JS550 to 750 Swap: Project "Missile"
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  1. #1
    PWCToday Regular b00st's Avatar
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    JS550 to 750 Swap: Project "Missile"

    INTRO
    First off I have to start off by saying thanks to JSNate, I must of asked him a million questions about the swap and he replied to every one and helped me out a lot. Thanks Nate!
    Also sorry for the lack of pictures, I raced to finish this project as I had started it late March of 2018 right before the season started. You can definitely tell that I rushed through the bodywork, but it's fine considering it looked like it went through hell and back three times when I first bought it.

    BACKSTORY
    I got my '85 JS550 for $500 in June 8th 2017.

    I literally went on Craigslist and bought the cheapest stand up I could find. It was basically stock. By July 8th the engine was blown up. What had happened was I was dumb and replaced a broken ziptie on my fuel line with a hoseclamp. This caused an air leak and she went lean and it took off like a missile right across the lake as I fell off. It ended up nailing some guys homemade boat launch that was made up of a bunch of stones, go figure. It sucked up a ton of rocks and ran for about 15 more seconds before I shut it off. At that point I figured the worst, so I tore the engine apart and found out that the crank bearings had cracks in them but they didn't give out yet. It also destroyed my prop. After realizing it needed a new crank, bore, pistons, and head, I figured the 550 wasn't worth rebuilding (yet). That's when I browsed the forums and came to the conclusion a 750 is what I wanted. I previously owned a '95 750SS and loved the thing so that influenced my decision a little too.
    So what did I do? September of 2017 I hopped on Craigslist and bought the cheapest 750 I came across. For $350 I got a mint '94 small pin dual carb SSXI jetski.

    THE SUPER QUICK "BUILD"

    Like I said, I did not take a ton of photos, but I did take pictures of the problems I ran into. I wanted to give back to the forum a little for all the knowledge I have gained from this site.
    Exhaust: First problem I ran into with my swap was the exhaust. You have to run a 650 exhaust with your 750 engine in order for it to clear without chopping the whole ski up. When I bolted the 650 manifold onto the 750 engine there were two studs that were longer due to the design of the old 750 exhaust manifold:

    I solved this by buying 2 shorter exhaust studs from a 650/750.
    The other problem was getting the rest of the exhaust to clear inside the ski. In this
    pic the exhaust is touching the under side of where the hood seal sits. This is because the pipe was sitting way too high.

    Some people did not have this problem with other aftermarket 650 exhaust systems, but I was on a budget (Of course, look at the skis I bought) and went with a stock X2 650 exhaust. As a solution I had a buddy machine me a spacer so it would lower the exhaust enough to clear the hull.

    It works very well honestly and I haven't had any problems.

    Battery: Once I had the engine where I wanted it I went to put the battery in it's spot and noticed the dual carbs were right in the way of where it normally sits. To fix this I cut the rear portion of the battery box out and re-glassed over it. I then took a raiser/spacer plate that come with PWC batteries and bolted it down further back towards the "firewall" to act as a new battery tray.

    There are these really thick portions in the battery tray and that is what I used to screw the battery raiser/spacer down to. You will know what I am talking about if you were to do this yourself. I then put the straps down over the battery in its new location. This gave me enough room to clear the dual carbs. You could avoid all of this and just buy a smaller battery, but I bought this battery brand new right before I crashed and blew up my 550 so I wasn't going to go that route (You get the theme of the build now don't you ??).

    Electronics: Next was the ebox. I stuffed all the 750 electronics in my 550 ebox minus the coil. I have the coil mounted externally as seen here:

    I mounted it with long bolts and springs because I was in a rush to finish and didn't have time to make a bracket for it. I see a lot of people post about how it's bad to mount them externally and other people say it's fine. The coil for the 750 is sealed so it shouldn't be an issue, but if it ever does fail prematurely I will be sure to give an update. As a precaution to prevent water damage, I put some sealant around the wires that go into/out of the coil itself to try and water-proof it a little more.
    As soon as I had the exhaust in (this pic was prior to adding the exhaust spacer) I knew the ebox would not fix in its stock location anymore. What I did was I machined the mount that the ebox bolts to so it would move the ebox closer to the "firewall" to clear the exhaust pipe. I had to move the ebox so close to the firewall that I had to relocate the stock fuel reservoir switch, since the ebox basically sits where the switch goes on the inside of the hull now.

    As you can see the ebox clears in the picture above. It does look like its very close to the exhaust from the angle of the picture above, but I measured it being 2.5" away. I also heat wrapped the whole exhaust to prevent the ebox and hull from getting warm.

    Fuel System: Now onto some fuel related issues. My issue here was that the 750 fuel lines were a size bigger than my 550s. I researched online about this and people were saying their 750 fuel line was the same size as their 550, but this was not the case for me. (Also I am unsure as to why). What I did was I took the fuel pickup tube off my 750 jetski's fuel tank, drilled a hole in the top of my 550 fuel tank, and sealed and bolted the 750 pickup tube down on my 550 tank:

    The 750 fuel pickup tube lines were way longer than the 550's pickup tube lines and would not fit into the 550 fuel tank. I shortened the two fuel pick up lines (leaving one longer than the other to retain the reserve function) and connected the fuel filters back on to the bottom of them. After installing the 750 pickup tube, it left me with an extra hole in the tank from where the 550 pickup tube used to screw on to, so I took the gas cap off the nose and found out it screws right onto where the 550 pickup tube used to be (as pictured above). I blocked off the nose filler port on my 550 tank since I fiber glassed over the nose gas filler cap anyways so I did not need it. Now I fill my jetski by taking the hood off and putting gas in where the 550 pickup tube used to be. I forgot to add that I also used my 750 jetski's reservoir switch due to the fact the fuel lines going to it were bigger as-well. Like I said previously, I had to move the reservoir switch due to the ebox being in the way. I do not have a pic on hand but I mounted it on the fuel tank right where the two fuel lines are coming off the pickup tube in the picture above.

