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Old 06-29-2015, 08:03 AM
Ben Ben is offline
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Outboard motor help - 2004 Merc 4 stroke surging rpms

I figure there are enough tech guys on here who may be able to make suggestions. Taking boat in right now or getting service to my place is not looking good w/holiday coming....

I got a new"er" pontoon / motor last year. No issues, other than I changed the voltage regulator per a recall / update after realizing it was not charging. Then is ran great all year.

Not sure if it matters, but it is a Bigfoot series / model.

2nd or 3rd time out this year, if run wide open (5000 rpm) for 2 minutes, it starts surging, maybe 2500 rpm - 4500 rpm back and forth. If I drop to 4000, same thing. If I drop to 2000-2500 it goes away, and I can run slower for a while. I went back to 4000 and it started doing it after a minute or two. It does seem I can run 1500-2500 rpm w/o issue, although I've not tried for more than 5-10 min. I was able to run 3000 rpm for ~5 min 2x this morning w/o issues.

Any suggestions? I have not done much with this one yet...

My initial thought is to go after fuel filter(s) and see if that does it. We have had A LOT of rain this spring, so I'm also wondering if there is water in the gas. I did drain the little cup that I think acts as a fuel / water separator out, but it still had issues after that.

Draining / refilling the fuel tank isn't very practical, It is on the water.

Thanks.
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Old 06-29-2015, 08:18 AM
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76S&S 76S&S is offline
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From what I've heard it sounds like a fuel issue. Possibly water in the fuel but what you describe is also often linked to losing prime in the fuel line. I would start with replacing the primer bulb and also dump a can of sea-foam in the tank.
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Old 06-29-2015, 09:57 AM
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i agree with '76. replace the bulb and check the fuel lines. you can pull out some gas, put it in a glass and let it sit and watch for any separation of water.
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Old 06-29-2015, 11:41 AM
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damaged442 damaged442 is offline
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Ethanol in the gas or do you use non-ethanol?
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Old 06-29-2015, 12:18 PM
Ben Ben is offline
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Outboard motor help - 2004 Merc 4 stroke surging rpms

Std pump gas used.
Although I used that last year too, unless speedway made a product change. More ethanol?
I'll do the separation thing and grab a bulb next time I'm out and about.
Thanks.
Additional comments appreciated too.
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Old 06-30-2015, 09:38 AM
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Check the number 3 plug. I had some thing and this was the fix
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Old 06-30-2015, 04:28 PM
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captain planet captain planet is offline
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I hate to oversimplify, but is your fuel tank venting properly? If your running hard and air can't get in fast enough to replace the fuel being consumed that can cause surging.
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Old 06-30-2015, 04:48 PM
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Operating temp consistent over rpm ranges? Also depending on vintage, computer could be putting into safe mode due to temperature or fuel quality/octane issues.
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Old 06-30-2015, 08:56 PM
lsupcar lsupcar is offline
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I experienced this identical issue with a 60 hp merc bigfoot last weekend.
Felt is was a fuel issue and went to the dealer for a new filter and fuel line. While there I talked to a tech and described the issue and the little red rubbery chunks in the filter sight bowl. He said it was the fuel line deteriorating internally. Externally there was solar deterioration of the line and bulb which is why I planned to replace the line anyway. Before I told him about the debris in the sight bowl he thought it might be from water in the tank which would require draining the tank. With the added info, he said if someone brought it in, they would have only replaced the lines and not drained the tank.
I changed the filter and line and all is well.
Good luck.
Bill Z
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Old 07-01-2015, 12:19 PM
Ben Ben is offline
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Cool. Thanks to all.
Im hoping to get back to this tomorrow or Friday.
Lots on the list now. Not great timing.
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