1977 Johnson 70hp Runs on the hose, but not in the water

[eclark53520]

Engine will start and run like a top while on the hose, but put it in the water, and it won't start.

Where do i start?

 

[eavega]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

Have you performed a compression test? An engine with weak compression or with a non-functional cylinder may start and run on muffs without any problems but fall flat on its face when it has to deal with backpressure on the exhaust system when in the water. Perform a compression and spark test to eliminate any problems there first.

Rgds

 

[eclark53520]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

I know compression is good. Just checked that, running 118 on all three.

I do have a sneaky suspicion that the middle cylinder isn't running...

Spark is good, and when i spray fuel into the carb idle doesn't change...

I'll have to get a true spark tester to make sure the spark is good on that cylinder. Right now i pull the plugs and check spark there.

Thanks for the quick reply!

 

[eclark53520]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

Can i spray the jets with carb cleaner without taking the carbs off the engine? I can see the jets...i just don't know if the carb cleaner would harm the reeds or anything else running through the engine...

 

[eavega]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

Spraying carb cleaner into the carburetor will not clean the jets properly. If the carb is suspect take it out and clean it and rebuild (if necessary) it properly.

Rgds

 

[eclark53520]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

Alright, i got the carbs off, and while cleaning them i was looking at the leaf valves...obviously not touching them at all, i rotated the flywheel. The upper and lower valves make the same sound...kind of a low thud when they open, while the middle valve makes a loud sucking sound when the valve opens...

Is this normal?

 

[eclark53520]

Re: 1977 Johnson 70hp Runs on the hose, but not in the water

I just wanted to post back with my results.

After rebuilding the carbs as per the manual - The engine would start in the water but would not run at full power. It would bogg and sometimes die out while attempting to advance the throttle past about 1/4-1/3.

While troubleshooting the ignition system i found a ground wire on coil #2 that was on top of a wire loom, and not directly against the ground metal. Fixing that gave me intermittent full power. Sometimes it would get on plane and act normal, other times it would still bog out.

Since that ground was messed up i looked over ALL the grounds on the motor. Lots of them were dirty, but the worst one was the power pack ground. It had lots of the liquid neoprene between it and the ground metal(engine block). After cleaning ALL the grounds on the engine, including the power pack ground, i have not had any symptoms since.