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  1. #1
    Bay Rat cvbdrew's Avatar
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    951 engine help!

    Yesterday morning, I woke up to a glassy bay. So I grabbed my GoPro and made some hot laps around the bay. Unfortunately, as I was wide open through a corner, the engine cut out. The way it died felt like I burnt up the top end. I pulled it out of the water today and began investigating. First thing I noticed was the plugs were black and dry. They had less than an hour on them. I decided to pull the top end and dig a little further. The cylinders look fine. The rings were still floating in the pistons. However, the front piston has burn marks on it. The oil lines are new and the pump is still working. I know it's hard to make a judgement call from these pics, but I'm looking for some advice.

    1. Should I pony up and buy some new pistons and rings?

    2. Should I just replace rings?

    3. What may have caused this?... Too much fuel at full throttle and not enough oil? Carb adjustment advice?

    Thanks,
    Drew





  2. #2
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    looks like it seized on one of the pistons. Did you check the compression before removing the cylinder?

    also when you buy the pistons make sure you buy the ones for a DI they will be more expensive than a carburated engine.

    I'm not sure what caused your seize, I'm nothing familiar with the DI.

  3. #3
    is that a big bore? what kind of pistons are they.
    tops of your pistons from the little i can see look a little dry and not much wash

  4. #4
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    Quote Originally Posted by winterstick View Post
    is that a big bore? what kind of pistons are they.
    tops of your pistons from the little i can see look a little dry and not much wash
    Could be cause its a DI they run leaner,
    some top pics can help

  5. #5
    KrunchovXPL-GTX-RX's Avatar
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    Uh, that is not a DI. Though the pic does not show the whole engine a big clue is the oil lines running to the Reed Covers. A DI has three oil lines and the injector fittings are on the other side under the pipe.

    What kind of oil is neon blue?

    Ok, to answer some of your questions:

    One, there is no such thing as TOO much fuel at WOT. Well, there is, but it will not kill an engine, just make it not run as fast as it could.

    Your pistons do not look like it did not have oil (un-lubed pistons are REALLY trashed, you see).

    I see that the piston is a single ring. What kind is it? How old is that engine and what was the piston clearance when it was built?

    The odd thing is that it stuck right there at the wrist pin, rather than on the exhaust port side where you tend to get more heat. I also would have expected it to stick the rear from what you describe.

    I assume it was a fairly cool morning. You could be looking at a cold siezure if the engine did not have time to warm all the way up, or if the water was too cold, warm up at all. You can ride in cooler, even cold weather, but you have to be mindfull of the temp and take it easy. Your carbs are also leaner in cold air which can bring your EGTs way up even though the engine will be cool to the touch.

    To answer the last question, that front piston stuck pretty hard so it is trash. You will need to get that aluminum off the sleeve wall too.

  6. #6
    SURFnTURF's Avatar
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    Di engines don't have studs so very easy to tell the difference , They look like rotax pistons , mrc makes the as well but they don't look familiar. I think your clearances are off,can you trust the machinist that machined the bores ? and with what clearance's ?

    You have had water through that engine previous to this judging by the crank and rod colour.Maybe not much but some at some point.

    Cant tell from pictures but does the top of the pistons look washed or very clean ?Maybe a head gasket leak ?

  7. #7
    KrunchovXPL-GTX-RX's Avatar
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    The Rotax single ring pistons I have run across have the skirt cut back on the side facing the carbs.

    Forgot about the DIs not having studs, oops.

  8. #8
    Bay Rat cvbdrew's Avatar
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    First off... It is NOT a DI. Thank you to those who cleared that up... I should have pointed that out first.

    The story behind that motor: I bought the ski with a rod punched through the original engine. It was ruined. I found a 951 on pwctoday.com from a member that ran it in his stand up. He claimed to have gotten it from a GSX that caught on fire... hence the dis-coloration of the motor. He had it rebuilt and put 10 hours on it in his stand up. I don't know the piston make. I don't know who built it. I don't know the tolerances. I would prefer to go back to the 2 ring pistons during rebuild. I noticed this morning that there is some burn marks in the front cylinder as well. The blue oil is cheap shit from Pep Boys that I'll never buy again. Tried to save a buck and it cost me in the long run. I should note that the engine did NOT seize. I could still turn it by hand. It would fire but only for a few seconds, so I knew something was wrong. The morning it stopped was about 60 degrees. No, I didn't do a compression test first. I'm perfectly comfortable rebuilding this engine. I didn't even miss a day of jet skiing because I have another machine. Living on the water, I'm out there every day. However, this is my nicest ski so I'd like to fix it asap.

