What should the compression be on a 1969 20hp johnson outboard?

Question

What should the compression be on a 1969 20hp johnson outboard?

Answer 1

I can't say for sure what it is specified at BUT

... generally...

all 2-strokes should be about 9 or 10 to 1.

at 9 to 1, sea level, you should get about 9x14.7 = 130 psi (absolute) or 115psi (gage)

at 10 to 1, sea level, you should get about 10x14.7=150psi (absolute) or 135psi (gage)

EDIT:

yep...right around 9 or 10 to one is where they all run...and yes, if they are all degrading down to 8 or 7 to one evenly, you are just getting a bit of blow-by but beware, enough blow-by will result in local overheat/scraping and you'll probably need to bore it out or resleeve

Be sure to check after it has warmed up - a cold cylinder will not hold near as well as a warm one

Answer 2

It can vary from 100 or so up to 140. I've seen decent running motors with as low as 80 psi!

What you really want to see is both cylinders equal, or within 10% of each other.

Make sure when you do the test that the trottle is wide open.  

Answer 3

OMC repair manual for your engine says the cyclinders should run 100 psi new.  Repair or Replace when the compression falls below 65 psi.

There, straight from the manufacturers mouth.

Answer 4

i dont know what these people are talking about

i have a 150 evinrude

the right way to check it is s***w the gauge in one cylinder at a time and turning over the motor NOT CRANKING and it will show you cyl. by cyl.

on my 150 its only suppose to be 105

Answer 5

because of variation in gauges and techniques, the difference between cylinders is more important than the raw numbers.

if one is more than 5 or 7% below the other, it needs attention.

on a 1969 johnnyrude, I'd be satisfied with anything upwards of 100.  90 would be ok if they are both the same.

my '79 3-cyl is 121-120-120;  the V-6's are low comp by design.

throttles open/throttles closed...my experience, makes very little difference with a 2-stroke.

But you do want to start with a fully charged battery, spark plugs out and the leads grounded to the block.

Crank the engine for a few seconds, maybe 4-5 turns, then repeat.

Answer 6

110 psi