8v92 Johnson & Towers 735 HP Good or No?

Just MY opinion, Your Mileage May Vary

Run Forrest Run!!!

If I am not mistaken BR is an engineer. I am not but am a former Detroit owner. Since BR didn’t say why he doesn’t like them I will tell you the pros and cons as I know them. I would assume they need a rebuild. You said an MY so will you be idling along mostly or running at cruise speed. If running at cruise mostly then you probably need to figure a rebuild at 2500 hours or so. If running at hull speed I think you will have much longer time between overhauls. The pros of Detroits especially for you who can work on them is that they are relatively simple and don’t have a computer that can be taken out with a lightning strike (unless DDECs). If they are DDECs I would walk away. They will tolerate things that a modern engine won’t and get you home. The cons. Old technology, heavy, inefficient, parts availability is an issue and available techs that are knowledgeable about them are aging out. All of this being said I don’t want another boat with them because of product support is aging. I hope I have helped. Others with more experience and knowledge will chime in.

What year are the engines? Are they DDEC? What version of DDEC? These are not inheritently bad engines. It depends on many factors as others have said. How have they been run? Are they propped right so as to make full RPM and not be overloaded? Have they been properly maintained? I think you would find average hours on these before MOH right around 3000 hours. And some will go thousands of hours more, some less. You can still get most parts for the engine internals (piston kits, heads, rods, rings, etc). But DDEC components are no longer made any can be difficult to find. It can also be difficult to fine someone to troubleshoot DDEC issues if you experience them. Some of the external parts J&T made are not available either like the heat exchangers depending on what you have. There are lots of these engines around so used parts are usually available for lots of items.

The DDEC's came out around 1986 (ish) era. I'm not sure what year J&T started with them but almost certainly those 1991's are going to be DDEC. Likely the DDEC version II I believe. Might be the III series but I don't think those were until the last years in like 93-95. Don't quote me on any of these years but I think they are close.

You need more than your SAM surveyor. You need a mechanical survey by an expert on these engines.

Doesn't matter to know the condition of the engines....interesting. Sounds like you'd made up your mind before asking here. Good luck.

I have old J & T DD 471s ramped up by J & T in my Ocean Yachts, MOTOR yacht not sport fish which tend to get pushed to the limits for long periods of time.
They run great. Hours are unknow. Meters not working when I bought the boat broken at 1575hrs. Still not working , but I keep a log on run time . I've put in 830 hrs so far on them, and still no major problems. Did have to replace exhaust elbows, turbos, water pumps as they rusted out.
If they look clean, fire up when cold with minimal smoke and you sea trail the heck out of them before purchase as I did you should be in good shape. Parts are still around. J & T is still around and can get them for you.
71s are the better motors than the 92s overall, but if in good shape and the were properly used and maintained it's worth a shot IMO. As long as it's a MY not a sportfish.

I am curious about folks comments about smoke at cold start and that's indicator of some issues? What issue are you suggesting more smoke at cold start indicates? When I had my 12v92's if I didn't have the block heaters on for a day prior to firing up it would look like the marina was on fire . They would do this even right after a MOH with everything replaced. Just have the heaters on and all is good. There have been lots of threads on this and people spend thousands on injectors and special air filters and other gimmicks to no avail. Block heaters are the best thing you can and should use.