Re: 1974 merc 115 compression
10 to 20 psi diff in compression readings on an Inline Six is NOT good news!
I'd recommend a Severe Duty decarbonizing treatment (fogging the motor thru the carbs and also spraying in the spark plug holes with the engine tilted up) with Seafoam or equivalent then checking again. Even better, take 'er out after the decarbonizing treatment and run with a load of cleaner in the fuel.
Check out this carbon-treatment FAQ:
http://forums.iboats.com/showthread.php?t=158076
Take compression readings while the motor's warm. Make sure the throttle blades are fully open when you take readings or you're not getting accurate ones. If your compression readings improve some value less than 10psi difference between the highest and lowest, you'll probably be OK. But I would still monitor periodically so you can detect any deteriorating trend.
Be sure you pull all Six spark plug wires and you'll need to jump +12V to the yellow-wire terminal on the starter solenoid to get the starter to crank over while you're in gear/full throttle. Watch out for the moving prop!
Be very careful of sparks around the carbs; you can use an automotive-type remote starting switch or make up your own with cheap parts from Radio Shack.
Note if you undo the rear cowling bracket you may be able to drop the lower cowling down enough to get your compression tester in to #6 cylinder. Or get the special short compression gage Merc (and others) sell, which will screw right into any cyl with no issues.
You also might consider contacting a local dealer who's set up to do a leakdown test, which is a much more accurate indicator of internal condition than a compression test. After the decarbonization, of course!
HTH & keep us posted.........ed
