1978 Johnson 6 HP with Low Compression on 1 Cylinder

[SteveRoy]

This motor has had limited use over the years, and has performed well to date. But, all of a sudden started running on only 1 cylinder. The spark was OK on both cylinders. The compression on the top cylinder tested at 90 psi and on the bottom cylinder at 30.

I suspect broken reeds. Do you reach them from the carburetor & intake end or the top end?

I would appreciate any help I can get.

 

[tx1961whaler]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

You can see the reeds with a flashlight once you take the carb off. However, I highly doubt that the reeds have anything to do with your problem. I suspect that either the rings are broken, or the piston has a hole in it.

 

[R.Johnson]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

Most likely, just a leaking head gasket, not uncommon on these small motors. Reed valves have nothing to do with compression in the cylinder. The gasket is very easy to change.

 

[tx1961whaler]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

Yeah, you're probably right. Busted rings and a holed piston would be making a nasty racket that he would have noticed.

 

[R.Johnson]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

Holed pistons, or broken rings are rare on these small engines.

 

[SteveRoy]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

Thanks guys for your ideas.

On the weekend, I think I'll pull the head off and check for the blown head gasket. I'll let you know what I find.

 

[kenmyfam]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

head gasket is a good place to start.
By the way, welcome to iboats.

 

[SteveRoy]

Re: 1978 Johnson 6 HP with Low Compression on 1 Cylinder

The Head Gasket was blown. R.Johnson of Laporte City, Iowa nailed it.
I replaced the gasket and she's running like a new one.
Thanks to all that replied.