"Hal: you said you are new to
"Hal: you said you are new to boating. I have a similar boat 1987 18.5' 4.3L 2 bbl carb. I use an SS 21 pitch prop. When I bought it 2+ years ago, I gave it a set of plugs, cleaned the flame arrestor, changed the fuel filters and it made a vast improvement. Look on the engine ID label for the plug # and gap, use what the label states and gap the plugs. Change the oil--Mercruiser 25W40 weight oil is what mine calls for. You can only get it at a dealer. Change the filter, mine is AC PF52. Prime the filter with clean oil about 3/4 full and coat the gasket lightly when you install it. Priming it reduces the time of dry starting after an oil change.
A compression gauge is about $20+ and is easy to use. All the cylinders should be nearly the same reading. What is important is that they are similar and not a big difference in any one cylinder. Any good repair shop mechanic can check it for you and install plugs, etc.
Chris asked about water in it. For water logged hulls; do you find water laying in the bottom even if it hasn't rained and the boat was dry when parked? Is there mold in the hold areas? Feel around the floor with your foot for soft spots under the carpet.
If you want to check for waterlogged stringers, look in the engine compartment and ski locker compartment for the 1/2 - 1" holes that the mfg. drilled to squirt expansion foam inside the stringers. You'll see a slight bulge there where the foam came out. Probe it with an awl or thin blade screwdriver to see if there is water in it.
I used a 1/4" drill bit and bored a shallow 1/4" deep hole through the fiberglass covered transom and stringers about 1.5" above the hull floor and found wet wood. Water actually dribbled out of it until I replaced it all.
The floatation foam was sopping wet under the floor of mine. I recently replaced the entire wood floor, stringers and transom.
The hole shot is now instantaneous and on plane at 12-20 mph. My WOT rpm range is 4400-4800 and I can trim it to get to 4600+ rpms and reach mid 40s+ speed."
