen", are you talking about the fuel tank pick-up tube screen? (this will be at the bottom of the tube)

the carb looked pretty clean but I just looked from the top I didn't take it apart.
Is this 1998 engine carbureted or is it equipped with TBI?

I looked at the alternator and saw the positive wire was pretty looks and looked burnt, so I fixed that and put the wire back on tight.
You may also want to check the alternator Negative cable. This cable joins the alternator chassis to the cylinder block.

I have not taken it back out yet but my big question is, could that alternator wire being loose and causing the battery not to charge properly cause it to not run WOT?
If the starter motor was able to spin the engine well enough for a "start", voltage is likely OK.

I would think if it was fuel related it would at least run the same if not better, but it getting worse has me stumped. I have not checked the plugs because they are a pain to get to but that is my next step and then a full carb rebuild if that doesnt work. Any suggestions? I havent checked a lot yet, but before I go changing a bunch of parts I figured I'd ask the experts.
Suggestion:
When trouble-shooting as you are doing, use the tried and proven P of E (process of elimination).
This means to test and/or change out one part ONLY at a time.
As you isolate the issue, you'll be better prepared to trouble-shoot in the future...... whereas by throwing a bunch of parts at an issue, you may eventually correct the problem, but you may also never know which part did the trick!

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InstaGator said:

I have not checked the plugs yet.
I would do this as part of your P of E.

They gas was yellowish, but not mucky.
Yellowish color may indicate that the gasoline is going foul.

We keep fuel stabilizer in the fuel and run it every week or two so the fuel does just sit in the carb.
This would not be quite the issue if you were to use Non-Ethanol gasoline.

I have checked the fuel filters and they looked okay.
Unless you are using the Parker/RACOR system, these are inexpensive. I would replace it.

I will check cap and rotor today, the wires look good though.
By chance has the distributor clamp-down bolt become loose? In other words, is there any chance that the distributor housing has moved?
Perhaps put your TB system into BASE mode and check the ignition advance timing! Then take a look at the progressive and TA.

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Sorry for posting so late. I found the problem and fixed it a few weeks ago. On the way back to the dock I noticed the volt gauge reading low so I checked the alternator and found the power wore loose and burnt. I fixed that and took it out and it ran great. Thank you for all of your help. You saved me a lot.of.time ando money