1980'ish black max 150 losing all power after running for 30 minutes

cmw1

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Jun 23, 2025
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Hi all,
I have a 1980'ish black max 150 that runs decent at idle and runs very well at high speed, for about 25-30 minutes (sometimes less). When it goes to die it acts like the fuel is being slowly cut off and it Boggs its self out. I can squeeze the bulb and it will start back up after a few seconds and will run rough until I let it sit for a while. Any suggestions or help would be great...I can take anything apart and work on it but I have a very rough understanding of boat motors
 

jimmbo

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May 24, 2004
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13,773
I suspect an Ignition Issue, due to Heat, possibly Corrosion. Verify that all Connections are clean, and secure. Switchboxes in the 80s Engines were Prone to Failure from Heat.
However, a Fuel Starvation can produce similar Action. A collapsed Primer Bulb happens when the Tank isn't venting. Try running with the Fuel Cap loose/off.
 

cmw1

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Jun 23, 2025
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I suspect an Ignition Issue, due to Heat, possibly Corrosion. Verify that all Connections are clean, and secure. Switchboxes in the 80s Engines were Prone to Failure from Heat.
However, a Fuel Starvation can produce similar Action. A collapsed Primer Bulb happens when the Tank isn't venting. Try running with the Fuel Cap loose/off.
So I took a look at the switch box and it seemed to be fine. I've read that these motors were notorious for having the stator go out after heating up. Not sure how to check that without just replacing it (which i dont want to do if it isn't necessary). I took the cap off the fuel tank when the bulb collapsed and it didn't seem to make a difference so I am leaning towards an electric issue....do you know a good place to find a wiring diagram. Stamp on motor claims its a 1981 mercury black max 150, serial number 5904448.
 

Chris1956

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Mar 25, 2004
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28,248
If the primer bulb collapses, it will starve the engine of fuel, which can damage the engine.

Put a hair dryer's hot air on the switchboxes and run her on a flusher. See if it dies. If not, the switchboxes are good.

Just so you know, that motor will continue to run even if overheat is detected.

Maybe you could run the motor with the cowling off and connect a voltmeter to the stator wires at the switchboxes. Read the voltage and then run her until she dies, monitoring the voltages.
 

cmw1

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Jun 23, 2025
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Just an update...ran rhe motor again this weekend and had much better luck as long as i drove mostly in a straight line....forgot to mention this happens mostly when pulling a tube and making sharp turns
 

Chris1956

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Mar 25, 2004
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28,248
Loses power when you turn? Sounds electrical. What shape is the wiring in? Are there any wires rubbing on metal, when you turn?
 

jimmbo

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May 24, 2004
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13,773
I guess the main Electrical Harness could be flexing/twisting when Motor is being steered
 

cyclops222

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Mar 21, 2024
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2,376
I really dislike black rubber or plastic fuel lines.
And buildup of GOOK is unnoticed. Especially small diameter sizes. Under 3/8" size. I replace them at about 5 years.
 

cmw1

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Jun 23, 2025
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Ok, new update

All the lines are now brand new including the fuel filter/separator. The Tank has absolutely no vent issues as I can easily blow through the lines and get virtually no resistance and hear it coming out of the vent. I think i have a couple problems going on. Where the fuel lines connect to the motor it has a quick disconnect that is leaking and im assuming allowing air into the lines and causing part of my problem as the primer bulb looses all pressure on rare occasion. And my second problem is if the fitting wiggles around and gets a good connection it will suck the Primer Bulb flat and i think thats caused by the 1 way check valve on the intake of the Primer Bulb (it was brad new) but when I took it off and blew through it, it was super easy at first and then I had to blow pretty damn hard to get it to move. Im thinking that with the positive pressure the bulb has its adding pressure to the check valve not allowing it to open properly and slowly creating a vacuum on the bulb until it is flat and starves the engine of fuel....I could be crazy but thats my working theory.

I've all but ruled out an electric issue!
Its still possible I have some sort of an electric issue but given that in either situation of the motor dying (flat bulb or no pressure) if I pump the bulb or open the valve on the bottom of the water separator allowing air in i can get the motor to immediately start up and run properly!
 

cyclops222

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Mar 21, 2024
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2,376
You have a lot of items that need checking out somehow. Keep doing 1 at a time. Only change 1 new item at a time and test run the boat on the water. To verify it is good. That way a reject new item is caught first time.
 
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