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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 14392
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a 1987 mercruiser 5.7 in my boat. The engine is

Customer Question

I have a 1987 mercruiser 5.7 in my boat. The engine is rebuilt from another boat. When I run it at home it runs great but at the lake and I open the secondaries on the carb it takes off at first running great but then slowly starts to bog and die off. I turn the key off and restart it and it idles fine but when I try to take off again it bogs down and will die.
I have replaced the carb, coil, fuel pump, rebuilt the distributor and installed new cap, rotor and plugs. I'm at my wits end with this boat can someone please help?
Submitted: 5 months ago.
Category: Boat
Expert:  Tan replied 5 months ago.

Hello. My name is Tan.

Have you tried running the engine on a different fuel source? When you have debris in the fuel tank it will act exactly as yours is acting. We need to eliminate the boat side fuel system from the engine to be sure we have good fuel supply. You have done everything on the engine but you have not eliminated the fuel source as the possible cause.

Can you run the engine on a different fuel source like a 5 gallon gas can? Connected directly to the water separator filter?

Customer: replied 5 months ago.
I did use a different fuel tank and it acted the same bogging out and dieing
Expert:  Tan replied 5 months ago.

Ok Where did you have the other fuel tank connected? Did you happen to bypass the weater separator fuel filter in your testing?

We need to check voltage at the ignition coil when it starts to act up.

If we are loosing voltage it will do the same thing the other test is to check the fuel pressure at the throttle body fuel inlet port when engine starts to act up. We have to verify fuel pressure and ignition when engine acts up.

Have you noticed what the volt meter is reading when the engine starts to act up?

Customer: replied 5 months ago.
I haven't tried that yet I'll go do it now
Expert:  Tan replied 5 months ago.

Ok

Customer: replied 5 months ago.
Ok I installed a inline fuel pressure gage and when the boat started bogging down the fuel pressure stayed between 8-9 lbs so it's good. But I did check the power to the coil on the power side. It was fluctuating from 13v down to 8v then back to 14v and back down to 4v while it was running. While the engine was off but had the key on it showed 12v at the coil it was erratic while running. Should it be doing that or should the power be a consistent 13-14v at all times?
Expert:  Tan replied 5 months ago.

The voltage should be steady. Fluctuation of voltage will cause the computer to go haywire. We need to check all the connections at the alternator and the ECM. Loose or corroded connections can cause the issue. Also a loose or dirty connection on the alternator.

Customer: replied 5 months ago.
Ok I have installed a HEI distributor I ran a hot wire from the battery to the distributor. I disconnected the alternator and the starter while it's running. I check the voltage from from the battery and it's still bouncing up and down. I only have a hot wire going to the distributor and the ground to the block. I did change the ground from the battery to the block but it didn't fix it.
Expert:  Tan replied 5 months ago.

Have you load tested the battery? So the voltage at the ignition coil is still going up and down is that correct?

I hope you mean a hot wire to the coil.

Expert:  Tan replied 5 months ago.

Did you bypass the water separator fuel filter? You can have plenty of fuel pressure and still have fuel starvation.

Customer: replied 5 months ago.
I did bypass the fuel filter, I ran new fuel line from the tank to the fuel pump to the fuel pressure gage then to the carburetor.
Expert:  Tan replied 5 months ago.

When the engine bogs down does the voltage at the ignition coil drop?

Customer: replied 5 months ago.
when the key is on it shows 12v but when running at an idle it goes up and down from 12v down and back up. When I throttle up the voltage drops below 12v and slowly goes down to almost nothing. When I bring it back to an idle it goes back to 12v but still goes up and down.I tried a different battery also and still does the same
Customer: replied 5 months ago.
it seems to be the voltage to the distributor is dropping causing to to bog down. I just don't know why it's doing it.
Expert:  Tan replied 5 months ago.

Lets try this disconnect the main engine harness where the boat harness connects to the engine harness. Connect you volt meter to the positive side of the ignition coil. Then with a jumper wire run from the battery cable connection at the starter. Now jump start the engine. Watch the voltage on the volt meter does it still fluctuate. Leave the starter and the alternator connected. What we are doing is eliminating all other outside sources to the engine as possible voltage issues.

Customer: replied 5 months ago.
I have removed the main harness to the engine. I also disconnected the wires to the alternator and ran a jumper wire straight to the distributor as I'm using an HEI distributor. I start it and then remove the starter wire when running. So the only wires that are connected are the distributor wire from the hot side and ground to the block. And the voltage is still jumping up and down
Expert:  Tan replied 5 months ago.

Have you run the boat connected this way?

Customer: replied 5 months ago.
I did, it the voltage is still going up and down while running. And when throttled up the voltage drop gets worse
Expert:  Tan replied 5 months ago.

