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Asmodeus2112

Losing power at higher RPM's

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Asmodeus2112

2001 LXi with Mercruiser Black Scorpion. About 900 Hours.

Last Sunday we were out and boat ran great and we were wake boarding for quite a while, engine operating between idle and ~2200 RPM. After the last rider session was over we were heading back to the launch and the water was super smooth with almost zero traffic, so to entertain the kids in the bow I took it up to about 4K RPM. Engine pulls strong and all was well. Backed off throttle to about 3K and was cruising along at that RPM for about 5 minutes when the engine started stumbling and losing power until is was down to about 2,500 RPM. I pulled throttle back, but the engine started behaving normally again. Pulled back up to about 3K and it ran fine for a few more minutes then stumbled again. Cruised the rest of the way at about 2,500 RPM without issue.

I have been slowly making repairs/maintenance to the boat but haven't gotten to the fuel system yet, so I believe that is likely where the issue is. The fuel pump does whine, so I have it on the list to get a new in line fuel filter, replace fuel filter/water separator, check fuel pick up in the tank and anti siphon valve etc. Will pick up fuel pressure gauge tonight as well.

Boat lives on a trailer, so looking for any info to increase the likelihood of repairing it first or second shot as test drives are not easy, and the family is usually with me so I hate to disappoint and have to return to the launch due to mechanical issues. TIA

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Asmodeus2112

Ok, so I'm working through the fuel system and haven't found any obvious issues. I replaced the cap and rotor a just before these symptoms appeared, do you think an issue there could cause problems like this? I wish the boat was in the water so I could test easily, but trying to cover as many possibilities as possible before spending the several hours to drop it in and test.

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MadMan

It's always difficult to figure out if you have a fuel problem or an ignition problem. Ignition problems usually show themselves at high rpm, like >6000. So this isn't really that.

Did you check that fuel pressure?

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Asmodeus2112

It's always difficult to figure out if you have a fuel problem or an ignition problem. Ignition problems usually show themselves at high rpm, like >6000. So this isn't really that.

Did you check that fuel pressure?

Not yet, ordered a gauge and should have it tomorrow or Tuesday. Started working from the fuel tank forward. Fuel pick up looked fine. My Merc Black Scorpion has two fuel pumps, and the first one is the first thing the fuel line runs too and it did have some debris in the inlet filter screen, but nothing like what I have seen pictures of affecting performance. I want to pull the main fuel pump to see if it has a screen as well, but most pictures I can find and diagrams I don't think that it does. It's not easy to pull the main fuel pump as it is assembled inside the "cool fuel" cooler assembly. Not impossible, but I think I'll wait until I get fuel pressure readings and maybe take it to the lake once to see if perhaps the debris in the boost pump could cause the issue. Has anyone really had seafoam solve any problems? In most of the threads I have read people always though that out, but it doesn't ever seem to clear anything up.

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