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Opinions on 750 to 900 conversion

Discussion in 'XJ Modifications' started by firefyter911mi, Nov 6, 2012.

  1. firefyter911mi

    firefyter911mi New Member

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    I have a few questions regarding the 750 to 900 internals swap. Can you keep the 750 cylinders and bore them out or is it better to just use the 900 cylinders? Would the 900 head bolt to 750 cylinders? Would there be any benefits to just keeping the 750 head and porting it? I plan to make a true 900 not just a big bore kit to 818cc. I will be building over winter full bob and stretch. Just looking for more power n space as i am 277lbs and 6'5. I have other bikes but i love this 83XJ750mk she is my fun bike. Thanks for the input guys
     
  2. MiGhost

    MiGhost Well-Known Member

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    The fun of building a 900/750 engine.

    To use the 900 top end with the 750 bottom end. You will need to use the crank, rods, pistons, cylinders, and head from the 900. Do not try getting away with just boring out the cylinders to fit the 900 pistons. This will leave the cylinder sleeves far to thin to be reliable. The sleeves will heat up, and crack leaving the cylinders worthless.

    The 900 head will bolt onto the 750 block, but the compression chamber is sized for the 900 piston. This will leave a gap around the cylinder that will expose the head gasket to a greater potential for getting blown out between the cylinders. Just in case you are thinking about using the 900 cams in the 750 head. It will not work. The 900 head was redesigned to add an addition bearing surface on each end of the cams.

    Porting the heads on these bikes is more effort than it will be worth for the minor gains that it will give. These heads are designed very well, and fall just shy of being a racing head. That said. Cleaning up any casting flash is always a benefit.

    While you are inside the motor. Take the time to upgrade the starter clutch with the 900 springs, and rollers. A 900 clutch assembly would be recommended also. It uses thicker plates. Plus an additional friction plate. 8 instead of the normal 7. You will also need to use the 900 clutch side cover.

    I am curious as to why you want more power. I have the same bike with the full tour package. Fully loaded, two up hauling 350+, and has more than enough power to try to walk out from under me when under heavy throttle.

    Ghost
     
  3. MiGhost

    MiGhost Well-Known Member

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    One other thing.
    As you are doing a bob on the bike.
    Let me know what parts you would be getting rid of. I am looking for some to restore my bike.

    Ghost
     
  4. interstatejoe

    interstatejoe Member

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    More power = more fun
     
  5. firefyter911mi

    firefyter911mi New Member

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    Thanks for all the info! Clears up a lot of ideas in my head, im always brainstorming lol. I love the bike but ive always liked the feel of more motor plus my girl rides a zx600r and i just cant keep up when she feels like hitting the gas. I was debating the 818cc big bore kit but feel 900 is the better option what do you think? Yeah depending on what parts you need let me know if they're off the bike id be more than willing to deal. Where abouts in Mi are you? SE Michigan here
     
  6. firefyter911mi

    firefyter911mi New Member

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    Interstate hit the nail on the head lol
     
  7. firefyter911mi

    firefyter911mi New Member

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    Also would the 900 middle gear give me lower cruising rpm say round 70-80mph? And if so is it significant enough to warrant a swap?
     
  8. MiGhost

    MiGhost Well-Known Member

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    Despite popular misconception. There will be no cruise rpm benefit to using the 900 middle drive. The gearing is the same as the 750.

    Trying to make the midnight keep up with that ninja is going to become very expensive. very quickly. A 30 year old cruiser will never match a modern sport bike for speed.

    Will PM you on the parts.

    Ghost
     
  9. RickCoMatic

    RickCoMatic Well-Known Member

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    When that trusty-ole 900 reaches the Red Line; those modern Plastic-wrapped Bullets are still "In the Black" with room to spare.

    A superbly tuned 900 will clock a-buck-twenty-five or thirty.
    A stock SuperBike will get 165+ with ease. 175-180 tweaked.
     
  10. firefyter911mi

    firefyter911mi New Member

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    lol thanks guys so then ill prob just build a 900 on the side for fun n keep the 750 running strong
     
  11. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    NOW you got your head on straight!!!! LOL

    Dave Fox
     
  12. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    What I got a kick out of was the desire to "hop up" the 750 so it could keep up with a ZX600R. As in, 600cc. Chain driven, isn't it? And well under 500lb, I bet.

    Hmmm....

    nevermind
     
  13. worlds-fastest-xj-750

    worlds-fastest-xj-750 Member

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    I know im new to the site, but if speed is want u want u can get it with bolt on and tuning. I have an 82 seca 750, and i am 240 and 6'4 and i can hit 135 sometimes 140 if i ahve a strong enough tail wind going down hill. And all i did was a 4into1 vance and hines pro ssport pipe and opened up the airbox with pods, and rejetted it with 140 mains only. :lol: oh almost forgot i trimmed some fat off the bike, not me lol i like my jumk food. Itook the center stand off thats like 10lbs the loss of 1 muffler pipe obviously from the exhaust upgrade thats like 3lbs, ut the rear fender excess off where the plate goes (only a few ounces) im gunna shave down the seat eventually, took off those big ugly dolphin nose fairngs with the windshield. And thats about it.
     
  14. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    So in reality, you didn't get it with bolt-ons, you got it with bolt-OFF's.

    dave
     
  15. worlds-fastest-xj-750

    worlds-fastest-xj-750 Member

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    Rotfl yea pretty much. Im not gunna stop there My goal is to get the seca to do 150 steadily. So im just not sure where to go from here. Prolly a bore, mabe cut the valve seats, mill the head. Idk but i was told these bikes have steel crankshafts so if they do that means they can handle the combustion pressure increas with a bore job. Just sux that u cant change the gearing :(
     
  16. worlds-fastest-xj-750

    worlds-fastest-xj-750 Member

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    Thats aweful why would anybody swap a 900 top end , cuz i was thinking bout it to. My 750 does a buck thirtyfive and stock its 1/4 time is 12.3 at 106 86 hp stock and top speed 126 with a jockey riding it.
     

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