Johnson

[Fisher128]

What spark plugs should I use on a 73 Johnson 25 hp outboard

 

[jimmbo]

Champion Spark Plug Parts search shows 4 plugs

841
J8C
871
RJ8C

iboats sell the J8C and the RJ8C

 

[F_R]

Somebody at Champion oughta be hanged. That is a high compression, high RPM motor and too-hot plugs can destroy it. J8 is on the verge of doing just that.

The correct plugs are J4C, and yes iboats sells them.

 

[Fisher128]

Is the gas mix ratio 50:1 on this Motor

 

[Mohawkmtrs]

Yup.

 

[Fisher128]

I just purchased this motor from a guy and it took it out for the first time yesterday and it was running great had it at full throttle I was hulling but and then it was like it jumped out of gear and now it will go into gear at low idle but when I give it throttle it don't move it just ramps up do I need another lower unit for it

 

[racerone]

Try another prop.--------Or maybe look for a shearpin ( if so equipped ) as several lower unit will fit on that motor.

 

[lindy46]

Most likely a spun hub in the prop. As racerone says, try another prop or have the old one re-hubbed.

 

[F_R]

Since it drives at low speed, it most likely is a spun prop hub. It can be tested. Make a mark on the prop nut, and another on the propeller, aligned with the first mark. Run motor till suspected slippage occurs. Stop motor and look at the marks. If no longer aligned, it slipped. If aligned, repeat the test because there is a rare chance it could have stopped slipping in the same position.

 

[Fisher128]

Ok I'll give it a try thanks

 

[thatone123]

NGK's are my go to plug for all outboards in general. Never any problems with them. I use a lot of B7HS or BR7HS on various compact OMC outboards I have. I change outboards a lot as I get bored and tired with different ones and always looking for the portable outboard "Treasure most hard to find".

 

[oldboat1]

^^pretty much like thatone says -- except I would substitute Champions for NKGs!

The J8s are pretty high in the heat range -- if nothing else, might mask other fuel or ignition issues. Issues you describe also sound to me like a prop hub issue, but can't rule out a worn shift dog. If it seems to go out of gear with some fracas, it might be a shift dog problem.

 

[Fisher128]

It did spun the hub iv been searching for a new or used prop don't have a lot of money how do you know what pich and size of the prop you need the old one is old and ruff looking its got chips missing on the propellers

 

[boobie]

There should be some numbers on the prop. Look closely.

 

[Fisher128]

I bought a 14 ft aluminium craft boat it had a 1961 4 hp evenrude on it that was broken so I got rid of the motor and got this 73 Johnson 25 hp motor and put on it do you think the boat is the reason the prop spun the hub I would hate to replace the hub every time I take the boat out on the water

 

[boobie]

NO......

 

[boobie]

Edit

 

[F_R]

The attached cutaway picture shows what happens when a hub spins. As made, the rubber cushion is bonded to the brass hub, and the whole thing is smashed tightly into the prop. The rubber provides drive line protection if you hit a submerged object. So what goes wrong? Mostly age and deteriorated rubber which comes loose from the brass hub. That and a lot of rock dredging. No, the boat has nothing to do with it.