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3 Cyl Johnson: Starts up fine, idles fine, runs slowly fine, but can't go fast

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DrunkJ14

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I have a 3 cyl 70HP Johnson, I think it's a late 1970's model. When I put her in the water for the first time of the day, everything works fine. I get to my fishing spot, turn off the motor, and sit for maybe 2 hours. When I'm ready to leave, the engine starts just fine, it idles pretty good, and goes slowly just fine too. But, when I try to take her wide open, it just dies. Before I found out how terrible it was for the LU, I was reving the engine in neutral, then slamming it into gear while it was still reving down, and that seemed to allow me to go fast... I've been told to clean-out/re-build my carb so far... any advice would be appreciated...
 
Where did you find that model number at? The model of J70ELCNB is a 1982 70hp Johnson. A 1982 model has three (3) coils... how many coils does your engine have?

You indicated that the cold engine ran fine until you shut it down. When you shut those engines down, all the heat rises to the powerhead area which can raise havoc with the stator (beginning of ignition and charging system). If the stator is faulty and melting down, when hot it will result in an AC voltage to the powerpack capacitor which is turn results in weak, erratic, and eventually no spark/ignition.

Look under the flywheel and closely inspect that stator if it is indeed a 1982 model... if there is a sticky looking substance dripping down on the timing base and powerhead area, the stator will require replacing.
 
The model number was at the top of the motor. I'm certain it's a 1982 now. 3 spark plugs.
Good gravy, a bad stator sounds a lot worse than dirty carbs, but it's better not to waste my time on something that I NOT the problem...
My fish locator has the system voltage as a readout, and it will read up to 17V when I'm full throttle, but reads around 13V when I'm in idle, if that means anything... I think that's OK,since you need higher voltage to force a charge on the battery.
I'm an electrical guy, so I understand a little bit in that realm. I know a capacitor is used in tandem with a diode bridge to take the AC voltage produced by the rotating motor and convert it to DC voltage.
so I'm a little confused why you think AC voltage is a problem at the capacitor?
I'll check the stator, and get back to you.
 
If it is the stator, am I SOL? I will need a new motor?
Hard to believe, since it runs so smooth otherwise...
Why would revving it up in neutral, then slamming it into gear allow it to go fast?
 
Without taking the flywheel off, it doesn't look like anything is sticky... I think the 17V at wide open is a good sign the stator is ok. I'm going to try cleaning all the filters and putting some cleaner in my fuel. I also noticed my LU was leaking a bit, I wonder if that could have contributed to my issue...
 
That line in my post #6, instead of reading "If the stator is faulty and melting down, when hot it will result in an AC voltage to the powerpack capacitor which is turn results in weak, erratic, and eventually no spark/ignition."... should have read "If the stator is faulty and melting down, when hot it will result in a AC voltage drop to the powerpack capacitor which is turn results in weak, erratic, and eventually no spark/ignition.

Your mention of the 17v.... you're speaking of the charging voltage to charge the battery.... I'm speaking of the voltage that energizes the powerpack.

The stator is a two part unit.... a series of small coils that throws AC voltage to a rectifier which converts it to DC to charge the battery. There would be three larger coils on the stator that provides voltage to the powerpack. One part could be perfectly okay even if the other part was completely shot.
 
Ok, all things being honest, I got worried about taking off the flywheel when I saw all the warnings and 19 degree advance mumbo jumbo.. Is there anything I should worry about when I take off the flywheel besides losing parts?
I did get all the fuel cleaners and junk, but I want to get new seals for my lower unit before I go and run around in it...
Let's say I find the stator is not leaking, what do you think would be the next place to look?
 
your best bet may be to take it to a good shop and pay for a diagnosis...a couple hundred now could save you a lot more later....
 
I appreciate the suggestion, and will probably take it to a repair shop eventually, but I find learning about how the motor functions to be fascinating. It's almost worth the price I will pay for the knowledge I gain.
 
Ok. Replaced lower unit orings, replaced lower unit fluid (old stuff looked ok), put a bunch of seafoam carb cleaner in my gas, and made sure my fuel lines were free of kinks. Started up a little rough. Killed on me a couple times. After a few minutes, it started to purr. I attributed it to the seafoam. I took her out, and sure it enough, no trouble going fast. After a fun time bebopin around, I stopped, shut off the motor and sat for about 20 min. She started right back up, and idled fine, but it killed when I tried to go fast. I put the throttle just at the edge of killing the engine, and pointed myself back to shore. As I was putting along, she started to take off! I drove around for a while, but noticed a strange sound, like the prop was cutting air, rather than water. It was a wavy day, but I hadn't noticed the sound before. It seemed to be a lot louder when I was turning left. Strange. I stopped for another 15 minutes, then started back up. Same thing, could not go fast, so I left the throttle at the verge of killing, and she eventually took off. Seems like there is still an issue, but maybe this is just how old engines are?
 
Sounds good. Where do I spray the seafoam to get it into the carbs? Right into the air intake? Or do u have to remove the throttle linkage and get deeper into the carb?
 
DrunkJ14 said:
I got worried about taking off the flywheel when I saw all the warnings and 19 degree advance mumbo jumbo..
Click to expand...

Taking the flywheel off does not affect any of the timing, synchronization, etc. The flywheel key aligns the flywheel perfectly with the crakshaft so there's no chance you'll distrub anything.

The important thing is to absolutely torque the flywheel nut to factory specifications (105 foot pounds) when reinstalling the flywheel, otherwise the key will shear which throws the engine out of time.
 
UPDATE: Took her out again last night, and everything ran beautifully. Never had a problem going fast, even though I stopped 3 separate times to fish. I think a re-build is in order later this year, but for now, in-gas seafoam has fixed my issue.
 
UPDATE 2: Starting to have trouble starting up as well. Once she starts, I rev it up in neutral, and hit the choke a few times and that seems to get her going. My thought is that the quick burst of air is cleaning out a small hole (of the jets?) and allowing the necessary fuel to get through...
 
Ethanol is a carbs worst enemy. I add a little extra 2 stroke oil to the non ethanol fuel during the cold months when I use the boat less, and never drain the bowls or run the carbs out of fuel. I find that the fuel left in the carbs actually keeps the carbs lubricated better than being dry. I use a little sta-bil or marvel mystery oil to help repel the water that accumulates in the tank and bowls through condensation (temp changes during winter). This is cheaper than rebuilding the carbs each year. I am on my fourth year on these rebuilt carbs. The best other option and the recommended one is to crank and run the motor at least weekly. This is usually not feasible and is often neglected. I try to crank mine monthly at the very least.
 
Rebuilding your carbs is tedious, but cheap. And necessary. It sounds like you might have small debris moving around because you don't always have the same issue.
 
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