Where to start with an Old Merc

Digging In: Some Tips on Evaluating and Fixing Up an Old Merc by Bob Grubb

Perhaps you have recently acquired an old Merc, or are considering purchasing one. You may be wondering where to start to get it running again, or how to tell if it is even fixable at all. I've put together a list of general suggestions designed to help you evaluate a motor, and give you a general idea of what could be needed to bring it into working order.

Give it a general look-over. The ideal is a clean freshwater motor. Beware of battered or corroded motors or motors with discolored paint on the powerhead from overheating. Motors with broken or missing parts or battered fasteners from constant tinkering are also negative signs.
Check that the motor turns over and feels like it has decent compression(s).
Check the lower gear case for cracks or breaks. Check the lower gear case lube. Rusty water or no lube are very bad signs. Crank the engine over or turn the prop, and watch the end of the prop shaft for any visible run out (bent shaft.)
If, at this point, you've not found any problems you consider serious, remove the spark plugs and check for excessive internal looseness. I like to use an allen wrench to insert through the spark plug hole against the top of the piston. Turn the flywheel so the piston is about mid-travel. At this point, try moving the flywheel slightly back and forth to try to detect lost motion between the crankshaft and the piston. Any significant movement of the flywheel without corresponding movement of the piston means disassembling the engine to find the looseness. It can be either in the piston pin or crank pin or both. Repeat this procedure on all cylinders.
Try moving the flywheel sideways. There should be no noticeable looseness in this direction. Check up and down, there should be some end play, but generally not more than .015".
Next, turn the motor over at a slow cranking pace and you should hear a soft pop from the spark plug hole each time the piston goes down and opens the intake port. This is a necessary indication that you have crankcase compression. Badly worn bearings, bad crankshaft seals, or leaking reed valves are causes of not having this. Another possible cause is, of course, little or no compression at all. I find gauges vary, but generally the smallest models should have 80 pounds or more, medium size models 100 or more, and many larger models 120 lbs. or more. Any engine with two or more cylinders should have similar compression on all cylinders.
If poor compression is your only internal problem, you may be able to restore the motor without tearing the motor down by using an engine cleaner (available from us or your local dealer.) This can only help if the problem is caused by piston rings that are stuck from carbon deposits.
If anywhere along the line the engine has failed a test, it must be dug into to correct the problem or laid aside as a source of parts for other engines. We are not out to ruin any equipment. On the other hand we find it best to not unnecessarily tear down power heads. Many times, unnecessary honing and poor cleanup results in a motor that is more "worn out" than it was before.
We recommend in general replacing the upper and lower crankshaft seals on the old Mercs, where this can be done without completely disassembling the powerhead.
The conventional magneto or battery ignition systems used mid to late 60's and before should be capable of jumping a 1/4" spark gap in the open air. Weaker spark than this usually means that condensers or coils or both need replacement.
Generally, fuel pumps should be rebuilt, replacing at least the diaphragm. Carburetors usually need disassembly and cleaning to remove old gas residue and debris.
Water pump impellers should be checked by disassembly and usually will need to be replaced.
Lower gear case sealing was originally not up to today's standards and usually has only gotten much worse over time. You should keep close tabs on the gear lube. (I check my own motors after each use.) Making these units seal as well as possible usually involves repairs to the shafts, as well as seal replacement.

Hopefully, following these guidelines will provide a good performing engine (or help you to decide that your engine is not a good candidate for restoration.)

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