Is this a new distributor?

Customer: replied 5 months ago.
No it's not but I removed from a good running truck. I have used 3 different distributors the first was an old points then I rebuilt the original one which is an electric mercruiser IV thunderbolt and now have an HEI distributor in it
Expert:  Tan replied 5 months ago.

Well Dave I am out of ideas. I will opt out maybe another expert has an idea.

Sorry.

Expert:  Jason replied 5 months ago.

Hi Dave, my name is ***** ***** I am another expert here.

Can you run the boat for me today for a quick test?

Customer: replied 5 months ago.
I have ran it today and still having the same issue
Customer: replied 5 months ago.
I took the hold down fork off the distributor thinking maybe it's not getting a good ground. I sanded it and made sure it's metal to metal and it's still doing the same thing
Customer: replied 5 months ago.
I have noticed that each time I changed the distributors it got a little better when running it at the lake. I have changed it out with an HEI distributor and it's puts out more voltage than the others.
Expert:  Jason replied 5 months ago.

HEI distributors are not used in boats. Can you take a picture of your distributor so I can see what you actually have.

Customer: replied 5 months ago.
As soon as I get home from work I will get a picture for you. I know HEI distributors are not used in marine engines in fact this one came out of a truck. I was just using it for trouble shooting
Expert:  Jason replied 5 months ago.

The "standard" GM HEI distributor is the one with the ignition coil that is in the distributor cap. Such as this one in this link (click the link for a picture) https://static.summitracing.com/global/images/prod/mediumlarge/pro-66945bkc_ml.jpg

Is that the distributor that's in the boat now?

Customer: replied 5 months ago.
Here's a picture of the distributor
Customer: replied 5 months ago.
it is I was trying different distributors so see if it fix my problem
Customer: replied 5 months ago.
But it hasn't changed anything
Expert:  Jason replied 5 months ago.

Yeah that distro isn't going to work, not only isn't it going to work that is either adding to your problem or is the problem. HEI distributors advance their timing via vacuum. Without advancing timing the engine doesn't make power. Boat engines do not make vacuum, and there isn't a vacuum line hooked up to yours anyways.

What other ignition systems do you have at your dipsoal, because the HEI isn't going to work.

Customer: replied 5 months ago.
I have a Mercruiser IV Thunderbolt distributor that was installed in it but it was doing the same thing with the voltage jumping up and down
Customer: replied 5 months ago.
I understand the HEI distributor isn't made for a boat I was trying it out to see if it had the voltage problem with it
Expert:  Jason replied 5 months ago.

What I want you to do is get a piece of scrap wire, an stick alligator clips on each end. Clip one end to the igniton coil positive, clip the other to the positive battery terminal. This will ensure a solid 12V to the coil at all times. Let me know if it runs better or does the same thing.

Customer: replied 5 months ago.
I did that, I have a wire ran from the battery hot to the coil hot and it still runs with the voltage jumping up and down
Expert:  Jason replied 5 months ago.

What exactly are you measuring? With the jumper I'm place it would not jump up and down.

Customer: replied 5 months ago.
Im using a multimeter on the battery terminals and while the engine is running its fluctuating very fast from 12v down to 9v back to 12v than to 4v. and when I throttle up it gets faster and drops below 12v to 4v-2v and starts to bog out
Expert:  Jason replied 5 months ago.

Good morning.

With the jumper in place the voltage should not be jumping up and down. Are you positive the voltage was jumping up and down with the jumper in place?

Customer: replied 5 months ago.
I don't understand why it's doing it.
Expert:  Jason replied 5 months ago.

It's a poor connection someplace, do you have a battery switch on the boat or do the cables connect directly to the battery?

Customer: replied 5 months ago.
I bought a starter switch wired to the starter to cancel out the ignition, I also bought a new distributor, I'm using the jumper to power it,so the only wires to the engine is the power wire to the starter and using the starter switch to start it, jumper wire to the new distributor, and ground wire from the neg side of battery to the block...... Infact I have two grounds from the battery and still having the same issue with the voltage jumping up and down
Expert:  Jason replied 5 months ago.

If you hook the meter directly to the battery, does voltage fluctuate there or is it holding steady?

Customer: replied 5 months ago.
No with the meter hooked to the battery it still fluctuates
Expert:  Jason replied 5 months ago.

If your meter is fluctuating on the battery then you either have a bad battery or a bad meter. Go stick the meter on one of your car batteries, start it up, and bring the rpms up to 1500 or so and see if the meter voltage reading is fluctuating or not. That will tell you if the meter is junk or if the boats battery is junk, does that make sense.

Expert:  Jason replied 4 months ago.

Hi Dave, any progress?

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