90 hp mercury runs awseome for 90 seconds then cuts out.

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cb110478

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I have a 2004 90 hp mercury 2 stroke on a 17.5 ft nitro boat. It will come out of the hole pretty good, plane out, and act like it's going to run great. After about 90 seconds of being on plane the engine will cut out like it's not getting enough fuel. If I drop the throttle down to about 3000 rpm it will cruise just fine all day long, but when I try to get on it, it will start cutting out again after about 90 seconds.

I took it to a mechanic once already, he told me the engine was "way out of adjustment and the float was set too low on the bottom carb". It worked great for the next fishing trip then it sat in my garage for 3 weeks and all the old problems just came right back again.

I've rebuilt the fuel pump, checked the tank vent, checked the tank screen, checked the ignition coils, replaced the fuel filter, and cleaned all 3 carbs checking the float needles and jets for trash.

I'm at a loss of what to do next. I though about putting a cheap electric fuel pump inline and putting a toggle switch on it to see if that helps. At this point I can't even tell if its a fuel issue or a fire issue. maybe i just need to pull the prop off and attach a new motor to it. can anyone help me out? Also I noticed what looked to be a shut off solenoid, could that be getting weak and causing my problem?
 

daveswaves

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Sounds like a bad high speed winding on the stator, when it warms up it goes open circuit.
 

cb110478

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Sounds like a bad high speed winding on the stator, when it warms up it goes open circuit.


Ya know. I thought of that, but I didn't check it out because I had too many people tell me that it couldn't be the problem because as soon as you go back to idle for a few seconds and then hammer it again it will run fine for another 90 seconds. It seems like the only thing I haven't checked out though. Thanks for the input. Is there a way I can test that stator, or do I just have to throw money at it and hope it fixes it?
 

daveswaves

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Ya know. I thought of that, but I didn't check it out because I had too many people tell me that it couldn't be the problem because as soon as you go back to idle for a few seconds and then hammer it again it will run fine for another 90 seconds. It seems like the only thing I haven't checked out though. Thanks for the input. Is there a way I can test that stator, or do I just have to throw money at it and hope it fixes it?

Yes, if you go to the CDI electronics web site, click on support and you will find all the test criteria for your engine.
 

CharlieB

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Sounds more like it is running out of FUEL.

Temporarily install a clear hose between the fuel pump and carbs, start the motor and watch for any air bubbles passing through, indicating a leaky fitting/hose allowing air to be sucked into the fuel line between the fuel pump and tank.

Continue to run this motor LEAN, WILL soon melt a piston in the upper cyl.

If no air bubbles, disconnect the fuel line at the carb, crank the motor and abserve the fuel stream, it should be a strong pulsed flow, any less is a restriction in a line/fitting or a weak fuel pump.

Either way, running lean will smoke this motor.
 
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daveswaves

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Sounds more like it is running out of FUEL.

Temporarily install a clear hose between the fuel pump and carbs, start the motor and watch for any air bubbles passing through, indicating a leaky fitting/hose allowing air to be sucked into the fuel line between the fuel pump and tank.

Continue to run this motor LEAN, WILL soon melt a piston in the upper cyl.

If no air bubbles, disconnect the fuel line at the carb, crank the motor and abserve the fuel stream, it should be a strong pulsed flow, any less is a restriction in a line/fitting or a weak fuel pump.

Either way, running lean will smoke this motor.

Thats true Charlie, but he has looked at the fuel route and is no further ahead, he has never looked at the stator. Its a simple test from the CDI site and he can eliminate the stator. If its good, great continue with the fuel path. If its bad he has found his problem. Its good to have 2 horses in the race. :)
 

CharlieB

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

A weak hish speed ignition will limit power and RPM will 'hang' at the limit of the voltage.

A weaken fuel supply will limit power to a fixed point in the throttle opening, any further opening of the throttle and the fuel mixture goes LEAN in the upper carb(s) and melts piston(s).

You GOT TO test both to determine where the fault is.

You rebuilt and checked most of the fuel system, install the clear hose between the fuel pump and carbs, start the motor and watch for any air bubbles indicating air sucking into the system.

