Perhaps the spark plug system is not sending electricity to the spark plugs.
125 is the least
anything over 100 on all cyl is fine and a good motor
I should be equal on all cylenders and about 110 to 120
compression rings on pistons are worn out or broken
You've probably warped the head and therefore the motor is unable to achieve any compression due to the air escaping out. If you're lucky, it's a machining and replace head gasket; if unlucky, you've done your motor. You've most likely got a blown head gasket. This would have caused overheating as the problem began and worsened, and the no start/lack of compression now. along with head gasket check timing more than likely jumped time if the belt is still good this causes no compression
What ever it cost you for a motor rebuild. It varys from shop to shop.
depending on hours 120lbs------165lbs there should not be more than a 5lb difference between cylinders
23:1 compression ratio
your 9.8 hp mercury is a 1964, unless its a white motor then it could be a 1963
It could be a thermostat that is stuck open or maybe a bigger problem...like the blend door is either jammed or broken as well.
Fuel, compression and spark.
It should be some were between 120 to 125 on all 6. That would be on a motor in good condition. 115 to 120 is not bad. Just make sure they are witnin 20percent of one another.