Johnson 150HP - Loses Power After a Few Minutes of Running

pianoko

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Nov 14, 2010
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Motor: Johnson 150HP - 1988 - Model: J150STLCCA

The motor is very difficult to start. It takes several chokings. Once it starts, it does not run cleanly. It has episodes of surging during idle. Once we get going, it runs pretty flawlessly WOT for about 4 minutes.

After a few minutes, it starts to lose power - almost like someone pinched the fuel line, then let go. It will do that a few times then lose power, like we are at half throttle. If we keep it at WOT, it will periodically surge, but will never maintain full power.

To me, it seems like a fuel/air issue. We tried squeezing the bulb while running, and that had no impact. We also gave it a good SeaFoam treatment.

Our next step is to rebuild the carbs. I do not know a lot about these motors, but my concern is the fact that it runs fine for several minutes.

Any ideas? Thanks in advance.
 

Fl_Richard

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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

With an 88 model the first thing you should do is rebuild the carbs. Remember if fuels not going through them neither is lubrication. You will kill that motor if you already havent.

I'd give it a compression test before running it again. Then I'd get the carbs done. then and only then would I take it out.
 

pianoko

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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

Well, that clears that up. We have ordered the carb rebuild system. Does this sound like a typical carb problem? How will gummed carbs cause this type of issue. By no means am I disagreeing, I am just trying to understand how it works. Thank you for the quick reply.
 

Fl_Richard

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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

Carburetor rebuilds are basic maintenance items. They should be periodically thoroughly cleaned and reassembled with new kits which include new gaskets a new float and fuel valve.

The issue with two stroke carburated motors is that the fuel is mixed with oil then sent throughout the carbs to the crankcase where it lubricates first then is burned. The carbs have different fuel paths for idle, mid speed and WOT. The issues your describing could definitely be all caused by mucked up carbs. Ethanol in the fuel is a solvent and will clean all the accumulated crap out of your fuel tanks and deposit it in your fuel filters and carburetors. (You do have a good canister fuel filter before the engine right?)

If this is a carb problem - The stumbling your experiencing is a little piece of debris intermittently plugging a vacuum or fuel passage. When your running at WOT and the engine feels like it's starving for fuel it,s leaning out. That is a low fuel to air ratio. Running lean increases combustion temperatures because of pre-ignition and heats up the pistons; this will eventually melt them to the cylinders. The first sign of this is lowered or uneven compression from cylinder to cylinder.Loss of power happens when they're getting too hot and beginning to bind. (a precursor to "seizing".) That's why I suggested a compression test.

BTW that engine is really easy to service yourself. If you can spin wrench you really should get the OEM factory service manual. It will pay for itself the first time you use it.

The problem could also be a fuel restriction somewhere else, pickup tube in the fuel tank could be the culprit, a bad fuel pulp could also be the culprit. But with a motor that old the carbs get my foam finger pointing at them first. Regardless of whether they are the actual final problem remains to be seen but it's time to do them regardless.
 

emdsapmgr

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Dec 9, 2005
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11,551
Re: Johnson 150HP - Loses Power After a Few Minutes of Running

Check the fuel primer hoses to make sure they squirt fuel when the key is on and pushed in. Temporarily pull the hose off the top of the upper carb to check it. A bad primer solenoid makes it nearly impossible to start when cold. If the engine idles with an occasional miss (called a lean sneeze) the carbs need to be rebuilt and the carb passages cleaned well with a carb cleaner solvent. If the engine runs fine for 4 minutes at WOT, the carbs are flowing fuel fine. You could have an ignition problem which is cutting out one or more cyls. Weak ignition components tend to fail once the engine heats up to normal operating temps. You can check for spark when the engine is running using an inductive timing light on each plugwire. That will show the quality of the spark to each plug. Squeezing the primer bulb is a good test-it forces extra fuel into the carb bowls. You probably don't have a fuel starvation issue based on your results.
 

pianoko

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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

This is good news. I was concerned about damaging the motor (still am slightly). We will test out the solenoid and use a timing light to look at what is going on with the ignition system. We will rebuild the carbs and replace the plug wires (we already replaced the plugs). Then we'll take it out and see where we stand. Thanks for your help, and I will post any other findings.
 

jonesg

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Feb 22, 2008
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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

When you take it out for a test run...

if its continues bogging down, hit the choke to see if it picks up power.

If it responds with more power theres a fuel delivery problem (or air leak)

If it bogs down its not fuel.

Order the genuine OEM manual,
www.outboardbooks.com
not seloc or clymer.!
 

Fl_Richard

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Re: Johnson 150HP - Loses Power After a Few Minutes of Running

What about the ---- "The motor is very difficult to start. It takes several chokings. Once it starts, it does not run cleanly. It has episodes of surging during idle"
 
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