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Mercury 5.0 stalling issue

Old 09-14-2018, 04:55 PM
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Default Mercury 5.0 stalling issue

I have a mercury 5.0 inboard .when I start the boat it runs for a few seconds I add throttle in netral and the boat stalls. Ran fine all season. Any suggestions.its carb motor 5.0 thunderbolt ignition system.

thanks
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Old 09-15-2018, 07:13 AM
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Could be a lot of things. Check the distributor cap and rotor. Moisture has a way of accumulating in there. Could also be the carb, maybe time for a rebuild. When is the last time you replaced spark plugs? (probably not that)
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Old 09-15-2018, 07:19 AM
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Condition of fuel?
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Old 09-15-2018, 08:19 AM
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electric choke or mechanical ? , if electric, the choke may not be closing ,remove the flame arrestor and check when the motor is cold the butterfly is closing.
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Old 09-15-2018, 09:39 AM
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Probably the Idle Air Controller valve has failed.
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Old 09-15-2018, 10:15 AM
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You didn't read the OP did ya ? Carb engine.
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Old 09-15-2018, 11:12 AM
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There is no iac. The gas is 2 weeks old I'm not sure maybe I didn't put enough staibliser. Sparks I don't know I've owned it for 1 year no maintaince records. Ran fine and now it's acting up.

so when I start it I push throttel forward 1 pump starts fine. Idles It then starts idling rough I pump throttle ones more runs for few seconds then dies.when I pump throttle it boggs engine down. Almost like I have no gas
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Old 09-15-2018, 02:04 PM
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I had a similar issue this early this year - same engines. After all the regular items to check, I took the carb off and removed the top half, figured it may be gunked up or full of ethanol residue. The blue rubber piece on the accelerator pump was split. I had some pieces from an old rebuild kit and happened to have one. No issues since.
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Old 09-15-2018, 02:06 PM
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Does the engine have a mechanical fuel pump or electric?
Do some more investigating. When it dies, take off the flame arrestor and get an led flashlight. Look into the primary barrels and pump the gas with the choke propped open by hand. If you see a squirt of fuel then the problem is not lack of fuel. But if no fuel is squirted when you do that, it could be the fuel pump or some carbs have a small filter in the carb inlet, this can get clogged (Not too likely if there is large water separating fuel filter, which most I/Os have as standard). You could also dump out that large filter into a mason jar and see how it looks, should look like this, if it has a layer of water on the bottom, you have water in the gas tank.
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Old 09-15-2018, 05:39 PM
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Ok changed the fuel filter didn't solve the problem.
Next looked with a flashlight into carb. when I press throttle there is a squirt of gas that goes in the bowl. Here is what I see happening if I move the throtel more squirts of gas into bowl the engine will run for awhile longer and then dies out. My thought is it burns through the gas in the bowl and then dies out.

It has an electric fuel pump. I think it might be an oil pressure sensor.

Here is why. when I turn the key on I don't here the fuel pump turn on ...does anyone know I, does the fuel pump turn on automatically or does it turn on when the gas in carb is depleted?
I started to investigate the oil pressure sensor. When I took the connector off of the oil pressure sensor and jumped the leads minus the sensor and I turn the key I hear the fuel pump turn on and if I start the boat it runs but dies. Could it be this sensor. Will the motor stay running if I jump this sensor or does it need to be in the circuit?
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Old 09-15-2018, 05:47 PM
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Originally Posted by 1breakinit View Post
electric choke or mechanical ? , if electric, the choke may not be closing ,remove the flame arrestor and check when the motor is cold the butterfly is closing.
its mechanical choke
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Old 09-15-2018, 06:18 PM
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Originally Posted by mitchell master View Post
You didn't read the OP did ya ? Carb engine.
What's a carb? Yea, missed the carb part. Just went reflexive on 5.0 won't run = IAC,
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Old 09-16-2018, 05:18 PM
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Idle mixture and idle speed. Simple adjustment.
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Old 09-16-2018, 06:28 PM
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Idle mixture adjstment. Would I need to do this if before it ran fine with no issues. I dont want to mess with that yet as im not sure thats the issue ..as it starts idles fine then dies in like 10 seconds almost like starved for fuel. . if i pump more fuel before i start it will run longer maybe 15 to 20 sec. Just to add once I press the throttle forward it will die right away when it's started and running
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Old 09-16-2018, 09:48 PM
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looks like its time for a carb service .
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Old 09-17-2018, 05:33 AM
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Originally Posted by sk7499 View Post
Idle mixture adjstment. Would I need to do this if before it ran fine with no issues. I dont want to mess with that yet as im not sure thats the issue ..as it starts idles fine then dies in like 10 seconds almost like starved for fuel. . if i pump more fuel before i start it will run longer maybe 15 to 20 sec. Just to add once I press the throttle forward it will die right away when it's started and running
It's made to play with. Probably lean. Fatten it up a little, won't hurt anything. Should be about 1-1/2 turns out to begin with. Adjust idle speed in gear, then adjust idle mixture. Lean it until it stumbles, go back 1/2 turn. Recheck idle speed in gear, recheck idle mixture. There is no preset idle mixture.

A carb does not need a constant supply of fuel. The float bowl holds enough fuel for the typical carb to idle for 30 seconds or so. Just like a lawn mower, when you shut off the fuel and run the carb dry.
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Old 09-17-2018, 08:23 AM
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ignition on prior to starting the electric fuel pump with pump fuel
once the motor is running the fuel pump needs the oil pressure switch to read oil pressure to pump fuel. use an electrical tester to see if your fuel pump is getting power after the engine is running.

could be a bad oil pressure switch.

if it's the oil pressure switch it will consistently start up fine then die 30 seconds or a minute after starting.

if it's the oil pressure switch, you should be able to repeat this same sequence every time regardless if the engine is hot or cold.

something that's simple to test without removing any parts
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Old 09-17-2018, 01:32 PM
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I'm close so when I messed with idle and mixture screw I got it running for over 5 minutes. The thing is it runs at 1000 rpms and every so often surges to 1200 rpms then back down. When I apply throttle fast it will stall it. If I add throttle slowly there is a bit of hesitation and a loud hiss noise then it kicks in.

We'll at least is started now I'm being pushed with maybe it's gummed up dirty carb but I use fuel stabliser each fillup.

Also to note when I take flame are stored off and spray chock cleaner into carb it revs up then down doesn't stall.

Any ideas everyone has been so helpful I appreciate the help so far.


If it gets running right do I need to keep adjusting the idle and mixture all the time as when I left the boat before all this occurred it was fine
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Old 09-17-2018, 02:46 PM
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It is possible that the switch that powers the pump (which is sensing the presence of engine oil pressure) is either defective or you have a bad or loose connection there. The way some electric pumps work, they get power from the S terminal as the engine is starting, and then oil pressure builds up. Once the engine starts, and the starter is disengaged (No current from S terminal) the contacts on the oil pressure switch close and power the pump. Take a look at this, not specifically a boat set up but same concept:
Electric Fuel Pump: How to Do It Right
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Old 09-17-2018, 02:59 PM
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OK an update...kept messing with idle and fuel mixture screw .I got it to idle at 650 and when I push throttle now it responds. I let it warm up 20 minutes ran smooth responds to throttle quickly. I thought I had it fixed turned it off waited 1 min pumped gas with throttle once and turned key ...it won't start...did I flood the carb by adding throttle when engine was warm. I'm planning on doing sparks coils full tune up over the winter ...I just can't get this thing out of the marina with its issues.


sorry I come from fuel injected Generation and I'm not to familiar with carboarators but I'm learning
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