compression for a 68 johnson?

[Isaacm1986]

I am looking at buying a 40hp 1968 johnson (I am into these old motors), I already have 3 parts motors, this 40hp runs good, I am just wondering if anyone knows what the compression should be on this? I am going to look at it later this morning, and I am taking my compression checker. I am thinking like 80-90# is what i should look for, is this right?

thanks for the help.

 

[billy4hp]

Re: compression for a 68 johnson?

I am looking at buying a 40hp 1968 johnson (I am into these old motors), I already have 3 parts motors, this 40hp runs good, I am just wondering if anyone knows what the compression should be on this? I am going to look at it later this morning, and I am taking my compression checker. I am thinking like 80-90# is what i should look for, is this right?

thanks for the help.

As you might already know there is no published compresion numbers listed in the OEM manuals. I have found most earlier engines has less compression compared to a later version and this is normal (say a 70 9.5 usually have around 75-80 psi on my gauge but a 82 9.9 read 110 psi). I know that's not a apples to apples comparison, but you get my drift.

So if it had 80-90 psi and both are even, I'd say that part of your prospective engine is alright. And if it has not been done, doing a decarb usually gains you some psi if they are a little low...

Good luck,

Billy

 

[steelespike]

Re: compression for a 68 johnson?

typical ideal minimum is 100 psi and even. Hand cranked may be tougher to get to 100.a little below 100 and even wouldn't bother me.Many times a decarb treatment will improve compression.

 

[jbjennings]

Re: compression for a 68 johnson?

I'll bet when you test it, if your gauge is correct you will have over 115psi on that motor. If you see a noticeable difference on cylinders, like more than 10% of the totat compression, I'd pass on it. I'm thinking it will have about 120 or 125psi or more on each cylinder.
JMO,
JBJ

 

[1946Zephyr]

Re: compression for a 68 johnson?

The compression on these models will be lower than the 60-61 models, so this is one thing to keep in mind too. In 62 and later engines, the 40's had a 2 ring piston, versus a 3 ring. However, the later design that you have is actually a better engine. Like JBJ says, make sure that the readings are within 10% of each other. If it hasn't been ran in a while, then it may need to be decarbed and ran for a while, before you can get an acurate reading. You'll want those rings to be moist of fuel mix and working properly.