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  1. #1

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    1994 650 and 1996 780 projects...

    OK. I'm brand new to PWC's. Bought a couple jet ski's after talking about it for a week, found a pair on craigslist with a trailer for what i thought was reasonable. Neither had been run for several years. I immediately found Greenhulk and read page after page of threads and sticky's pertaining to my ski's.

    After much more detailed inspection both are pretty much junk. Pulled both motors. The 650 looks like it's rebuildable.....top end work only, so far......

    The 780 broke the Mag rod, chipped the sleeve and broke a silver dollar size piece out of the block on the exhaust side. I'd guess it stopped running pretty violently.

    I'm trying to salvage what I can and wondered if it were possible to swap the bottom case/block with the 780 top end after I get the jugs honed and new pistons and get ONE running ski out of this deal.

    Will post pics of both tonight or tomorrow. All input is appreciated and welcome.

    Thanks in advance:


  2. #2

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  3. #3

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  4. #4

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    I posted the same picture twice in the first set, but both the 650 and 780 look nearly identical with regard to condition. After posting the initial post last night I took a closer look at several of the other 650 780 project posts and it sure looks like I am able to use the 780 jugs and the 650 block/case together. I have a "friend" that works for Western Powersports and has been around two-stroke engines his entire life-maybe he knows for sure; I'll need to see if I can reach him.

    I also read quite a bit (at least the posts on here) about rebuilt engines and there clearly was not a definitive answer about where to purchase a rebuilt engine. I know folks don't want to suggest a company and then have a bad experience with that company, which may reflect poorly on their reputation around here, but with all of the members of Greenhulk, there sure was a lack a participation on that subject-good or bad.

    In the first set of photos pictures 5 and 6 (left to right and down) are the pictures of the 650 engine. I checked to see if it would turn by hand before I took it out and it did roll over pretty easily. When I was checking the Crank Phase/Index I found a big divet in the top of the MAG piston so I began taking all of the cylinders off and found that all of the Pistons and Rings are shot and the Cylinders were equally bad. It looks like this one has been rebuilt once before already since the Mag and PTO were both stamped 25 and the CEN was stamped 50. Of the three cylinders the CEN was in the best condition.There was no other obvious damage to the engine and that's why I started on this one first. What I did note is that there was about 1/2 a gallon of oil in the case when I took this one out of the ski since I had the cylinders off to inspect the top of the pistons, it ran out all over the place. I spent the next 20 minutes upside down in the ski with rags and a bucket soaking up the gas/oil mix. It was nasty.

    The Second set of photos are of the 780 engine. The damage here is pretty obvious. I checked this one to see if it would crank by hand and it wouldn't, so I put a battery on it. It gave a big THUD and arced a little but no movement, so I disconnected the battery and began tearing this one down. The MAG was the siezed cylinder with big chunks of piston stuck to the top of the cylinder head and the obvious chunk out of the block. After I took the cylinder off, I had to pry the piston out of the block since the rod had broken and was wedged into the cam. In the one photo you can see the end of the rod is still stuck in the cam, so I'm assuming that the crank/cam is twisted. I'm not sure how to check for sure since the normal 120 degree's can't be checked. Suggestions are welcome on that. Right now I'm assuming the crank/cam is twisted and since the bottom had a huge chunk out of it, the entire bottom is trashed. I noted above that the 650 had a ton of oil/gas in the bottom when I tore it down; the 780 hadn't a single drop of oil in it. NOTHING. Could it be someone tore this down and saw the damage and then put it back together? I pulled the engine mounting plate (sled???) off and there is no damage at all to the bottom of the case, so it didn't run out AND the inside of the ski was all but dry, albeit a little nasty/dirty, but no oil.

    So, as I posted last night......Is it possible to combine the good 650 bottom and the 780 cylinders and pistons to make a good running ski? I would be putting the engine back in the 780 since it is two years newer and has the trim option.

    Suggestions and Comments are welcome.

  5. #5

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    I thought I'd add a few more pictures. Also there are a few items that I have questions about. The 650 engine looks like (from what i've read on here) that it has been converted to the Pre-Mix, based on the picture that shows the silver plate on the oil pump. ???? Also the PTO's are a little different between the 650 and 780.Click image for larger version. 

