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How many hours on the engine?
Yes, a compression test by all means. Each cylinder should be at about 200 psi with no more than 10% difference between lowest and highest reading. Testing should be done on a cold engine, all spark plugs out, throttle full open, and a fully charged battery. But even if compression is low, a valve adjustment and possibly a decarb will normally bring them back up to specs. IMO valve adjustment on this engine are a serious pain. That's one of the few things I pay professionals to do about every 400 hours, although the next time up, I may try it myself.
Pull any fault codes from the ECU. It's best to have a dealer hook up the Honda Diagnostic System (HDS) or Dr. H to get a detailed readout of hours and operating history and any fault codes. If that is not available, you can pull fault codes yourself using a paperclip. Send and e-mail to me and I will reply with a write-up of the procedure. [email protected].
There are two service bulletins on this engine - #56 and #70. Try to find out if either or both have been done from the seller or by calling Honda Customer Service at 770-497-6400. Give them your engine's SN and ask if they have a record if either SB has been performed. If you need a copy of either SB, send me an e-mail.
#70 is pretty simple. If you get an alarm and the MIL light goes on, you then pull fault codes. If you get a fault code "1", replace the heated oxygen sensor (HO2S).
#56 involves the HO2 sensor also. The original exhaust tubes (from SN BAGJ 10000001 to 1500324), under certain circumstances, would allow water and mist to get to the HO2 sensor and destroy it. The SB explains how to change out the exhaust tubes for an upgraded design. All the parts come in a kit. On Boats.net it's somewhere around $150. But the labor is fairly intensive. Several folks on this forum have done the job themselves.
Also, your engine may have the problem with the mid-shaft bushing failing. It's pretty easy to inspect every time you change out your water pump impeller. There is a fairly good video on YOUTUBE showing how to change it out and the part numbers needed. https://www.youtube.com/watch?v=44ZaDzNck1I
If you purchase the engine, then the first thing to do is change the oil & oil filter, change the lower unit oil, change the low pressure fuel filter and change the high pressure fuel filter. The high pressure fuel filter is critical for these engines to run right.
Having said all of the above, these are very good engines IF MAINTAINED CONSISTENTLY AND CORRECTLY. I'm approaching 1900 hours on mine, and I expect it to go well over 3000 hours. The USCG regularly puts over 5000 hours on these engines.
Since you're looking into buying a used unit, definitely check how many hours it has and the year. Both info will help you figure out the condition of the engine.
Where do you plan on purchasing it?
I replied to your e-mail. Let me know if you don't get it.