If not for bad luck I'd have none....finally got boat working and Big block goes boom

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by WetPleasure, Sep 21, 2021.

  1. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    So as title describes I've been through the ringer restoring a 1999 260 da. Bought it from a friend for cheap with a blown up carb 7.4 in it that blew up on him after only a couple trips after he repowered his blown 5.7.

    How can a person blow so many motors. He blew up the original motor with only around 375-400 hours then blew up the repowered big block with only handful of hours and then in a twist of irony after I finally got the 7.4l "performance tuned" motor that I bought advertised as used with low hours I let him take it out figuring if anyone knew how to run the boat it would be him and after a few hours it blew up on him on the way home. Low compression on 3 cylinders. 2 on starboard side one on port. Completely blew out a spark plug like ive never seen before. I'm way too deep into this boat already. Hindsight being what it is, I would have bought a new crate motor from the get go. Had a shady mechanic install this and they lied about tuning it and giving it good work through. New mechanic has been great and we sorted out multiple issues including rusty carb and fuel pump (motor must have had water in fuel at some point), leaking sea water pump, bad thunderbolt ignition module, bad cap and rotor and lastly, an apparent case of bravoitis. I'm now into this boat for 31k which includes full cosmetic refit so now crate motor is out of the question. Anyone got tips for rebuilding this 7.4? Apparently "performance tuned" means it's got peanut port heads on oval intake. People poopoo peanut ports but apparently they make great torque and this application was created with cruisers in mind. Most likely has cast internals but rebuilding I can change all that so what is best way for me to get some more power out of this? I know it's a big heavy cruiser but not gonna lie, was disappointed with it topping out at 35mph at 4250rpm which was right on the 42-4600 rpm spec with the 24 pitch bravo 3 props. We did have 4 guys on board during the sea trial but still if that's top end is my realistic cruise only gonna be 25mph? That sucks for a guy coming from center consoles.

    Anyone rebuilt one of these? Supposedly even with cast internals the motor should be good to go north of 400hp. It's guys going crazy trying to get 5-600 hp with blowers that need the forged stuff. Open to all ideas. Quote to rebuild is appx $3000 without builder seeing it yet. Mechanic should have it out tomorrow and rebuilder is in the same industrial park. Reputable rebuilder here in Tampa though i do need to make sure he is aware of marine specific needs like not going too crazy with cam and making water reversion possible. Anyone ever see a spark plug blow out like this? No water in the oil which is good but what would cause this? Friend said they cruising along heading back to ramp when it started making a sound of air like a steam engine which must have been the plug blowing out of the cylinder and piston pushing air. He immediately shut it down and then it wouldnt restart.

    PXL_20210920_182428024.MP.jpg
     
    Last edited: Sep 21, 2021
  2. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    Sorry to hear. I just finished building my 454 after a couple hiccups. Feel free to check out a couple of threads I started.
     
    WetPleasure likes this.
  3. paulswagelock

    paulswagelock Well-Known Member

    Oct 25, 2010
    pa
    2018 SDX 270 OB 300 Verado
    Verado 300
    Water inversion into cylinder(s). Check if riser height is sufficient.
     
  4. El Capitan

    El Capitan Well-Known Member

    Jul 9, 2014
    Chicago IL./Vero Beach, Fl
    1970 SRV 180 w 2.5L Mercruiser.
    2000 Ford Expedition Eddie Bauer
    120HP Mercruiser
    I know someone. My wife. Electrical appliances and her cell phone last 6 mos at best.
     
  5. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    Triple checked it. Approximately 20". Dead flat seas on the day in question and brand new flappers installed 2 weeks ago
     
  6. Nater Potater

    Nater Potater Well-Known Member

    445
    Oct 19, 2020
    Southwest Idaho
    1992 300DA Sundancer
    Twin Merc Alpha Gen II I/O's with 5.7 V8's
    You noted towards the bottom of your first post exactly where I was going; could those engines have had non-marine-spec cams? No water in the oil isn't necessarily a for-sure indication that you haven't had water get into the jugs.
    'Not sure on your setup, but there's a range of Thunderbolt ignition systems (smarter people - chime in) that have very specific routines for setting the timing. Seeing the plug blown out like that could be from detonation. The fact that your buddy has more than one lunched engine makes me think it's something common to all, whether it's one of the externals, or how those are being set up.
     
