Replacing Johnson piston?

[ZK]

Accidently posted this in the Mercury section... Sorry...

This is about a 1983 3 cyl Johnson.

Sorry I haven't posted here in awhile, guys, I have literally been on my boat on the water any spare time I've had...

Which was great...

Until a week ago, when I got stuck between Racine and Kenosha, and could only get my boat to idle.

I took it back to the marina, and did a compression test, my top cylinder has 30PSI, and the bottom two each have 120PSI.

I took the head off to see what was going on, and a reed got into the top cylinder, and put a hole in my piston.

The cylinder walls have no scoring...

I have no experience with 2 cycle motors, how difficult is it for me to replace this piston, and what are the steps?

Also, do I need to pressure test this motor now that the head has been taken off?

And do you have any more tips?

 

[ezeke]

Re: Replacing Johnson piston?

You will need a factory service manual to do the job correctly yourself.

You have to remove the powerhead and tear the engine down to replace the piston.

The market value of your entire motor is less than the cost of a replacement powerhead, and unless you are very fortunate, less than the cost of repair by a good mechanic.

 

[ZK]

Re: Replacing Johnson piston?

You will need a factory service manual to do the job correctly yourself.

You have to remove the powerhead and tear the engine down to replace the piston.

The market value of your entire motor is less than the cost of a replacement powerhead, and unless you are very fortunate, less than the cost of repair by a good mechanic.

I'm not too worried about tearing a motor apart. It'll only be one of several hundred I've done.

And a new piston and head will be $160.

I'm not sure exactly why I'd need a factory service manual. And I should have one anyways, but I don't.

And where is a decent place to find a replacement powerhead? I've checked all over the internet, but I'm unable to find anything at a decent cost.

 

[rolmops]

Re: Replacing Johnson piston?

Since you really cannot use the engine as is,you might as well tear it down.That way you can also find out what damage that reed did on its way to the piston.
A single piston can be gotten for well under a hundred dollars.In fact the piston,the rings and needle bearings that you will need will be about a hundred.Add to that the gaskets and without labor you should be able to wrap it up for between 130 and 150 bucks.
I would buy buy the factory manual because it has step by step guidance for this type of repair.It will also give you the exact torque specs for every nut and bolt you loosen and have to tighten again.
The other thing you have to buy is reed leafs. They are easily replaced but I have no idea about the cost.

 

[tx1961whaler]

Re: Replacing Johnson piston?

Here are the steps:
Remove the powerhead from the engine.
Remove most of the parts from the powerhead.

Crack open the crankcase
Replace #1

Re-seal the crankcase.
Re-assemble the powerhead.
Put the powerhead back on the rest of the motor.
That's about it. I'm sure the service manual has about 50 pages of useless information in it concerning tearing down and rebuilding the motor, but I wouldn't worry about those details.
Good Luck!

 

[ZK]

Re: Replacing Johnson piston?

That's about it. I'm sure the service manual has about 50 pages of useless information in it concerning tearing down and rebuilding the motor, but I wouldn't worry about those details.
Good Luck!

What I am still worried about is the pressure testing, since this motor uses the vaccum from the motor to draw fuel through the carbs to the chamber. Someone was telling me you have to block all the exhaust and intake ports and pressurize the motor, otherwise the motor just won't work...

Is this true?


And pistons and bearings for $100?

BMP sells the piston kit for $140 or $160, I forget. But I found Wiseco pistons from their website for $120.

And even then, for about $410, I can have the motor bored out .030, and have larger pistons installed...

Would this even be worth it?

I keep checking the internet everyday, since I really do want a 115HP motor, as that is the highest my boat is rated for... But ideally, I'd just buy a whole boat, swap motors, and sell the other boat and motor to recoup some cost... But that's turning out to be more of a nightmare than anything.