1981 Johnson 115hp 4 cyl

[TDAY44]

I have been working on a 1981 Johnson 115hp 4 cyl motor since winter, it has a problem with power cutting out intermittently. It will run great for 30 minutes or so then power drops off dramatically . Later power just returns to normal. I replaced the 4 spark units based on popular opinion from friends and youtube .
It runs stronger but still the same drop in power .
I found a rotted vacuum hose coming off of the carburetor intake. It runs underneath the engine so I cant see where it goes. Is this normal ?
I am looking for a vacuum hose routing diagram for this engine . I don't want to have to take the engine apart to lift it off the pan if I don't have to . It may not be related
Any help would be appreciated .

 

[airshot]

Find a factory service manual for your motor, all that info and more will be in there. I buy one for every motor I buy, well worth the few extra bucks.

 

[Crosbyman]

chat gpt says...
On a 1981 Johnson 115 hp 4-cylinder (crossflow V4, 2-stroke), a sudden power loss after running fine for 20–30 minutes, then returning to normal later, usually points to an intermittent heat-related or fuel delivery issue. Here are the most common causes:

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Fuel-related causes​

1. Fuel pump diaphragm getting weak
   – After 20–30 minutes of demand, the diaphragm may soften and fail to keep up fuel pressure. Once the engine cools, it works again.
2. Vapor lock / fuel line heating
   – Old fuel lines routed near the powerhead can heat up, causing vapor bubbles and fuel starvation.
3. Anti-siphon valve restriction (at tank)
   – A sticky or partially clogged valve reduces flow after long demand but works again after sitting.
4. Clogged fuel vent or pickup
   – Tank vent blocked → vacuum builds up, fuel flow slows, engine loses power until the vacuum dissipates.

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Ignition-related causes​

1. Power pack / ignition coil heat failure
   – These Johnsons were notorious for ignition components breaking down when hot. A weak coil or bad power pack can cut spark on one or more cylinders after warming up, then recover when cooled.
2. Stator or timer base insulation breakdown
   – The stator under the flywheel can overheat, reducing voltage to the power pack until it cools.

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Mechanical / cooling​

1. Overheating triggering S.L.O.W. mode (later models)
   – On 1981s, you don’t have modern S.L.O.W., but overheating can still cause loss of power. A sticking thermostat could let it overheat temporarily, then recover.
2. Exhaust leak inside cowling
   – Hot exhaust gases leaking under the hood can cause poor combustion and heat-related misfire.

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Things to check​

• Spark test after it bogs down: Carry a spark tester; see if all 4 cylinders still fire when it acts up.
• Fuel bulb firmness: If it’s soft when the problem happens, you may have a fuel restriction.
• Vent the fuel cap: Next time it loses power, open the fuel cap—if it suddenly runs better, it’s a venting problem.
• Look for melted/oozing stator: If brown goo is dripping from under flywheel, the stator is breaking down.
• Thermostats / water pump: Make sure it’s pumping strong water and not overheating.

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Since it’s intermittent and comes back after cooling, I’d lean first at ignition (coil/power pack/stator), and second at fuel delivery (vent/pump/anti-siphon).