1975 Johnson 15hp

[Creedxs]

I have a 75 Johnson 15hp on a 14ft flat bottom jon boat. The engine runs great but it will not get my boat on plane no matter what trim position I put it in. I have to have someone sit up front to get close to plane any ideas?

 

[flyingscott]

Check for spark and compression. That motor can run great on one cylinder. The 74-76 models were the worst of that series. Leroys ramblings has some.good info on those motors.

 

[jimmbo]

Compression
Good Spark(can jump a 5/15" gap) on both cylinders? That engine has a Points-Magneto Ignition system, Condition of Points, Proper Gap is 0.020"
What condition are the Spark Plugs,
Carb clean, Float set correctly
Adequate Fuel getting to carb? fuel pump-fuel filter, fuel hoses, fuel Tank pick up clogged?

Those are a few

 

[Creedxs]

Ok i rebuilt the carb and new fuel pump and spark plugs. It sounds like it hits wot as it should just seems to lack the torque. Oh and it will occasionally jump out of gear not sure if that helps. Im new to

 

[oldboat1]

You may need to replace a worn clutch dog in the gearcase. Or maybe the brass connector between the upper and lower shift shaft was assembled incorrectly.

There are some pretty beefy jon boats, so it's possible redistributing weight is the best solution. You might buy an extender for steering and throttle -- operate from the middle seat.

 

[Creedxs]

How would I know if its the clutch dog? And i have built a deck up front to get me on plane when im by myself just want to get the most out of it but only getting 10 to 11 mph with just me compression is good and spark is good

 

[oldboat1]

If it feels like you ran over a log, it's likely the clutch dog.

Better measure spark and compression and post results (or risk getting lambasted here)

Sounds like you might be a little underpowered, or expectations a little too high.

 

[Creedxs]

Compression is 90 on both cylinders and checked spark with spail indicators on both then remove the plugs and its sparking through the plugs bright blue spark and you maybe right it may not be enough motor for my boat.

 

[jimmbo]

Can the Spark Jump a 5/16" Gap? Spark at an Uninstalled Plug means nothing

 

[flyingscott]

My 9.9 will plane out my 14 ft starcraft it sounds like your motor is running on one cylinder.

 

[Creedxs]

How would it only be running on one cylinder if its firing on both? Maybe a stupid question but im confused

 

[oldboat1]

Look for a 5/16" spark like Jimmbo suggests. Use this type of tester:




You may have weak spark or perhaps intermittent spark, which usually indicates a problem with the ignition system. Measure the spark, and post results.

 

[Creedxs]

Where do get a tester like this

 

[oldboat1]

Most auto parts stores. Not expensive (maybe $15 US). Avoid the inline testers -- want open air type, and adjustable. Remove all plugs initially to make sure you are spinning the flywheel fast enough -- and make sure you have a good power head ground for the tester.

 

[flyingscott]

How would it only be running on one cylinder if its firing on both? Maybe a stupid question but im confused

Do this when it is running pull the wires off one at a time. The idle should change as you pull the wire.

 

[Creedxs]

So i noticed today that the cavitation plate had a peice broke off so i out a hydrofoil fin on and it gets up on plane and is running 15 to 18 mph does this make sense? Would it make that big of a difference?

 

[Creedxs]

So i noticed today that the cavitation plate had a peice broke off so i out a hydrofoil fin on and it gets up on plane and is running 15 to 18 mph does this make sense? Would it make that big of a difference?

 

[oldboat1]

(Now if you operate from the middle seat you could see 20mph!)

 

[Creedxs]

Thanks for all the help guys i was at a loss but now I'm able to put it to use