1968 20 HP Johnson Outboard Compression

[CANE CUTTER]

I have two questions:

1. What should the compression be on this motor?

2. How do you get an accurate reading?

Thanks.

 

[JB]

Re: 1968 20 HP Johnson Outboard Compression

1) Above about 80psi and within 15% of one another.

2) Readings will vary from guage to guage, technique and with a few other factors.

Most accurate would be:

Warmed engine.
Screw in guage.
All plugs out.
Spin engine 5 or more times to get max reading and do the same on both cylinders.

But. . . the important reading is the difference between the cylinders and that can be read on a cold engine as long as both are measured exactly the same way.

 

[K.B]

Re: 1968 20 HP Johnson Outboard Compression

'Zactly

All gauges will read differently. You can use different ones, but keep your data straight as to what you read on which gauge and whether it was warm engine reading or cold engine reading. Pull cord reading that has a slower turning (with me anyway!) or elec. start turning. That will show a diff. in lbs./in. also. Getting within 10-15% of ea. is most important.