Boatthecape24 wrote:I may buy a 2005 170 MONTAUK.
Q1: should I hire a marina to go through the boat and the engine at a cost of $500 to determine what needs to be done to the boat?
As for hiring a "marina" to perform a survey, I would not hire a "marina."
Instead, I will give you some free advice. I think this will be a much better value proposition for you than spending $500 at "a marina."
The condition of the boat is generally easily assessed. It is an open boat, there is practically nothing hidden. The hull integrity is the most important aspect, and this can be assess just by a careful visual inspection, looking for any indication of damage to the gel coat surface such as crack, penetrations, soft spots. Observe the hull in the water at its static trim with the engine trimmed down, and the usual gear aboard. The boat should float so that the engine splash well drains are not submerged under water. There may be a slight lateral trim to the side on which the battery is installed; this is natural. There should not be a significant stern-down trim, but a slight general trim to down-by-the-stern is intentional to help drain water off the cockpit.
Boatthecape24 wrote:I do not know if the Mercury 90-HP engine could provide a comprehensive diagnostic report and running time.
The c.2005 Mercury engine will not have a diagnostic port.
For more background on the various models of the "Mercury 90-HP four stroke" engine, read my article that describes the several variant versions and their features:
A Brief History of the Mercury 90-HP FOURSTROKEhttps://continuouswave.com/forum/viewtopic.php?t=2405The "problem" with carburetors is also rehashed in the above linked article.
Because the engine has carburetors, I suspect that the engine you are considering is most likely to be the Yamaha-made engine. The power head was made by Yamaha in Japan. The power head was sold to Mercury, who adapted it to their electrical harnesses and connectors, then mated it to a midsection and gear case made by Mercury, painted everything black, and put the Mercury brand on it. Mercury was unable to manufacture any 90-HP four-stroke-power-cycle engines of there own until about 2006, when they adapted the basic 1.7-liter four-cylinder in-line block of their VERADO series of supercharged engines to become a 90-HP model with natural aspirated air intake.
You can identify the version of the 90-HP FOURSTROKE from the cowling shape. Read more in my linked article above.
The best immediate assessment of the engine will be to check the engine oil sump lubricant for color and contamination; check the gear case lubricant for indication of ingress of water; make a very careful visual inspection of the engine block, wiring harness, rubber hoses, spark plugs, and other components, looking for signs of any leaks, rusts, damage, and so on.
If the gasoline fuel is four years old, discard the fuel--probably only six gallons at most--and get fresh, non-ethanol gasoline. Put the boat battery on a good battery charger for a day, bringing it up to full-charge. Using a hose attachment, attempt to start the engine--following the exact pre-start procedure given in the engine manual. The engine should start and idle smoothly.
If a sea trial can be arranged, attend the sea trial and evaluate the boat and engine on the water. If the engine was properly prepared for storage and is in proper running condition, there is no reason why it would not continue to function for many years.
In general, this boat is almost 20-years-old, so it may need some restoration. You are in the best position of anyone to determine if the boat has received good care and proper maintenance, as you know the seller. Also, you know best about your own abilities or willingness to have the time and skill necessary to restore the boat.
But for me the most attractive aspect is the very modest asking price. If you do not buy this boat I am sure someone else will without hesitation.
Read this discussion from 2021 regarding a c. 2005 Mercury 90-HP four-stroke-power-cycle engine with carburetors:
Mercury 90 FOURSTROKE Carburetor Enginehttps://continuouswave.com/forum/viewtopic.php?t=6171A wide range of opinions will be expressed. Pay attention to the comments from participant SOSMERC, who is an actual Mercury mechanic.