Can anybody tell me what the compression should be on a 85 Johnson 150 outboard motor be? thank you?

Difference between cylinders is more important that actual numbers.

You want the lowest to be within 5 to 7% of the highest.

Anything upwards of 100 would be considered acceptable by most.

Engines of that age are usually between 115 and 125 lbs. but they should all be about the same or with in 10-12 lbs. of each other. If one is like 70 lbs. remove the head on that side. If the piston has failed, you will know right away. l do not think l would invest in repairing the power head. Often it is #3 that is bad. But it is the damage to the crank that kills a lot of these engines. Good Luck!!!!!