Use an inductive timing light or inductive tach and watch for any change in the flash/RPM when the condition occurs. Check each cyl, it is possible to have a fault in a switchox or the stator.

I highly recommend verifying the fuel system first as continuing to operate this motor under WOT load could melt a piston while testing ignition first.
 

cowboyguy63

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

:cool:check out wireing behind oil tank, ive seen them burn a wire going to the stator,and as it turns for adv. timming, it shorts out u will see a burn and melted wire
A weak hish speed ignition will limit power and RPM will 'hang' at the limit of the voltage.

A weaken fuel supply will limit power to a fixed point in the throttle opening, any further opening of the throttle and the fuel mixture goes LEAN in the upper carb(s) and melts piston(s).

You GOT TO test both to determine where the fault is.

You rebuilt and checked most of the fuel system, install the clear hose between the fuel pump and carbs, start the motor and watch for any air bubbles indicating air sucking into the system.

Use an inductive timing light or inductive tach and watch for any change in the flash/RPM when the condition occurs. Check each cyl, it is possible to have a fault in a switchox or the stator.

I highly recommend verifying the fuel system first as continuing to operate this motor under WOT load could melt a piston while testing ignition first.
 

CharlieB

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

More likely his fuel tank fitting is partially clogged and restricting flow, the motor runs out of fuel, sucking the primer bulb down. When he pulls the throttle back to a speed that the restricted fuel supply allows the primer refills, everything looks fine.

Pull the fuel hose to the carbs and crank the motor for 30 seconds or more. Remove the spark plugs, pull the lanyard and/or ground the plug wires to reduce the starter load and battery. Watch the fuel flow start good then rapidly fall off to a slow flow, then get back into those tank fittings and dip tube to clear the obstructions.
 

Texasmark

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Re: 90 hp mercury runs awseome for 90 seconds then cuts out.

Nice comments will add my 2c. Service manual also has methodology to test the stator also. Problem is, if it is an overheating condition, you have to get it hot to do the ohm test and that could well be impossible. Other thing, I'm reading here is an immediate change in the problem from the time it happens through the deliberate throttle back-off to reapplication of power and 90 seconds later bam again. Having spent a career in magnetic/electrical/electronic circuits and all, I never saw a coil (stator) fail, recover, and fail like that. Same for the CDM or other electronics If it was a correctible condition it would take time for all the elements to cool down before they were ready for another round....I don't get that from these comments.

One thing I did notice on my engine ('02 version of yours) is the rectifier/regulator module. I don't know if there is supposed to be thermal grease between it and the mounting plate...using the mounting plate as a heat sink but I put some under mine just because that is what you do and the plate was there, yeah verily it was something to mount the thing to, but also serves/could serve as a heat sink and it never hurts to keep things cool. It's white, radio shack has it and doesn't take a lot....if it squishes out when you remount the module you have enough.

On fuel problems sounds it like just that. The way to smoke the fuel problem is to pull the cowl and the plastic carb(s) cover and safely have another person spray Sea Foam aerosol into the carbs trying to get all 3 shot evenly when it bogs down. If you get any response go and chase your fuel issue.

You didn't mention the fuel line or the squeeze bulb which should have some resistance and respond (get harder) with you squeezing it while the engine is running....and heads up for any engine response when you do. I have had problems with squeeze bulbs and also "stuff" i put into my fuel tank to absorb water that plugging the fuel line at the entrance to the bulb with this white gooey stuff (apparently alcohol that had absorbed a significant quantity of water). Others have had squeeze bulb problems too and everyone recommends using the Quicksilver/Mercury bulb, not an aftermarket.

HTH,

Mark
 

1977mckee

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Did you ever check to make sure your fuel bulb is going the right way? The arrow on it should be pointing towards the motor.
 

jbcurt00

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It's been nearly 4yrs, if they haven't figured out the fuel/running problems yet, they aren't likely to and have started using this Merc as an anchor.

Please check the forum rules in my Signature links below, and don't post to topics that have been inactive for 90+days.

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