Air Nautique Engine Rebuild - Help

1998 Air Nautique 351 HO - Carbed 290 HP

I purchased this boat in March, it was stored in a heated garage and we lake tested it on a 60 degree day and the boat ran great. I took it home, put in my unheated garage and winterized it.

We used 4 or 5 times through May, June, and July. The boat ran good, was harder to start after it ran for a while, throttle had to be held wide open (flooding issue?). While cruising across the lake around 38 mph, the boat started slowing losing power. It had done this to me one time before on the way to the dock and I backed off it a bit, but didn't really think much of it as we slowing down anyway as we came into the buoys. This time however I started seeing some smoke come from engine compartment, we immediately shut it down and towed it in. The engine did not get real hot, I think I saw maybe 180-200 and that was after the engine was shut down without letting it idle down and allowing water to continue to cool it. Oil looked fine, no water in it and clean.

I took it home and started it again, and it did not sound good, I had a buddy look at it and he noticed the exhaust manifold gaskets were leaking, which was producing the extra sound and putting some smoke in the engine compartment. Ordered new manifold gaskets from skidim as well as bolts, replaced them (was sure to drain water from manifolds before removing them). Boat started up and sounded a ton better as it was not losing exhaust out the manifolds, but had a tapping sound on the right side of the engine (facing from front). Pulled the plugs and the 2nd cylinder (right side from front of engine), had noticeable metal shavings on plug and compression tested only revealed 30lbs, while the other cylinders were 130 to 140. Boat only had 500 hours on it.

I have a reputable auto shop that is going to rebuild it. My 2 questions are:

What year of 351w engine would this be considered in order of getting the right rebuild kit?

Any idea of what happened? I only put 10 hours on the boat, so I am unsure if there was something done prior to me buying the boat. I just don't see what factor would have caused this, but I am far from a mechanic. This is my 3rd nautique, and had such good luck before, but I have been diligent about maintenance. I want to make sure I am not missing something to prevent this from happening again. 500 hours seems low for this motor to be rebuilt.

Thanks in advance!

Thanks for info. I guess the guy rebuilding should be able to get a decent idea of what happened when he tears it apart.

Any idea of the year of the motor? The boat is a 1998, but I know when we was looking up the automotive equipment, ford hadn't put the 351 in car/truck for sometime before 1998. He wasn't sure what year of rebuild kit. I assume the latest year that the 351 was made?

Disagree with Gary on the roller cam... I would put one in 100% when rebuilding a basic lefty like you have there. Agree that the rebuilder should know what parts it needs and where to get them- if they don't, keep looking for another builder. You'll need to smarten up on gaskets, etc if you're only having the longblock prepped vs. having them assemble the full bobtail.

Thanks for the info. I bummed this engine only made it to 500 hours, looks like 1000+ is easily obtainable. I will let you know what the tear down reveals.    

Now it is making some sense. When I replaced the exhaust manifold gaskets it was missing 1 of the bolts on that side of the engine. The person before must have replaced the exhaust gaskets, lost a bolt and never got a replacement.

If I remember correctly, that bolt was right next to that cylinder.
Can an exhaust leak bend an exhaust valve? So it is possible I have low compression in that cylinder just do to a bent exhaust valve?

TRBenj wrote: Disagree with Gary on the roller cam... I would put one in 100% when rebuilding a basic lefty like you have there. Agree that the rebuilder should know what parts it needs and where to get them- if they don't, keep looking for another builder. You'll need to smarten up on gaskets, etc if you're only having the longblock prepped vs. having them assemble the full bobtail.

Boy your disagreeable today no where did I say don't use one I was telling the OP the difference between his marine engine and 95+ truck engines so when they go and get parts.

Bri892001 wrote: JoeinNY wrote: ...Its possible you just toasted the exhaust valve with an exhaust manifold leak. In which case an actual engine rebuild would not necessarily be needed. . Just curious how the manifold leak leads to the a burnt valve? It it letting the exhaust escape too quickly?

The valve warps when it's shut down and "cold" air cools it off. Have not paid attention,but in years past that is why when the engine was shut off on race cars with shorty stacked exhausts they'd cap them,low buck operations would use a shop rag,pro's had caps

Yes changed it at beginning of season, I either used 5w40 or 10w40 and motorcraft filter. Oil looks clean.

nautique_wake wrote: Yes changed it at beginning of season, I either used 5w40 or 10w40 and motorcraft filter. Oil looks clean.

No ZDDP oil for the flat tappet engine? I've seen cam lobs worn down to almost nothing when non ZDDP oil was used. One of them was only run 5 hours before the cam went south. It will be interesting to find out what happened when the engine is torn down.

I agree on the roller cam...   It cost all of $200 to have one put in the short block that I bought last year.    If you are considering a rebuild instead of just finding out what the issue is, I would recommend a short block, they are less money from a shop that does them all day than the cost of the machine work for a 1 off build.   

HO already has them.

Gotcha, you guys know these boats!

Please tell us that you didn't take off 3 1/2" exhaust risers and put on 3"

Oh, what have I done? If it is a 3" and not the 3.5" what will be the impact? Restricted exhaust flow? power loss? OR

The pictures are hard to read clearly but it appears to me that an Automotive gasket with a steel core was used and it looks like the cooling ports around that cylinder were leaking water into the cylinder.   A marine head gasket would have a solid and Stainless core.
The gasket on your boat has a perforated core which I have not seen designed as a Marine Gasket. They work, and I have used one set but if the boat is in an environment where rust can grow they will rot out. The Solid Stainless core gaskets will last the life of your boat. A perf core is a gamble as to when it will rust out. But they are a lot cheaper to buy.
What did that spark plug look like in the bad cylinder?
I have seen failed head gaskets leak into cylinders several times in the past and the valves and piston head are always washed really clean by the steam created.
Yours does not show that look? Could just be the pictures.
It also looks like the gasket may have blown between the bad cylinder and the cylinder next to it. Look hard at the head gasket it will tell you what happened in there.
If that much water was in that cylinder you very well might have a bent connecting rod on that bad piston.