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:

---

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.

---

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.

---

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.

---

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.

---

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).

 

[TDAY44]

S

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:

---

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.

---

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.

---

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.

---

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.

---

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).

So I replaced the four ignition coils, and the gas hose from the tank ( low hanging fruit ). That didn't help. I finally ordered the two power packs and it starts and runs great. I took it for a spin and was on the lake for close to an hour and then suddenly it acted like it was running out of gas and then died. I know there's gas in it. I had to paddle back to the dock .
Well the next day I went out to look at it and hooked up the water hose and the engine started right up. So I let it run for 5 or ten minutes and it seems fine. So How do I tell if the engine is over heating ? There doesnt seem to be a temperature gauge on the dash .

 

[Crosbyman]

get a cheap Infra red gun and scan the PH when it dies
old impeller ??? when was it changed last ??

ground the t switch if any...see if warning buzzer works

 

[racerone]

Test the overheat horn.

 

[saltchuckmatt]

To elaborate more on what Crosby and Race are saying, there is a tan wire coming out of each head. About 3 inches up that tan wire has a black rubber boot...you can either peal that back or do what I do and stuff a metal pick in it, than touch any clean ground while the key is in the on position.

If the horn starts blaring then your horn checks out only...whether your actual overheat sensor works is another test.

The old school method for testing head temp is wax strips...if they melt when touching the aluminum its to hot. Or do what I do and do it very carefully. Slowly see if your hand can get near the spot where the top of the head meets the block. If you touch there when the engine is running, it's should be pretty hot to the hand but not scalding....

Do not use this motor until it has been properly diagnosed.

IR guns are like $15 bucks now....time to invest.

 

[racerone]

If you do not know how old the impeller is you must install a new one.-----And look at the 2 thermostats and poppit valves.