    Throttle Cable: Now I did not get any pictures of this, but when swapping in the 750 engine your 550 throttle cable will be way too long. I fixed this by cutting the whole cable down to the size I needed. I then went to the local bicycle shop and bought a universal cable that fit inside of the cable housing and crimped a new end on it and hooked it up.

    Hood: After I solved all the issues above I went ahead and fired it up. It actually ran the first time which surprised me. I read online that the hood wouldn't fit with the stock air intake box and sure enough it didn't.

    I really wanted to keep the stock air intake since it said "750 Dual carb" on it (and I didn't want to drop $100+ on aftermarkets) so I modded the hood to make it fit. I cut a little bit of the inside air vent out of the hood and re-glassed it in order to clear. Here is a mid-progress pic of what that looks like:

    I know I might get flamed for not only keeping the stock air intake, but also narrowing some air supply to the engine from the hood, but I can tell you the engine performs amazingly and definitely exceeded all my expectations. I don't regret doing this at all. Plus I get to pop the hood off this old ragged s**tbox and surprise people with the huge "750 dual carb" logo lol.

    Final Result
    I literally only have one "good" picture of the ski. It isn't much to show like I said previously due to how bad the body originally was when I got it, and the lack of time I had to fix it. I could of sworn I had a ton of pics of it finished but I guess I was too busy riding it. Well here it is

    Specs: Chopped rear rails, dual cooling, 550 pump w/ custom prop (19* I believe), 7 degree bars, shaved nose gas cap, open pump grate, rhaas engine conversion plate, converted to x2 side dump exhaust.
    Not a lot done to it but even with the 550 pump it will rip your arms off if you floor it from the hole. I plan on getting tubbies and a ride plate this summer of 2019.
    Other future plans include an aftermarket exhaust/intake, lightened flywheel, different head, and maybe one day put the 750 pump I have from my donor ski into it.

    Well that has been my project "Missile", if you have any additional comments/questions lemme know. Thanks!
    Last edited by b00st; 03-15-2019 at 06:07 PM.
    '85 JS550/750: Small pin/Dual carb (Finished 6-7-18)[BUILD THREAD]
    '91 300SX: 38bn carb, Westcoast flame arrester, 650 head gasket

  2. #2
    PWCToday Regular b00st's Avatar
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    Re: JS550 to 750 Swap: Project "Missile"

    *Reserved for (better) future pics, wiring diagrams, and other useful info
    '85 JS550/750: Small pin/Dual carb (Finished 6-7-18)[BUILD THREAD]
    '91 300SX: 38bn carb, Westcoast flame arrester, 650 head gasket

  3. #3
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: JS550 to 750 Swap: Project "Missile"

    i really like the stock air box because they keep the water out. if you take both of the rubber boots out of the air box and remove 2 of the 3 screens on toop of the carbs, itll flow as well as any aftermarket flame arrestor and keep all the water out. also the top is on reversed, should read twin carb 750 with the twin carb in the back

    ive got a westcoast pipe that has been modded for more power and to fit better in the 550 hull id make you a good deal on when youre ready. itll be a big jump in power over both the stock 650 pipe and even a stock westcoast pipe.
    Last edited by fastgtfairlane; 03-16-2019 at 10:27 AM.
    91 440; PJS Viper 7000, jetsport exhaust mani, factory full pipe, coffman waterbox,solas 17*, jetco worked 440 pump, ocean pro nozzles. shredmaster ocean rideplate and grate combo, wide tray, footholds, deep strake mod, driveshaft fill,650sx quick turn

    build thread part 1: http://www.pwctoday.com/showthread.php?t=331343
    build thread part 2: http://www.pwctoday.com/showthread.php?t=364872
    build thread part 3: http://www.pwctoday.com/showthread.php?t=396154

  4. #4
    PWCToday Regular b00st's Avatar
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    Re: JS550 to 750 Swap: Project "Missile"

    Quote Originally Posted by fastgtfairlane View Post
    also the top is on reversed, should read twin carb 750 with the twin carb in the back
    Thanks for pointing that out, I forgot to talk about that. I noticed it was backwards prior to me putting the rest of the ski together and I bolted it the correct way before I finished putting the ski together.
    Also I will definitely reach out to you once I'm ready to upgrade the pipe! I always hated the fact I had to go with a stock 650 pipe at the time and choke the poor 750 out. I built the ski while I was going through college so money was tight throughout the build.
    Thanks for letting me know!
    Last edited by b00st; 03-16-2019 at 12:16 PM.
    '85 JS550/750: Small pin/Dual carb (Finished 6-7-18)[BUILD THREAD]
    '91 300SX: 38bn carb, Westcoast flame arrester, 650 head gasket

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