1985 Mercury 85hp (4 cyl) runs great for 20 minutes then boggs down

[Markslund]

Are the decals a brown / copper color ?-----I do not see and 85 HP listed for 1985.-----So what HP is it and what is the serial #.-------Are you sure you do not have an overheat buzzer ??----I am going to be blunt as usual.-----What electrical trouble shooting led you to replacing all the ignition components ?

Yes, those are the colors. I originally posted the title as an 85 hp and then mentioned in a follow-up post that it is actually a 75 hp. IT IS A 1985 75hp, 4 Cyl, MERCURY, S/N 6615200
I purchased the boat a year ago, and have not seen any signs of a overheat buzzer, or wiring anywhere on the boat.

As mentioned in my previous posts...After troubleshooting the first thought of a defective fuel tank vent, and fact that the motor would run fine for 20 minutes each time and then bog down, suggested to me and the mechanic I spoke with that there was something electrical heating up and then failing. He suggested that the stators in these motors often create this issue. Then while doing a good physical inspection, I noticed one of the wires coming from the trigger that had possibly been grounding-out on one of the CDI Switch Box mounting screws. I decided at that point to start fresh and replace stator, trigger, Switch Box, Coils, plugs. Still same issue. Then removed both carbs, thoroughly cleaned, replaced floats, seats, and gaskets.

A mechanic recently adjusted both carbs, and checked spark/timing and compression, as well as found the leak in the full pump assembly.

 

[Markslund]

Hi.Your issue is electrical I believe and NOT fuel.That's why bulb squeeze doesn't help.What model is your engine- Model #. You have a part (not familiar with mercs-that is heating up and then breaking down(like a bad coil on older performance engines). Weak spark, can't handle the fuel load.Here's a link to look up your specific engine and parts

Mercury Outboard Motor Parts at MarineEngine.com

Buy from the largest online inventory of genuine Mercury outboard parts including discount aftermarket parts. New and vintage parts available at MarineEngine.com.

www.marineengine.com

Thank you Mark Grant. I have replaced all electrical (ignition system components). Spark/timing, and compression just recently checked by mechanic when he adjusted the rebuilt carbs. He also found the leak in the fuel pump assembly. He suggested I take it back out on the water to test, but there is currently no open water. I am just not confident my issue is gone with what he found and am exploring other possibilities while I'm waiting for ice to melt

I had not yet replaced the water pump since it seemed like I have had a constant tell-tale stream. Thought I would do that and then also started thinking about replacing a thermostat (these should be normal maintenance items anyway , right). I just cant find a thermostat listed in diagrams for this motor.

 

[Chris1956]

That motor likely doesn't have a thermostat. Most (not all) inline engines from that era don't have one. They rely on the waterpump to cool more than the motor requires. They therefore run dead cold at idle and warmer at cruise. If the prop pitch is too high or some other extra load is put on the motor, they can overheat. Those motors usually did not come with an overheat alarm.

 

[racerone]

If wire from temperature switch is a tan color or tan with a blue stripe it might be for a warning horn.

 

[Markslund]

If wire from temperature switch is a tan color or tan with a blue stripe it might be for a warning horn.

OK. Thanks

 

[Markslund]

That motor likely doesn't have a thermostat. Most (not all) inline engines from that era don't have one. They rely on the waterpump to cool more than the motor requires. They therefore run dead cold at idle and warmer at cruise. If the prop pitch is too high or some other extra load is put on the motor, they can overheat. Those motors usually did not come with an overheat alarm.

Thanks Chris.

That motor likely doesn't have a thermostat. Most (not all) inline engines from that era don't have one. They rely on the waterpump to cool more than the motor requires. They therefore run dead cold at idle and warmer at cruise. If the prop pitch is too high or some other extra load is put on the motor, they can overheat. Those motors usually did not come with an overheat alarm.

Any idea what the Temperature Switch in the diagram is...how it works?

 

[Markslund]

Thanks Chris.

Any idea what the Temperature Switch in the diagram is...how it works?
View attachment 357694

 

[Chris1956]

That temp switch is likely an overheat switch. They usually ground when the motor overheats. This ground completes a circuit that sounds a buzzer.

You wire the ign accessory wire (battery positive, usually purple) to one side of the buzzer and the other side of the buzzer is connected to the overheat switch.