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    This is the MAG cylinder from the 780.

  6. #6

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ID:	276694 780 Carbs are painted BLACK
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ID:	276695 650 Carbs are painted BLUE

    Is there a difference? Have the 780's been rebuilt? Remanufactured? They are the only items on the entire engine that are black.
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    Here is the oil pump on the 650. Looks like what I picture in my head about blocking off the oil tank. IDK.
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    650 PTO
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    780 PTO

    Ideas? Facts?

  7. #7

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    It seems like I'm at the auction.......everyone's looking, but no one's buying!!!!!

    I would have thought with all of the knowledge here I would have had multiple responses by now.....maybe tomorrow when everyone gets back to work and has time to cruise the internet on company time!!!!!!

  8. #8
    I'm addicted to Polaris PWC ghostinstallations's Avatar
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    Ill try to answer some questions for you. Not in any particular order.

    -Yes, bottom ends can be swapped.
    -780 carbs are painted black from the factory, are oriented 180 degrees from 650/750's and have different jetting.
    -Reman Engine Info. I wont recommend a place, but I will say I jst purchased a reman from SBT and have only run it for an hour so far without issue. They will charge for damaged aluminium cases/jugs. With the kind of damage you have, I think you need to source a used motor instead of reman unless you can piece together a 780 with no broken aluminum.
    -Couplers are different but interchangeable. The larger is a rubber dampened coupler that was on the newer engines for less vibration.

    Not sure I understand your oil blockoff question. That is in fact the oil pump. When you no longer wish to use it, you pull the pump and internal gear and replace with a blockoff plate. They are reliable pumps and there is no reason to get rid of them. Premix is a pain. If that pump has been spinning with no oil line going to it, I would no longer trust it.


    Let me know if you have more questions. I love that you posted pic's. We love em. Sorry if I did not answer all your questions, just kind of hard to follow all your info.

    -Brad

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  10. #9
    BlueFishCrisis's Avatar
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    The 780 carbs are not that different from the 650s. They are installed 180 degrees different as Ghost mentioned, so you will need to select the correct intake manifold based on the carbs you go with. If you choose the 780 carbs, the throttle and choke connections will be towards the back of the ski while the 650 will be towards the front. The 780 carbs also have a built in in-line restrictor at the last return nipple, so you don't need one in the fuel line. Also, the jetting is much different form a 650 to a 750 so you will need to open the carbs up. A rebuild is always strongly suggested for the carbs as well.

    We try to help as much as possible, but future suggestion........ more posts, with less questions each, is better than a long post, with multiple questions....

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  12. #10

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    Thanks Ghost and BlueWater! BlueWater-I'm originally from Ohio, down around Tuscarawas County in a little town off I-77 called Dover. GO BUCKS!! OH-IO!

    I do appreciate your input on what I have going right now. Like I said in my initial post I'm brand new to the jet ski world and man have I stepped into it!

    So based on what I have read thus far in all of the threads on Greenhulk, I think that I should try to get the top end going to the machine shop and get her all back together. In the mean time, I'll be looking for the new-to-me or rebuilt 780 engine.

    I tested both of the stators and all checked out good based on the specs I found on the "most useful link in the world" on the Polaris PWC Knowledge Page. The 650 electrical box was totally dismantled when I bought it and after the "closer inspection" that I talked about found that the coil, regulator and CDI box are all missing from it.

    The 780 on the other hand had all of the aforementioned "stuff" intact in the box. My guess is that since the 650 only had top end damage with the burned pistons, etc, that the problem was in the Carb Adjustment and/or electrical since the box was torn apart and the top end burned up. What say ye? (or like we say down here, whatch-y'all think?)

    The 780 had all of the electrical intact (as far as I can tell) and there is fatal engine damage, so.......I should be able to get a new 780 motor, rebuild the carbs and plug-n-play her hopefully. What say ye?

    Anything in specific that I need to be looking for in getting the 650 jugs honed and the new pistons?

    Ghost, thanks for the tip on SBT, I've been looking at them for a couple of days now and I knew about that $250 welding fee. I located a bottom case on e-bay for $75.00 which apparently they'll consider as the bottom for my engine. If/when I send my engine in, i'll toss that case in the box too. That will offset my cost a touch.

    Thanks again for your insight!

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