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'05 Johnson 30 hp 4-stroke Loses Power

I

in-too-deep

New member
Hello,

Model J30TEL4SD
Ser. S 20026107
Built 7/2005

Starts great, idles great, but will suddenly lose RPM and power. Rarely runs perfect with all the power, most times seems like 9.9 hp instead of 30. Still runs smooth, not missing or cutting out, just pokey. Will do it with just me in the boat, or family and all the camping gear. Not dependent on load.
Choking does not help, squeezing primer does not help.

What I've done:
Different fuel supply hose and tank
Cleaned fuel filter
Sea Foam in the gas
Premium gas
Spark plugs
Flushed carbs as well as I could with out removing them
Took it to marine mechanic and they cleaned carbs and synced them. They ran it in tank and of course it behaved for them.

Must be ignition-related, but what parts do I throw at it first? Can I troubleshoot ignition parts before throwing money at it?

Thank you!
 
Does the engine rev up like it should, or is it bogging? If it is revving up normally, it is probably a spun prop hub. Have you checked the compression? Pull the plugs and check the color. If you haven't tampered with the timing, it should be fine. Given all the work you have done, the engine is not missing, backfiring, or experiencing any other issues; it only has a lack of power. Did this issue occur suddenly one day, or did it develop gradually over time? I believe the issue may be related to compression, a spun propeller, or a gearbox problem that is causing drag.
 
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Sttuppid said:
Does the engine rev up like it should, or is it bogging? If it is revving up normally, it is probably a spun prop hub. Have you checked the compression? Pull the plugs and check the color. If you haven't tampered with the timing, it should be fine. Given all the work you have done, the engine is not missing, backfiring, or experiencing any other issues; it only has a lack of power. Did this issue occur suddenly one day, or did it develop gradually over time? I believe the issue may be related to compression, a spun propeller, or a gearbox problem that is causing drag.
Click to expand...
It revs up great and runs very well... except when it doesn't.
I took prop off and the splines are all fine, and if it wasn't I'd have higher RPM and less propulsion right?
I will check compression, and now I'm wondering... maybe I should adjust valve clearances?
Thank you for the reply.
 
You don't need to look at the spines. You need to look at the rubber hub that secures the splined sleeve.
But, you stated in your 1st post "Starts great, idles great, but will suddenly lose RPM and power." A spun hub gains RPM as the boat slows. So unless you are totally confused & loosing track of what is really happening it's not the prop.
Valve clearance, I don't think so. They change as engine warms but not suddenly.
I'd go back to fuel source then check for spark while it is acting up. I've seen spark plugs fail when fully hot. Engine restarts & fails quicker & quicker. Runs OK again after it cools then starts the cycle over.
 
Droid said:
You don't need to look at the spines. You need to look at the rubber hub that secures the splined sleeve.
But, you stated in your 1st post "Starts great, idles great, but will suddenly lose RPM and power." A spun hub gains RPM as the boat slows. So unless you are totally confused & loosing track of what is really happening it's not the prop.
Valve clearance, I don't think so. They change as engine warms but not suddenly.
I'd go back to fuel source then check for spark while it is acting up. I've seen spark plugs fail when fully hot. Engine restarts & fails quicker & quicker. Runs OK again after it cools then starts the cycle over.
Click to expand...
Sorry, I missed the "loses rpms" part. The issue is likely to be related to either compression, spark, or fuel. I'd pull the fuel pump cover and re-check the fuel filter, recheck the plugs, and verify compression. Does it sound like the engine is dropping a cylinder when it loses power, like it is operating solely on one cylinder? Does it do this immediately after starting, or will it be okay for a period of time, either 10 seconds or a couple of minutes? I highly doubt that the problem is a valve issue; however, you might have a tight valve that isn't fully closing, which would show up in the compression test, although this kind of condition is rare. If the tappets were a little sloppy, you would hear a ticking noise, which would not cause any issues apart from the noise itself and possibly the compression release if it has them.
 
