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  1. #21
    I dream skis E350's Avatar
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    Re: Blown piston need diagnostic help

    My air leak was on front crank seal. But apparently the back (coupler) side crank seal often leaks. Your piston with the hole is the back cylinder, correct?

    Also do you have two carburetors or one carburetor?
    Last edited by E350; 10-23-2019 at 11:27 AM.
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  2. #22
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    Re: Blown piston need diagnostic help

    Yes was in the back and I have one carb

  3. #23
    I dream skis E350's Avatar
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    Re: Blown piston need diagnostic help

    Crank seals, at least on the JF650 are not hard to replace. If you have compressed air and something like the Astro Nano 1822 1/2" impact wrench (~$99), you just need to put the head on, screw the spark plugs in and hit the coupler with the appropriate coupler remover tool. Yes I bought the 650 the coupler removal tool on ebay. There is a tool expense if you want to work on these things yourself.

    But as others have said above, put the head on and screw in the spark plugs and do at least a simple pressure test first. Others will correct me, but:

    1. Put the head back on and screw in the spark plugs;
    2. Take the flywheel cover off;
    3. Take the flywheel off (you will need a tool to hold the flywheel, Cold Fusion's are great if they have one for your engine). Don't lose the Woodruff key;
    4. Take the stator off and move to the side.
    5. Go to Ace Hardware and buy two sheets of red gasket material in the plumbing section. (about 6"x6");
    6. Take the exhaust manifold off, lay it on one sheet of gasket material and mark the bolt holes on the gasket material
    7. Do the same for the intake manifold;
    8. Drill clean holes in the gasket material where marked;
    9. Bolt the gasket material between the exhaust manifold and the case and torque as specified (this is per @myself);
    10. Bolt the gasket material between the intake manifold and the case and torque as specified;
    11. Pressure the crank case to no more than 7 lbs. through the pulse line;
    12. Make dishwashing detergent and water solution in a spray bottle;
    13. Spray the gauge and where it connects to your pump and to the pulse line to look for leaks;
    14. Spray seals and every other joint on your engine, head, spark plug holes, gasket to case on both intake and exhaust manifolds;
    15. Look for bubbles and post up pictures here.

    This is not a "leak down" test. This is just a simple "leak out" or pressure test.

    And a lean condition is caused by air being sucked "into" the engine. That requires a vacuum test which is a little more complicated.

    But if I understood @wmazz correctly, you can have a vacuum leak without a pressure leak, but if you have a pressure leak, you likely also have a vacuum leak.
    Last edited by E350; 10-23-2019 at 01:20 PM.
    Learning Slowly...

  4. #24
    PWCToday.com Is My Home Away From Home Chester's Avatar
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    Re: Blown piston need diagnostic help

    A couple of things. Never, ever just do a top end. Did you replace the crank seals? Did you pressure test the engine after the first rebuild? Also, switching to premix or changing the mix ratio on a completely stock engine will have absolutely no detrimental effects on the engine. There is no way it will get lean enough to melt pistons. It's only a consideration on a performance engine with modifications.

    Chester
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  5. #25
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    Re: Blown piston need diagnostic help

    Lot being discussed here but when most people talk lean or rich they are are talking about fuel vs air. As already stated in here if the oil must pass through the jet it is taking up room that was previously occupied by the fuel. While I am sure the fuel is slightly thicker, that is not what I am referring to. As WFO ways it boils down to a fixed volume of fluid through a fixed orifice. If the fluid was 100 percent of fuel and now is comprised of some percentage of oil, it is now proportionally b leaner than before. I would agree it is generally not enough to melt the piston. But if the engine is borderline, maybe. I have never melted a piston and sometimes I wonder how people can can get the engine to run so lean to do it. There can be other things that can lead to elevated crown temps, non of which I think are a play here, but timing, plugs proximity to the crown, single ring pistons, poor combustion chamber, etc.

