Pre-purchase HELP...Yanmar 4JH3-TE

[undrwtrfun]

Pre-purchase survey of a 2002 Hunter 456 we are trying to buy revealed an issue with the engine. Of course the selling broker & their marina tells me its not an issue, but before we invest a majority of our savings on this boat, I'm hoping to gain some insight from your experience in the forums.

Problem: Engine failed to reach manufacturer wide open throttle RPM of 3700 while in gear under load. It reached 3,360 with no blow-by pressure at normal cruising & there were no excess vibrations or leaks.
She ran at 2,800 during sea trial & other than some black smoke at start up she ran fine well within normal ranges for temp & alternator output.

Background:
The 76hp 4JH3-TE is the original motor with 353hrs
Bottom is spotless, original prop with no record of any pitch changes.
Engine had not been run for 10months. Was winterized by Marina and hauled then prepped for sea trial.
Marina records show all periodic/scheduled maintenance was completed, but they have no records or RPMs ever recorded.
The surveyor did not spot tach (is that correct?) direct on the engine, so I don't have the data for no load or under load rpms at the engine.
The boat was hauled for bottom job prep before I could request any other inspection by a marine mechanic.
Questions:
-Is this something very significant that we need to be weary of before purchase?
-What could be the most likely causes?
-Are there any actions that can be done while hauled to verify condition?

Any help would be greatly appreciated!!!
Sorry for the lengthy post...

Matt

 

[reworb]

I would get an engine survey if I were you.

I just bought a boat that surveyed very well;, no engine survey as it had very low hours. On the delivery home the engine suffered a broken crankshaft. Fortunately I can afford to rebuild the engine, if you are not in a position to suffer some risk then get an engine survey.

Also have the prop checked, as the first thought when an engine can not reach the speced RPM is the prop size.

 

[Steve Dion]

I would have the mixing elbow removed and inspected. This is a common problem and is not an expensive maintenance item.

 

[kloudie1]

I would get a true tachometer reading before doing anything else. It would not be unusual for the boat tach to read 340 off at full speed. After seeing what real RPM the engine runs to, the prop and the elbow would be the next two suspects, echoing Steve and reworb. The spot that yoiu mention is probably a hand held tachometer to obtain true rpm. If there is no black smoke at full throttle, it may be that everything is fine.. The injection limiter on the governor could be a bit off as well, but without an accurate RPM reading, I wouldn't suspect anything as being a problem.

 

[Ron20324]

Your first thing should be to rev the engine in neutral and see if it can make max RPM. If it can, then it should be able to make that in gear, under way, in fairly calm water. If not, the prop is fouled or, most likely, not the proper pitch. Take about an inch off the pitch. I had to do that.

 

[Don S/V ILLusion]

All good advice but I like the "get an engine survey" best. It doesn't seem prudent to depend on internet advice, albeit all well-intended, for such an investment.

 

[sailaway78]

More than likely

As others have said, it is more than likely a tach not reading properly, a prop a little off pitch, or the mixing elbow somewhat fouled up. I have a friend who purchased a brand new H 36, and the prop pitch was off. But check the tach first. Being that few RPM off, doesn't seem like much of a deal.

 

[Rich Stidger]

If the boat has a non-OEM alternator, like a high-output upgrade, the pulley ratio of engine crank to alternator could be different from the factory unit. The alternator could also have a different number of poles.

This could be a factor because many tachs use a signal from the alternator instead of a signal from the crankshaft. I don't know how your Yanmar is setup. A different pulley ratio or a different number of poles in the alternator would cause the tach to read incorrectly if the tach uses an alternator signal.

 

[CPseudonym]

Do not buy ANY boat with an expensive(read hear ANY inboard engine) without a complete engine survey by a factory certified mechanic of your choosing... Not the marina mechanic, not the surveyor, not the brokers budy... Your factory tech paid by you with full oil analysis and sea trial with him on board taking readings underway.

I don't care if its a 35 year old Atomic 4 or a pair of brand new Yanmar 500HP's on a power boat. It is the cheapest insurance you will ever buy. I am convinced it would have saved more than one poster on this Forums wallet in expensive "unforeseen" problems.

Ask any of the guys hanging outboards on the back of inboard equipped boats because they can't swallow the bill to repair them properly. Most of them thought an engine survey was a waste of money.

 

[undrwtrfun]

Thanks to all for your sage advice. We are working to extend the accept/reject date in our contract so we can have a Yanmar certified mechanic do a full survey.
We've already rules out the tach as direct engine readings were taken and the prop & pitch have been verified as accurate according to manufacturer specs.

we are hoping it's something easy but will let an independent expert confirm for us.

Thanks again to all have responded & assisted. We really appreciate it.

 

[Stu Jackson]

Could also be the tach itself. Many have adjustments on the back where they can be adjusted for different alternators that Rich mentioned.

 

[CarlN]

The reason I'd do an engine survey on this boat is different. A ten year old engine with only 350 hours could have a variety of issues. Diesels need exercise.

Carl