Mercruiser 5.7 carb only starts with throttle open, dies in neutral

leesfer · Sep 22, 2011

I've got a Mercruiser V8 5.7 with the Thunderbolt V ignition system

The boat starts up instantly when the throttle is open but as soon as your bring the throttle back to neutral then the engine dies.

Here is a video of what is happening: http://youtu.be/IrlKw2uHJ_U

2 weeks ago I took the boat out and forgot to plug the drainage hole in the rear and about 12 inches of water came into the boat around the motor. After that I replaced the starter since the water killed it, I replaced the battery, and I've replaced the spark plugs.

The coil is okay, and I am getting spark, also the carb is spraying gas as it should when I pump the throttle. I am going to replace the distributor rotor and cap today along with the ignition control module.

Anyone else have ideas of what is going on?

mpdive · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Any chance your fuel got compromised? Take a sample first. If not run it off a remote tank to eliminate everything behind the fuel pump. We can go from there.....

leesfer · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

mpdive said:
Any chance your fuel got compromised? Take a sample first. If not run it off a remote tank to eliminate everything behind the fuel pump. We can go from there.....

What is the best way to take a sample from the fuel and check for water?

Don S · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Remove the water separating fuel filter, pour the contents into a clear container. Throw the old filter away, don't even think about reusing.
Water will separate from the gas and sink to the bottom. How much water do you have?

leesfer · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Don S said:
Remove the water separating fuel filter, pour the contents into a clear container. Throw the old filter away, don't even think about reusing.
Water will separate from the gas and sink to the bottom. How much water do you have?

I will have to do this after work today and see what I can find.

leesfer · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

If there is water in the fuel filter, what would be the next step?

mpdive · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

If you took on water there could be quite a bit and you may have to have the tank pumped. Don knows the preocess better than I. Get your sample first and update.

leesfer · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Bubba1235 said:
When was the last time you cleaned the falme arrestor? Sometimes, it really is the simple things.

The boat is new (to me) so I haven't cleaned it yet. I should probably just throw a new one on while I am at it so I know everything is good.

Though I have tried running without the flame arrestor on the carb and the same thing happens.

Bondo · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

leesfer said:
The boat is new (to me) so I haven't cleaned it yet. I should probably just throw a new one on while I am at it so I know everything is good.

Though I have tried running without the flame arrestor on the carb and the same thing happens.

Ayuh,... With the Info so far, start with the fuel filters, as Don says...

Are ya startin' it with the throttle only, 'n it's stallin' when ya go back to neutral to pick up the shifter,..??
Have ya had it runnin', "In gear",..??

Eric 3.7LX Alpha 1 · Sep 22, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

leesfer said:
The boat is new (to me) so I haven't cleaned it yet. I should probably just throw a new one on while I am at it so I know everything is good.

Though I have tried running without the flame arrestor on the carb and the same thing happens.

Flame arrestors are all metal and just need to be cleaned to be like new.

leesfer · Sep 23, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Bond-o said:
Ayuh,... With the Info so far, start with the fuel filters, as Don says...

Are ya startin' it with the throttle only, 'n it's stallin' when ya go back to neutral to pick up the shifter,..??
Have ya had it runnin', "In gear",..??

I have to have the throttle open to start the boat and when I pull back into neutral is when it stalls. I have had the boat running in gear the day that I got water into the hull (2 weeks ago).

mpdive · Sep 23, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

leesfer said:
I have to have the throttle open to start the boat and when I pull back into neutral is when it stalls. I have had the boat running in gear the day that I got water into the hull (2 weeks ago).

After close inspection of the video, I noticed it binding up when you started it. This is a sure indication of a possible timing problem. Have you made any timing adjustments? Did the hold down screw come loose? She is definately binding......Anyone else seeing that?

Edit.....And also notice the half turn of the crank when it's shutting down as in a misfire....

Bondo · Sep 23, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

leesfer said:
I have to have the throttle open to start the boat and when I pull back into neutral is when it stalls. I have had the boat running in gear the day that I got water into the hull (2 weeks ago).

Ayuh,.... 'n ya haven't been able to get it into gear since,..??

At the Shift plate on the motor, disconnect the cable to the controller, the forward cable, 'n Try it....

Why's the motor in there backwards,..??

What drive are We talkin' 'bout,..??

leesfer · Sep 23, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

mpdive said:
After close inspection of the video, I noticed it binding up when you started it. This is a sure indication of a possible timing problem. Have you made any timing adjustments? Did the hold down screw come loose? She is definately binding......Anyone else seeing that?

Edit.....And also notice the half turn of the crank when it's shutting down as in a misfire....

I haven't made any timing adjustments but could the ignition sensor be causing this?

leesfer · Sep 25, 2011

Re: Mercruiser 5.7 carb only starts with throttle open, dies in neutral

Here's an update for anyone interested or having the same problem I am having.

First, my ignition sensor was wet and had gone bad causing my sparks to be intermittent.

Secondly, the carb needed to be cleaned again because it was dripping too much fuel at idle causing a bad fuel/air mixture which was the idle issue.

Everything is fixed and good to go now and I hope this helps someone else looking for this information.

Mercruiser 5.7 carb only starts with throttle open, dies in neutral

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