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93 Johnson 8hp low compression

M

MartinZ

New member
First time working on a 2 stroke, J8RETB. I got this motor years ago and never messed with it until now.

So far, I can only get it to start by using a drill on the flywheel. It would only run at medium throttle or higher. I soaked and cleaned the carb really well and got it to almost idle, much lower than it was, but it will still die if I let it the throttle all the way down. I ran half a can of seafoam through it while tinkering with the idle adjustment.

After that, I checked compression and got 35 and 45 psi, so not good. The compression gauge is harbor freight, so maybe not reliable, and I checked it after the engine had been running for a while. Don't know if I should check with the engine hot or cold.

My next thought is the head gasket, because it's easier, and then the cylinders and rings. I'm guessing it's hard to start and won't idle because the compression is so low.

I don't know the name of them, but I have these two screws adjusted to try and raise the idle as high as possible.
 
Might be the head gasket. Top cylinder has a little scoring. Bottom cylinder looks pretty good.

I could get a cylinder hone and try to smooth it out. I'm not eager to tear the whole thing apart just yet.

I'll can try a new gasket and see how the compression looks.

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Check compression on your working lawnmower. If it is also very low your gauge is crap.
I ran a permanent magic marker around the sealing surface of the head. Do as Racer said, run it on a figure 8, using emery on a sheet of glass, lubed with soapy water. The high spots/magic marker will sand off first. When the last bit of marker comes off, you know it’s flat.
Those two screws you screwed with - they need to be adjusted in a proper manner/order. Send me an email addy, I’ll send you the link & synch procedure. The full service manual is too big to email, but I can send you a link to download a shared file.
 
I cleaned the head surfaces and put the new gasket on. I wanted to check and see what difference it made without machining the faces (that and I don't want to separate the powerhead from everything else if I don't have to).

The compression is up to 50 psi on both cylinders (I checked the gauge on another motor, it's good). The motor will idle very low now. I had it running long enough to find a new problem (more on that in a second). I put the pull start back on and was able to get the motor to fire and run before dying twice, but my fuel can is empty, so I'll have to run to the store before I know if pull start is now an option.

Before it ran out of gas, when I was starting it with a drill, I had it running at a very low idle and I checked to see if it would go into gear. Reverse works, but forward does not. I did replace the water impeller years ago when I didn't know anything about the motor, so I might have put something back together wrong. Moving the head by hand and feeling the prop, there is a lot of play both rotational and lateral along the prop shaft.
 
If it has the factory original gearcase it does not have a sliding clutch dog.-----It has drive balls.----Perhaps those are broken ??-----Have you checked the drive pin on the prop ?--Only one way to find out and that is to investigate..---
 
Alright, full tank of gas, completely reassembled, engine starts easy with the pull rope.

There's a switch on the tiller that does nothing. I assume it's the kill switch, but I'll look into it. Applying the choke at idle is how I'm shutting off for now.

Now to study the lower end before I start investigating the gearbox.
 
racerone said:
If it has the factory original gearcase it does not have a sliding clutch dog.-----It has drive balls.----Perhaps those are broken ??-----Have you checked the drive pin on the prop ?--Only one way to find out and that is to investigate..---
Click to expand...
The bad news is I only found 3 of the 4 balls.

The good news is I finally found the fragments of the 4th ball, so I didn't lose one.
 
Did you deck the head? It WILL be warped, you don’t have to machine it, just get a piece of glass, lay a sheet of wet/dry automotive sandpaper (100 grit) on the glass. Pour some soapy water on for lube & work the head in a figure 8. Wipe a Magic Marker around the mating sirface of the head. The high spots will wear off first. When the last bit of marker wears off, you know the head is perfectly flat.It costs nothing, but must be done.
Then assemble with your new gasket.
 
Edchris177 said:
Did you deck the head? It WILL be warped, you don’t have to machine it, just get a piece of glass, lay a sheet of wet/dry automotive sandpaper (100 grit) on the glass. Pour some soapy water on for lube & work the head in a figure 8. Wipe a Magic Marker around the mating sirface of the head. The high spots will wear off first. When the last bit of marker wears off, you know the head is perfectly flat.It costs nothing, but must be done.
Then assemble with your new gasket.
Click to expand...
Ok ok, I'll do it.
 
Im not trying to browbeat you, but if you have the head off, & purchased a new gasket, it is silly to reassemble without decking the head. Post #9 seemed to indicate you didn’t want to resurface either the head or block. If, after following Post #13, you still only have 50 psi, the engine needs to come apart. Do the free, or very cheap stuff first, before throwing money at what might be junk or parts motor.
 
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