    Does oil increase octane? I believe this is against conventional wisdom, but I admit I have never explored it with respect to two stokes. I do remember reading somewhere over the years there were some assertions that high oil ratios made more power. I don't think they narrowed this down to friction or better burn, but they were air cooled if I remember. Getting you crankcase oil in the combustion chamber in a 4 stroke will induce detonation. There is certainly a big difference in the oils. Even if it Does increase octane, it would do nothing for flame front propigation. Very important in two strokes. It is possible to burn more fuel and elevate egts while maintaining or lowering engine temp. In fact this is desirable. It means your combustion chamber is more efficient. There is more to most engine building than peak hp. Efficency, throttle response, and broad torque make very good engine characteristics. Most of my two stroke i think would be next to impossible to burn up the piston in. They simply don,t run when that lean. This is also why it is important to know which side of lean you are On when monitoring egts. Most engines will have both a hot a cold side of lean.

    Does two stroke oil burn? Yes if your combustion chamber is set up good. If you have a Lot of spooge in the exhaust, that is unburnt oil. My big bore sled is injected but probably has an overall ratio of 25:1 but zero spooge. I do not run it for extra octane. It is for extra lubrication. When you are close to 200 up On a 860c.c. twin, you feel better with some extra lube. I have experience no downside to this other than up front cost.

    I do not think a higher oil ratio would help the op. He appears to be lean.

  6. #26
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    Re: Blown piston need diagnostic help

    Why one cylinder? While I don't think the engine was run long enough to provide a definitive answer, It does appear to me both are lean. When engines share common intake tract and exhaust, wierd air flow dynamics will certainly cause unequal fuel distribution. Engine orientation, pto v mag, etc. I don't own a single twin that runs the same jetting in both carbs. If you have a single carb you need to yet for the leanest cylinder. This is why I prefer individual carbration.

    Confirming the health of the engines ancillary systems when doing a rebuild. Seems like common sense but I have seen so much of this over the years. I bought a 1990 1/2 ton chevy of a guy for a 1000 bucks about 10 years ago. Had a brand new gm engine, not a rebuild. Ran like ****. Decide if was the ignition. Pulled the plugs, brand new. Hmm. Pulled the distributer. The bushings where so shot, the tone ring was nearly completely gone. Really, you didn't notice that when you put in the new engine. Sheesh.

  7. #27
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    Re: Blown piston need diagnostic help

    A 2 cylinder 2 stroke engine is like 2 individual engines put together. Even if they have a common carb and exhaust, the cylinders act as independent engines. The crankcase for each cylinder is isolated from the other by a seal between the cylinders. If your PTO seals are bad, that would only affect the PTO cylinder.

    If you do a pressure test, you will be able to either rule the seals out or they will be bad and need to be replaced.
    2, 1998 TS 770 L, restored and running great as of July 2019 (NO THANKS to the person at SBT, who put the WRONG crankshaft bearing into the rebuilt engine I got from them)

  8. #28
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    Re: Blown piston need diagnostic help

    Yes, you are right. Air flow dynamics in the intake never result in unequal fuel distribution to individual cylinders. That is why direct injection and multi-port where never invented.

  9. #29
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    Re: Blown piston need diagnostic help

    For the OP. Since we have thoroughly polluted your thread, I will give some some more direct advice.

    As I have stated twice. It is hard to tell from the pics but you actually look lean on both cylinders. That weird gray color and tell tell pitting in the domes and piston crowns (and big hole) is from detonation due to being lean. If I were you.

    1. Take a look at the carbs. Easy enough task and a good step to do when building a new motor. If WFO is right, your ski may have Had the wrong jets installed from the factory.

    2. If that checks out. Go looking for an air leak in the bottom end. Pressure check, visual inspectio , etc.

    3. Probably need a new bottom end anyway. There has been a fair amount of small pieces of vaporized aluminum floating around in your bottom end.

    No sense in half assing it. Short cuts will only cause you more headaches in the future.

    TIP: Many two stroke cases have a couple of small ears on them. One on each case half. These are used to split the cases properly. Either buy a case spreader or make a nut and bolt that fit between then. Then use it to put pressure between the cases. Tap case line with rubber mallet. POP. Cases are split with no damage. Cool huh.

  10. #30
    PWCToday.com Is My Home Away From Home Chester's Avatar
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    Re: Blown piston need diagnostic help

    Quote Originally Posted by Jet Cowboy View Post
    Why one cylinder?...
    The hole is from detonation. It could also be caused by a twisted crank.

    Chester
    Speed is very expensive. How fast do you want to spend?

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