Hello, I am looking at a boat, 85 Ranger 350-v, really well cared for. My question is the motor, it is a Johnson GT 150, runs great, they told me compression is 85-90 in all cylinders. Is this good? I am not knowledgeable about that sort of thing. 

2 hours ago, Alan Land said:

compression is 85-90 in all cylinders

 

If the motor runs fine and all cylinders are the same + - a couple pounds you're likely OK. 

 

Call some local marine shops and see if anyone has the specs for that motor.

 

That's all that motor had new.  When OMC first started making detuned motors for the low octane gas, they didn't have the modern electronics today's motors have, they just chopped hell out of the compression.  You also have to remember, boat owners are bad about leaving gas in the tanks for months and months and still try to run it, so they had to allow for that junk gas also.  

The main thing you want to look for is the variation between the highest and lowest reading between cylinders.  A warmed up, good motor is going to be no more than 5%.  When it starts getting close to 10%, the motor is getting close to the end of its life cycle.  Also, a leak down test is a whole lot better and more accurate than a compression test.  Doing a compression test, you only want to let it hit six times per cylinder to get a fair assessment, and make sure it's the same number of hits for each or the readings are not going to give you the proper comparison.  Just cranking it until it shows the max it will show on the gauge is not a good test either because you are not getting a accurate indication of the blowby it has.  That's why a leak down test is better, it measures the blowby, which is the true wear indicator for the rings and cylinders.  

 

Also make sure you understand what the GT stood for on OMC engines.  GAS THURSTY.  There is no such thing as fuel economy with those old cross flows, and at full throttle, they love that stuff.  

 

One other note, everybody knows a fisherman won't tell a lie, but I would want to see my own test of the engine just to be sure.