Compression on Johnson- What should it be.

[Firewood]

Johnson v4 90 hp.<br />I don't have a manual for this motor.(I get it from the library when I need it).Anyway I ran a compression test on the motor today. It pumped up to 120lbs. seems low to me. I'm going to sell it soon and want to be able to tell someone that the motor is fine or not. Also isn't it alright for one bank of cylinderds to read different than another? Mine all hit 120lbs, just asking.

 

[Dunk]

Re: Compression on Johnson- What should it be.

Most V4's come in between 120 and 130lbs. The fact that they are the same means she's in good shape. The biggest difference people see in compression reading are the gauges. I have 3 compression gauges and all 3 read different on the same engine.

 

[petryshyn]

Re: Compression on Johnson- What should it be.

Yo Firewood. Yes there can be a difference beween banks or cylinders. The generic allowance is about 10% variation from the highest to lowest. In your case, she's in real good shape. Yes the amount of compression matters as well. The amount is quite subjective to the number of 'hits' or compressions that are recorded. The general standard would be 5-7 hits and the number must be consistant for each cylinder. Anything less than around 100 psi could result in a misfire. If the variation was higher than 10%, rough idle may be apparent. One thing that is often over-looked is having the throttle held wide-open while doing this test.If you measured the compression with them closed, you may gain a few pounds if you retest with them open. As DUNK says, there are differences in gauges. I would recommend a compression gauge that uses a screw-in type adapter, rather than a push-in hold-in type gauge.(much less subjective to poor posture!)Also, the hold-in type gauge actually increases the combustion chamber size and may result in a slightly lower reading. One question......why are you selling such a nice engine?

 

[Firewood]

Re: Compression on Johnson- What should it be.

I just picked up a new project boat, with a Mercruiser.The Johnson 90 is a great motor (its on a 19'Crestliner) I fish all day or all night on 12 gallons. She idles good,runs perfect temp.But I moved up to a 24', I need to get offshore more often, with a 19' I don't go out of the inlet often. Well I'm glad she check out good, One of my buddies will probably end up buying it off me and at least I have piece of mind.

 

[petryshyn]

Re: Compression on Johnson- What should it be.

if your buddy backs out, could you let me know?? thanx