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Droid said:
You don't need to look at the spines. You need to look at the rubber hub that secures the splined sleeve.
But, you stated in your 1st post "Starts great, idles great, but will suddenly lose RPM and power." A spun hub gains RPM as the boat slows. So unless you are totally confused & loosing track of what is really happening it's not the prop.
Valve clearance, I don't think so. They change as engine warms but not suddenly.
I'd go back to fuel source then check for spark while it is acting up. I've seen spark plugs fail when fully hot. Engine restarts & fails quicker & quicker. Runs OK again after it cools then starts the cycle over.
Click to expand...
Sorry, I overlooked the "loses rpms" bit. Compression, spark, or gasoline could be the source of the problem. I would remove the fuel pump lid, recheck the fuel filter and the plugs, and check the compression. Does it sound like the engine drops a cylinder when it loses power, as if it is just running on one cylinder? Another test would be to disconnect the line from the fuel pump to the carb and turn it over to ensure that the pump has adequate flow coming out of the carb feed hole. If that is not the case, please examine the pulse line that originates from the engine's crankcase or the valve cover thoroughly and replace it if it is dry, cracked, or has any air holes. If all is satisfactory, move on to carburetion.

The carburetors must be removed for proper cleaning, especially if it sat out the off-season without gasoline stabilizer in the tank. They need to be removed and thoroughly cleaned, or better yet, purchase a pair of rebuild kits. You can do this job yourself, or if you are not comfortable, take it to a shop to ensure they are properly done. Does the issue manifest immediately, or does it take a few seconds or minutes after accelerating? I highly doubt that the problem is a valve issue; however, you might have a tight valve that isn't entirely shutting, which would be detected during the compression test, but this is uncommon. If the tappets are loose, you will hear a ticking noise. This noise will not cause any problems apart from the sound itself, but it may affect the compression release if applicable.
 
Sttuppid said:
Sorry, I overlooked the "loses rpms" bit. Compression, spark, or gasoline could be the source of the problem. I would remove the fuel pump lid, recheck the fuel filter and the plugs, and check the compression. Does it sound like the engine drops a cylinder when it loses power, as if it is just running on one cylinder? Another test would be to disconnect the line from the fuel pump to the carb and turn it over to ensure that the pump has adequate flow coming out of the carb feed hole. If that is not the case, please examine the pulse line that originates from the engine's crankcase or the valve cover thoroughly and replace it if it is dry, cracked, or has any air holes. If all is satisfactory, move on to carburetion.

The carburetors must be removed for proper cleaning, especially if it sat out the off-season without gasoline stabilizer in the tank. They need to be removed and thoroughly cleaned, or better yet, purchase a pair of rebuild kits. You can do this job yourself, or if you are not comfortable, take it to a shop to ensure they are properly done. Does the issue manifest immediately, or does it take a few seconds or minutes after accelerating? I highly doubt that the problem is a valve issue; however, you might have a tight valve that isn't entirely shutting, which would be detected during the compression test, but this is uncommon. If the tappets are loose, you will hear a ticking noise. This noise will not cause any problems apart from the sound itself, but it may affect the compression release if applicable.
Click to expand...
It doesn't drop a cylinder. Still runs smooth, just less power.
I will inspect the fuel pump and pulse line. Might be on to something there.
Edit:
But wait.... choking doesn't help at all, so it can't be undersupply of fuel... ugghhhhhh

I had the marine shop clean and sync the carbs last year and it didn't help. They couldn't replicate the issue running it in the tank either.

Issue is erratic as it can be seconds or minutes into full-throttle before it starts acting up. Or, once in a blue moon, it runs perfect and I smile ear-to-ear. But that's rare.

I'm really tempted to put all new coils on, since I've already done spark plugs, but I wanted to ask here before firing the parts cannon.

Thank you.
 
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Droid said:
You don't need to look at the spines. You need to look at the rubber hub that secures the splined sleeve.
But, you stated in your 1st post "Starts great, idles great, but will suddenly lose RPM and power." A spun hub gains RPM as the boat slows. So unless you are totally confused & loosing track of what is really happening it's not the prop.
Valve clearance, I don't think so. They change as engine warms but not suddenly.
I'd go back to fuel source then check for spark while it is acting up. I've seen spark plugs fail when fully hot. Engine restarts & fails quicker & quicker. Runs OK again after it cools then starts the cycle over.
Click to expand...
I understand what a spun prop would do... like shifting into neutral. It's not that.
I did new spark plugs with no change.
I'm going to inspect fuel pump and pulse line again. I'm confident there's no restriction of fuel... but there might be an undersupply of fuel. Two different things. But not so fast... choking doesn't help at all, so it can't be an undersupply of fuel....

