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2000 Johnson 70HP - Engine Troubles

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stripey

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I've had an ongoing problem with this engine. Very intermittent and has been for a few years now. Basically it loses power and will only do around 5 knots at WOT. It eventually clears up
and runs fine. I've got a water sep I drain routinely, all new fuel lines and fresh fuel, etc.

I took it out the other day and ran it until it acted up. When it did, I pulled the airbox and sprayed some carb clean in each of the carbs to see if the performance improved. This was
my reasoning to rule out/in a fuel problem. All the spraying did was kill the motor in any of the 3 carbs.

Took it out again with a buddy, once it started acting up we pulled plug wires one by one. Pulled the top cylinder, absolutely no change to the motor, middle and bottom cylinders
were definitely working because when we pulled those the motor died way down and skipped, ran poorly.

After this we figured hmm...ok you need a coil for that top cylinder and wires just because. Did the coil and wires on Saturday, took the boat out Sunday and 20 minutes in, it starts doing it again!
Continued to run the boat for another half hour or so and it cleared, then I ran it for the rest of the day, all over the place, trolling speed, anchored up, fished, ran wide open for a while, etc
etc etc and it never did it again. Next time I go to use the boat it's almost a guarantee this problem will once again rear it's ugly head and then disappear. Not having faith in my
engine is making time on the water very stressful. Any thoughts would be greatly appreciated.

Thanks!
 
My 9.9 hp had the identical problem for several years. 1 cylinder would stumble or fully quit. finally the top cylinder power pack died long enough to be proven bad. Real aggravating problem...............Sure it was electrical because after the cover was pulled it would restart after quite a while of cooling down. Fuel problems are not like that very often.
curious. Does the engine ever get a soft or collapsed fuel squeeze bulb ? ever try running with the gas tank cap just loose ?

5 knots is more than 1 cylinder if the problem is electrical.
 
cyclops2 said:
My 9.9 hp had the identical problem for several years. 1 cylinder would stumble or fully quit. finally the top cylinder power pack died long enough to be proven bad. Real aggravating problem...............Sure it was electrical because after the cover was pulled it would restart after quite a while of cooling down. Fuel problems are not like that very often.
curious. Does the engine ever get a soft or collapsed fuel squeeze bulb ? ever try running with the gas tank cap just loose ?

5 knots is more than 1 cylinder if the problem is electrical.
Click to expand...

Hello and thanks for the reply. The fuel bulb is always good and attempting to pump it when this nonsense happens has no effect. Yes I've ran with the cap missing for an entire day!

I'd like to subscribe to the heat issue but as of late the pattern seems to be runs mint for about 20 minutes, then goes into it's nonsense, then eventually clears and runs flawless for the rest of the day. VERY aggravating!
 
A simple trouble shooting process will lead you to the answer.-----Note , spraying some carburetor cleaner into the carb throats does not clean the actual working parts of the carburetor !-----So report the results of a compression test.---Then take a timing light with you and hook it up to # 1 cylinder and run the motor.-When motor acts up observe the flashing light to see if there is an ignition issue.----Check all 3 cylinders when it acts up.-----This motor may be equipped with a S.L.O.W system to protect the motor in case of overheat.--------May be as simple as replacing a sensor or even the waterpump impeller once you gather the facts.
 
racerone said:
A simple trouble shooting process will lead you to the answer.-----Note , spraying some carburetor cleaner into the carb throats does not clean the actual working parts of the carburetor !-----So report the results of a compression test.---Then take a timing light with you and hook it up to # 1 cylinder and run the motor.-When motor acts up observe the flashing light to see if there is an ignition issue.----Check all 3 cylinders when it acts up.-----This motor may be equipped with a S.L.O.W system to protect the motor in case of overheat.--------May be as simple as replacing a sensor or even the waterpump impeller once you gather the facts.
Click to expand...

The intent of spraying the cleaner into the throats was not to clean the carbs, but to introduce a flammable substance to the engine in the hopes of pinpointing a fuel starvation issue on a per carb basis.

The tstat is brand new, and we unplugged the sensor from it that would enable S.L.O.W. to engage, while the problem was happening. No effect. We also tested the idiot lights, they work, and they don't come on when this issue surfaces. More information...the guy on board with me at the time used to be a marine mechanic and he said it definitely was not S.L.O.W. mode.

I like the timing light idea. Sure beats getting zapped yanking wires! Can you provide more details on the timing light? I just asked my father about it, he said the inductive type may work but he has the contact type.
 
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