Re: Need some quick info on a Merc 850
Compression test does not require the motor to be running. Can be done two ways. You want to get at least 4-5 revolutions to get a good read. Removing the plugs makes it much easier to crank it over.
1) Manually crank motor over with rope around flywheel.
2) Crank the motor over by jumping the starter motor with a set of battery cables.
Most important aspect of the compression readings is the consistency from cyl to cyl. Generally the readings should be within 10% of each other. The higher the reading the more grunt the motor will have. Readings over 100psi are good. Higher is better of course but you won't see numbers higher than about 150psi.
As for starting it, you need the following. Use
extreme caution working around the motor as the flywheel and prop are exposed and can cause severe injury. Do not run the motor dry. Always run it in water or hook up a garden hose to a set of muffs. Make sure your gas is mixed at 50:1.
Using jumper cables, connect a battery (-) to the motor cowl - a good safe ground. Using 16ga wire, connect the battery (+) to both the RED
and WHITE terminals of the switchbox on the starter side (do NOT connect or remove anything from the distributor side of the switchbox). This will enable the ignition system.
Momentarily connect the other jumper cable from the battery (+) to the starter motor terminal (NOT the side connected to the cowling
). This will crank the motor over. You can operate the choke manually with the choke knob located on the cowling above the switchbox.
This is important. To shut the motor OFF, remove the +12V from the WHITE terminal on the switchbox. You could use a toggle switch to accomplish this. Do not disconnect +12V from the RED terminal while the motor is running or you will cook the switchbox.