Time to load the parts cannon. 🧑‍🔧
Thank you.
 
Since it is doing it on both cylinders, I would probably go for a new powerpack. If it was a coil, I think you would have a misfire rather than loss of power. did you put an OEM fuel pump on it? I am still stuck on a fuel issue as your symptoms leads to it. Does it act up only under full throttle when the fuel demand is high?
 
Testing is the path forward.----Run with a timing light.----One cylinder at a time.----Observe the flashing light when motor acts up.---When it loses power is it instant ?-----Or a surging ?
 
racerone said:
Testing is the path forward.----Run with a timing light.----One cylinder at a time.----Observe the flashing light when motor acts up.---When it loses power is it instant ?-----Or a surging ?
Click to expand...
It is instant. ```````````````--------------------_______________________________ Then it might come back, then drop again, or it might run perfectly at full speed for many minutes.

Timing light is a good idea.
 
Sttuppid said:
Since it is doing it on both cylinders, I would probably go for a new powerpack. If it was a coil, I think you would have a misfire rather than loss of power. did you put an OEM fuel pump on it? I am still stuck on a fuel issue as your symptoms leads to it. Does it act up only under full throttle when the fuel demand is high?
Click to expand...
Only full throttle, but when it happens, power loss is noticed at all RPM's. It's like someone turned a dimmer switch down. Still runs smooth, just less of it. If I gently apply choke to try to increase fuel, it quickly stumbles like a typical choke response. I would think choking would improve, or do nothing if it was starving for fuel. But I was wrong once before, I was told. ;)
 
in-too-deep said:
Only full throttle, but when it happens, power loss is noticed at all RPM's. It's like someone turned a dimmer switch down. Still runs smooth, just less of it. If I gently apply choke to try to increase fuel, it quickly stumbles like a typical choke response. I would think choking would improve, or do nothing if it was starving for fuel. But I was wrong once before, I was told. ;)
Click to expand...
That engine also has a "LIMP mode," which would line up with your symptoms.
"The 2005 Johnson 30 hp 4-stroke engine can enter a limp mode, often referred to as S.L.O.W. mode, which is designed to protect the engine from damage due to overheating or other issues. If this occurs, it's important to diagnose the underlying problem, such as checking sensors or cooling systems, to prevent further damage." search here

This system is officially called S.L.O.W. (Safe Low Oil Warning) or a similar term by the manufacturer, but the boating community commonly refers to it as "limp mode". When it activates, the engine's RPM will be limited, and the motor will enter a restricted "limp home" speed.

Common causes for limp mode activation
  • Overheating: One of the most frequent triggers is an overheating condition. This can be caused by a faulty thermostat, a failing water pump, or restricted water passages from debris. A strong telltale stream (the water indicator jet) does not always guarantee that the engine is cooling properly.
  • Low oil pressure or level: The safety system is designed to protect the engine if it detects low oil pressure or an improper oil level. A faulty oil pressure switch can also trigger this alarm mistakenly.
  • Contaminated or low-quality fuel: Water in the fuel or stale fuel can cause performance issues that the engine's computer may interpret as a serious problem, triggering limp mode. A clogged fuel filter is another common culprit.
  • Sensor malfunctions: The engine's electronic control unit (ECU) relies on various sensors to monitor critical parameters. A fault with the temperature sensor, throttle position sensor, or another critical sensor can cause the engine to default to limp mode as a precaution.
  • Water in the fuel/water separator: If the outboard's fuel/water separator is full, it can cause the motor to lose power and enter limp mode.

What to do if your motor enters limp mode
  1. Reduce throttle to idle and safely get to shore or a dock.
  2. Check for obvious issues: Look for clogs in the water intake, a strong cooling stream from the telltale, and inspect the fuel filter and fuel/water separator.
  3. Check the oil level and quality: Ensure the oil is not milky (a sign of water contamination) and is at the proper level.
  4. Disconnect the battery: Some issues can be temporarily reset by disconnecting the battery for several minutes, though this will not fix the underlying problem.
  5. Consult a service manual or mechanic: For a more precise diagnosis, refer to the service manual for your specific model. Since the engine is 20 years old, it's best to consult a marine mechanic who can use diagnostic tools to read any stored fault codes from the ECU.
 
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