  7. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    So regarding the motor, it was bought used and supposedly 150 freshwater hours. Not a lot of hours for a motor from 1994 but nevertheless, many people with these cruisers just use them as floating condos, especially in the .idle if Texas where it came from. I really don't think the motor had been apart, a lot of clearly original parts on it all in good shape. My thought is possibly bad manifolds risers but man, we took them off at one point and inspected them and they looked GREAT totally in line with a freshwater boat that only had 150 hrs or perhaps they had been changed. Really a mystery as to what happened at this point .

    Many people swear by mercruiser vs out oards but I am firmly in the "fuck mercruiser" camp lol!
     
  8. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    Hawk, your a goddamn animal! Just read through your entire post! Sounds like you've had it worse than me! Though your doing work yourself so your saving dollars that I'm burning through!!!

    So you started with pea ut poet heads but didn't use them. You used the cam out of a 454 mag right? Saw your last post saying you hit 35mph. Not familiar with your hull but it's a 290 and mine is a 260 so suffice it to say I should get more than 35 mph out of the same.motor if it's built right.
     
  9. Nater Potater

    Nater Potater Well-Known Member

    445
    Oct 19, 2020
    Southwest Idaho
    1992 300DA Sundancer
    Twin Merc Alpha Gen II I/O's with 5.7 V8's
    And yet, it went 150 hours without blowing up. Your buddy put a couple of hours on it after it was installed, and it blew. That takes me back to my earlier statement about it being something common to either the accessories, or how they were set up, timing in particular. What octane fuel are you running, and how old is it? What rpm is it running at, and have you verified the tach? It might be something simple like over-revving it.
     
  10. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    Yes it went 150 without blowing up however, who knows how long it was sitting before I bought it. We all kno boats don't like sitting. Tach is accurate. It's possible it wasnt timed properly I suppose but it was running solid for him for several hours before blowing up. The problem I have is that the new mechanic who is great isn't the one that installed the motor. He was hired to fix a broken gimbal ring and then look at some stuff and we have just been peeling back the onion. Now he will get a fresh start on the motor
     
    Nater Potater likes this.
  11. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    Meh, it wasn't so bad. Just one real hiccup that I wasn't expecting and it was my own fault with the cam timing. I'm just glad she's running right now.
    The heads that came on the boat were the peanut ports, but I ended up using L29 heads to bump up the compression and get better flow. This isn't going to raise your speed, only get you up on plane quicker. Your prop is what will change your top speed.
     
  12. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    So the peanut heads supposedly are torque monsters paired with the oval port intake. What I seemingly need is to be a me to get another 1000 rpm out of this thing. WOT spec is 42-4600 and on the sea trial we were at 4250 with me, my friend and 2 .mechanics on board. I do need to do some fiddling with emptying out the freshwater tank and waste tank which I was going to do my first trip out when i could run her somewhere to get her pumped out. This boat is awfully heavy for only 300ish horsepower but again, I'm a center console guy and this boat is a 1999. 300hp was a lot back then. I don't like the Yamaha 425 because it weighs 1100 lbs and only puts out 425hp so you can imagine the disdain I have for this big block that weighs 1100 lbs and only puts out 330ish! Should turn .y swim platform.on this thing into a flotation bracket and throw on a couple of 300 yammies! Hahaha
     
  13. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    Unfortunately I think you're chasing something you're not going to find. When I want power and speed I either hop in my 69 Camaro or now my new F350 w/ 1,050 lbft of torque lol
    I think you need to either be comfortable with the fact this is a cruiser and not meant to be a speedboat or you need to get a Fountain or something with twin 502 ci + motors for the adrenaline shot. Don't get me wrong, I'd love an offshore speedboat.
     
  14. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    I don't totally disagree with you, it's also possibly that the motor was already slightly down on the cylinders in question and not making power like.it should. I was just researching and I saw a boat test on a newer version 260 dancer check in with a top speed of 43mph with a 5.7. only topping out at 35 with a big block seems odd. One issue though is I don't have enough time running the boat. I bought it as a blown Up turd from my friend. I w run the. Oat a total of 5 minutes on a sea trial last Friday lol. We were fighting with the tabs and trim a bit as well too so that could be it. 35mph CRUISE speed and too end of 43-45 would be acceptable to make me consider keeping it.
    But I'm just trying to minimize losses at this point. For whatever reason the resale value on these things still totally sucks in spite of the boat boom that is going on.
     
