Looked at a 29 Fountain today with twin 454's. Engs were rebuilt abt 40 hrs ago.

After rebuilds, the boat sat for a couple of years and the fuel went bad. Boat was taken out and one eng ran lean. 2 pistons were replaced (#4 and #6). Cyl walls and valves looked good.

Eng was ran on test stand for abt 20 mins and compression test done afterwards reportedly passed. Eng was ran on trailer for abt 20 more mins while getting timing and carb tweaked.

Got the boat on the water today for the first time (after pistons were replaced). Estimate the fuel tank had abt 25 gallons of the old gas still in it. Another 100 gallons of fresh gas was added along with a couple bottles of octane booster and a bottle of Lucas fuel conditioner (wasn't my idea).

When we started the engs, the repaired eng had blue smoke coming from the exhaust. The eng also died a couple of times at idle after it was warmed-up.
Cruised around lake for abt 20 mins running abt 3k RPM. Engs seem to run OK.
When we got back to dock, eng was still smoking at idle and the idle was still a little rough.

Owner thinks smoke is from the bad gas and will clear up after the old fuel is ran out, but other eng doesn't smoke at all.

I really like the boat but the smoking has me worried.
What would make the eng smoke at idle but not while on plane (at least noticeably)?

I'm thinking it could be as simple as an intake gasket may have slipped and it's sucking oil from lifter valley under high intake vac conditions (I.E. idle).

If it was a valve guide seal, it would have smoked more upon initial start-up and cleaned-up after lake run, but it didn't.

Would 40 mins of idle and 20 mins of cruising be enough the seat the 2 new oil control rings?

Seemed like the smoke was coming from both exh pipes but it's really hard to tell from inside the boat with pipes under the swim platform.

May go over there and pull the plugs tomorrow just to see if I can tell which cyls were burning oil.

what kind of oil was used for break-in ? take a look at the plugs see if you see signs of oil...newer rings are really touchy on cylinder wall finish for seating..ran into that on a 502 I bought a few years ago...you do not want synthetic oil for break-in I use rotella and seems to work fine...thats with a roller cam...

Based on what you have stated, I would be very causious about buying the boat.
They should have changed all of the piston rings, not just the two. They should have had a valve job done on the heads. And the big question: Why did it lean out causing the 2 bad pistons and was that problem adressed and fixed? Bad gas is not the problem, but an intake gasket could maybe cause the problem, but it was just rebuilt and shouldn't be the problem. Are they carbureted or injected?

Thx for the replies guys!
Not sure what brand oil, but I'm pretty sure it was dino oil.
Cyl walls still had nice cross-hatch pattern.
Eng has roller cam.
Only the 2 pistons with low compression were removed.
The other head was not removed.
Lean-out was caused by plugged fuel filter.
Engs are carb'ed. (750 DPs)

My thought is the piston/ring assemblies are the cause of the smoke.

You are right, valve guide seals that are leaking generally clear up after initial startup. Worn valve guides generally will smoke on decel.

my guess is their is more damage than you think,if 2 pistons were burnt,the rest probably have issues,i have seen this many times,unless the deal is really good,walk away from this boat,or plan on a total rebuild.

I think you are back to pulling the plugs and see which ones are oil fouled, if you can tell. Make sure you mark them, and pay special attention to 4 & 6. It would be better if you could get a leak down test, or at least a compression test as I think there is still something wrong with that motor. I wonder if they honed those 2 cylinders out? If not, it sounds like the oil rings are not seating. If they were rebuilt 40 hours ago then the heads/guides shouldn't be a problem.
I think it will need to come back apart and done right, so if you can get the boat for the right price knowing that, then go for it.

+1
I would have never done it the way they did by just replacing the 2 pistons. They never even took the other head off, so you don't what kind of damage is on that side.

Well I guess the $99 question is; if it is the oil rings not seating, will they seat after a few more hrs? How long does it take oil rings to seat under normal circumstances?

I know compression rings need to have engine loaded to seat properly, but not so sure abt oil rings.

I would move on there are plenty of nice boats out there for sale. By the sounds of it this boat will become a child problem to the new owner.

my two cents.