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Losing power after pulling skiers, 89 sunsetter. 5.7 mercruiser


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I have an 89 malibu sunsetter.  After driving the boat for an hour or so , when pulling a wakeboarder, the boat loses power and will only run about 15 mph.  i can stop and wait 5 minutes, but then once we start back up it will pull fine for about 5 minutes and then lose power again.  Last season we changed out the thermostat, water pump, impellar, and exhaust manifolds.  We purchased the boat last year and prior to that it had set a couple of years.  It has a 5.7 mercriuser.  The motor temp will reach about 180 when this starts to happen and sometime a little smoe will come off the engine.  I was thining it was some kind of obstruction, but we seem to have good waterflow through the system.  Anyone have any other suggestions?

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Can you check the block temperature with an infrared thermometer particularly at the point in time that it won’t run correctly?

Edited by Bozboat
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Seems kind of strange that it would take an hour of running to get to an over heat condition. Once it loses power, how does it run - I'm looking for some idea of a fuel vs electrical culprit. I assume carburetor not fuel injected? And is this a new problem or were you fighting it last year as well?

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Yes, what he said.  When you crack it, listen for air whistling in there.  

Edited by hmh800
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thanks for the input.  Is has a carb, not fuel injected.  thinking about switching that over to help with power.  I will check the fuel vent line and see if there are any issues there.  The boat had set for about 3 years and I am trying to work out all the kinks in it.   

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@jlowe72- if there is a vacuum in the fuel system the anti spill vent fitting located on the fuel tank may be stuck shut.  Extended delay power issues can also be a faulty coil, heat is the enemy of the coil.  That can also manifest itself if any of the downstream ignition components are high resistance or in need of replacement.  As for carb, a rebuild is a lot cheaper than replacing it.

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21 hours ago, jlowe72 said:

thanks for the input.  Is has a carb, not fuel injected.  thinking about switching that over to help with power.  I will check the fuel vent line and see if there are any issues there.  The boat had set for about 3 years and I am trying to work out all the kinks in it.   

Find and fix your issue, I second the fact that it is very likely a fuel vent issue.  I'd recommend not going down the fuel injected rabbit hole.  It's just not worth it, lots of threads in this forum.  Carb'd is absolutely fine, lots of us running carb'd boats that are super reliable and have more than enough power.

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@Woodski already pointed to what I would check:

Ignition.  I had very similar symptoms when my ignition coil was on its way out.  It would get hot and resistance increased dramatically after I was running higher revs.  It eventually progressed into dropping a skier, cut throttle, and motor would cut out.  I was chasing carb issues and it was just the $20 ignition coil.

I agree with @solorex on power.  Our boat makes plenty of power with the carburetor even at altitude.  

I have a carb rebuild kit sitting in the box in the garage for the day that the carb gets finnicky.  It was well under $50.  

Edited by Pra4sno
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martinarcher

Another bet here on the ignition coil.  I've failed a few of them and that's exactly the symptoms when they go.  I keep a spare onboard and can change them in 5 min.  I just buy a new spare when they fail.  The Pertronix Flamethrower coils do last me much longer than the Accel coil I used to use.  

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Michigan boarder
On 3/31/2021 at 2:41 PM, jlowe72 said:

thanks for the input.  Is has a carb, not fuel injected.  thinking about switching that over to help with power.  I will check the fuel vent line and see if there are any issues there.  The boat had set for about 3 years and I am trying to work out all the kinks in it.   

Fuel injection will not get you any more power.  The limiting factor on your '89 is the pre-vortec heads, you are better off putting money towards a cam and heads than in fuel delivery.  EFI will help with cold starts, but will not help with hole-shot or top end.  A carb rebuild might though.

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