  15. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    Unless I'm mistaken, and I'm a FNG when it comes to boats for the most part..., but you'll either need to raise your RPMs or change your prop to get more top speed. I think you raise your rpms by changing your prop in a boat though. I'm sure you can change your outdrive gears as well, but I don't know if guys do that or if the gears are available to do it. Just like with anything else with gearing. Drop the gear ratio and you'll get out of the hole quick, but sacrifice top speed. Raise the gear ratio and you can go as fast as you want. It'll just take you longer to get there if you haven't increased your power.
    I have plenty of power to get up on plane quickly, but 4,200 rpms and 35 mph and I'm out of throttle. I'm sure I can squeak another mph or two out by adjusting trim, but that's basically all she's got in my case the way she's set up.
     
  16. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    I agree with you but imguessnwhatbin thinking is by changing some of the internal parts that should make the motor able to rev higher i.e. heads, intake, etc. But I am a total novice with this stuff. Just seems that we have th same basic motor but you have a boat that is 3' longer and certainly must be heavier and you are getting the same max rpm and mph as what I saw. Motor should be totally out today and going up to the rebuilder. PXL_20210922_133834013.MP.jpg
     
  17. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    Like I said, the max rpm and mph is dictated by the gearing, not the power. The power just gets you there quicker.
     
    Last edited: Sep 23, 2021
  18. WetPleasure

    WetPleasure Member

    54
    Jan 30, 2021
    99 260 sundancer
    7.4 bravo3
    this stuff is making my head spin. Everywhere you look to ask for advice on rebuilding there are replies that say "this has been discussed numerous times, please search" so then you search and all you find are posts that say "this has been discussed numerous times please search" or you find posts where people start to talk about what they had going on but presumably they get it fixed and go boating and never come back to tell about all the details.

    So as i mentioned my marine mechanic is located in the same industrial park as a reputable engine rebuilder. So the nice thing is he can put it on a pallet and strap it down and in 30 seconds drive the motor on his forklift to the rebuilders shop. Likewise, the rebuilder can easily be in touch with my mechanic about any marine specific issues and things to be aware of so i think i have that going for me. It just seems that i have an opportunity to add some power/performance now if i'm going through all of this trouble but i definately have some limiting factors. For one, boat is not closed cooling, everything has to be friendly with saltwater. engine space, not sure what intake is on this motor but most 260 DAs have 5.7l motors so a bit more room. The flame arrestor on this big block is up about as high as it can go without hitting the engine hatch. things i keep coming across talk about going from 454 to 489/496 via 4.25" crank and boring out to .030 or .060. Seems easy enough and reasonably cost conscious. Ending up in the 400-450hp range would be really nice and would be some semblance of a way of saving face when i go to sell this boat to command a solid price. I know shes a cruiser but can't help wanting her to run reasonably quick or efficient.
     
  19. HawkX66

    HawkX66 Well-Known Member

    Jul 27, 2020
    SE Virginia and NH
    "Tread Knot"
    1997 Sea Ray Sundancer 290DA

    2020 F350 6.7L
    454 L29 Carb w/ Bravo III
    I'm not sure of your questions at this point so that might be half your troubles with research. What is your specific question?
    Yes, a 496 is an awesome power upgrade depending on how you do it and what you are really looking for. Are you just looking to punch out a 454 .060" over and stroke it or do you want a purpose built 496 which is a different motor?
    Personally I know I have a cruiser and am not expecting high performance out of her. I don't want to have to run port to port looking for fuel docks. Your purpose sounds different than mine. In the end, rebuilds and information on them are everywhere. There isn't much special about a marine motor other than gaskets, stainless internal water/circulation pump and different cam specs.
     
  20. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    This isn't true, more power will allow you to run a taller prop which will give you more speed. Surprised Lazy Days hasn't seen this thread, He's owned that boat and knows what speed it's capable of. I could be wrong but I believe he was mid 40's with a small block. Maybe PM him.
     
    WetPleasure likes this.

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