    Knowing now that I need to do a complete rebuild now, I'm concerned with clearances and tolerances.... something I'm not good at. If I get the cylinders honed to clean them up, I can only assume it'll change the size rings I should purchase. Which leads to my main question... What pistons and rings should I purchase... brand? Where's a good place to buy them?

    Thanks for all the help fellas! I'm leaving this project for a few days. It's finally 80 here in upstate NY and I'm going out on the boat!

    -Drew

  9. #9
    KrunchovXPL-GTX-RX's Avatar
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    Quote Originally Posted by cvbdrew View Post
    First off... It is NOT a DI. Thank you to those who cleared that up... I should have pointed that out first.

    The story behind that motor: I bought the ski with a rod punched through the original engine. It was ruined. I found a 951 on pwctoday.com from a member that ran it in his stand up. He claimed to have gotten it from a GSX that caught on fire... hence the dis-coloration of the motor. He had it rebuilt and put 10 hours on it in his stand up. I don't know the piston make. I don't know who built it. I don't know the tolerances. I would prefer to go back to the 2 ring pistons during rebuild. I noticed this morning that there is some burn marks in the front cylinder as well. The blue oil is cheap shit from Pep Boys that I'll never buy again. Tried to save a buck and it cost me in the long run. I should note that the engine did NOT seize. I could still turn it by hand. It would fire but only for a few seconds, so I knew something was wrong. The morning it stopped was about 60 degrees. No, I didn't do a compression test first. I'm perfectly comfortable rebuilding this engine. I didn't even miss a day of jet skiing because I have another machine. Living on the water, I'm out there every day. However, this is my nicest ski so I'd like to fix it asap.

    Knowing now that I need to do a complete rebuild now, I'm concerned with clearances and tolerances.... something I'm not good at. If I get the cylinders honed to clean them up, I can only assume it'll change the size rings I should purchase. Which leads to my main question... What pistons and rings should I purchase... brand? Where's a good place to buy them?

    Thanks for all the help fellas! I'm leaving this project for a few days. It's finally 80 here in upstate NY and I'm going out on the boat!

    -Drew
    Drew,

    That front piston seized, you can see the marks there beside the wrist pin and it will have corresponding marks on the cylinder wall. When we talk about a seizure it is in reference to a point in time that the piston became so hot that it swelled up and literally got stuck inside the bore. The actual point of seizure does not normally last very long and the engine may even keep running after a mild one. If it shut the engine off it was not mild. Once one has seized it will then continue to do so at lower and lower temps until enough damage is done to make it not run anymore (said damage can be VERY dramatic).

    Once the piston cools back off it will then rotate and may even run more or less normally. The problem is if you seize (or stick one) hard enough you will bend the rod. Once a rod loses its structural integrity it is just a matter of time before it lets go (folds up/breaks, or both).

    I only use Rotax pistons in the engines I work on. Bad news is they only come in standard and .25mm over (1st over). If the bore is too big you have to re-sleeve it. You will have to have the bore measured to even have an idea what size piston would be required (the piston determines the rings, you see). It was probably bored already so who knows where it is now (though if you clean off the top of your pistons you may find a clue there). Once you know how big those sleeves need to be to clean up you can decide on a piston that comes in the size you need and THEN you can ask what clearence to run them at since not all piston types/brands are set up the same. Having fun yet?

    Take a VERY close look at that crank, mainly the rods. If there is ANY red coloration in the rods or they are not perfectly straight you should replace that crank.

    Right now you have an unfortunate failure of an engine that, well, to be honest, I would never have put in a boat without taking it apart. I suppose I have trust issues but then it did croak, after all... Sleeves, pistons, cranks, can be bought and the prices are not too outrageous. Stick a rod through the cases on a 951 and you will know true sorrow.

  10. #10
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    damn I feel like an idiot, I assumed it was a DI cause I believe they made a carb and a DI that year

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