 

[Markslund]

That temp switch is likely an overheat switch. They usually ground when the motor overheats. This ground completes a circuit that sounds a buzzer.

You wire the ign accessory wire (battery positive, usually purple) to one side of the buzzer and the other side of the buzzer is connected to the overheat switch.

Gotcha. Thanks!

 

[chris.olson]

On the older four-cylinder mercs, if it has had a stator conversion from 9A black stator to 16A red stator with an inline voltage converter for the ignition pack, I've seen some tech's just lay that voltage converter and connections in the bottom of the cowling because there's no place to really mount it. The grommet thru the cowling tends to go bad with age and the telltale hose leaks water into the cowling where it plugs into the grommet.

So what happens is, it runs like a bear for a bit the first time out on the lake. But that hose leaking water fills the bottom of the cowling with water, or sprays water on the connections and shorts out the connections on that voltage converter and the engine bogs out because it loses ignition voltage. And it won't run again until that thing dries out.

I'm not saying that's what your problem is, but I've seen it on at least a half dozen of the older four-cylinder two-stroke mercs that have had that stator conversion done.

 

[Texasmark]

The suggestion to depress the ignition key should answer the fuel problem...not necessary to do any spraying into the venturis.

Thanks racerone. I have tried another tank and fuel. And don’t have the timing light yet. I’ll pick one up soon, if I am confident I don’t still have a fuel issue. My wiring looks good.
Texasmark: Do you feel it is still necessary to shoot sea foam into carbs if my motor is slowing down when I press the electric choke?
Thanks guys!

 

[Texasmark]

Thanks Chris.

Any idea what the Temperature Switch in the diagram is...how it works?
View attachment 357694

It will plug into the rear of Item 4, the water jacket cover plate. I don't see a stat, popoff valve, nor OT sensor in your IPB. They would be on the rear of item 24....but I am curious about item 35. That could be your OT sensor.

 

[racerone]

Item 35 is the temperature switch / sensor.

 

[Texasmark]

Item 35 is the temperature switch / sensor.

Yes "senses" 195F and "switches" closed grounding the low side of and blowing the horn.

 

[racerone]

No report on test results for the overheat horn at this time

 

[Markslund]

It will plug into the rear of Item 4, the water jacket cover plate. I don't see a stat, popoff valve, nor OT sensor in your IPB. They would be on the rear of item 24....but I am curious about item 35. That could be your OT sensor.

Ok, I think common consensus is that item 35 is the OT sensor. You sound pretty confident that I indeed have a thermostat on this motor, while Chris1956 doesn’t believe it has one. ?? Thanks

 

[Markslund]

On the older four-cylinder mercs, if it has had a stator conversion from 9A black stator to 16A red stator with an inline voltage converter for the ignition pack, I've seen some tech's just lay that voltage converter and connections in the bottom of the cowling because there's no place to really mount it. The grommet thru the cowling tends to go bad with age and the telltale hose leaks water into the cowling where it plugs into the grommet.

So what happens is, it runs like a bear for a bit the first time out on the lake. But that hose leaking water fills the bottom of the cowling with water, or sprays water on the connections and shorts out the connections on that voltage converter and the engine bogs out because it loses ignition voltage. And it won't run again until that thing dries out.

I'm not saying that's what your problem is, but I've seen it on at least a half dozen of the older four-cylinder two-stroke mercs that have had that stator conversion done.

Interesting. I just don’t see any signs of anything getting wet. I wasn’t aware of a voltage difference in stators. When I suspected a bad stator and pulled out the old one, it was black. I just replaced with what I read was the CDI replacement. It is not black, and not really red either

 

[racerone]

I doubt you will find a thermostat on this 75 HP 4 cylinder motor.

 

[Markslund]

Thank you guys for your all inputs. I just wish I knew someone who either has my same motor or has actually had it in their hands and could definitively say yes, you have a thermostat, yes you have a thermal sensor, and yes there is or is not a “limp mode” on this model. I can’t physically see it and don’t have the experience to rely on. Thanks again…

 

[racerone]

If post #46 shows your motor there is no thermostat shown !!----It does have a temperature switch and a wise owner would test and see if there is a warning horn.-----If none is found install one !