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900 Engine in 750 Frame?

Discussion in 'XJ Technical Chat' started by cds1984, Jan 15, 2011.

  1. cds1984

    cds1984 Active Member

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    Hi,
    Looking at the mountings on the XJ900 compared to the XJ750 they look to be in the same sort of area but... the XJ900 has a longer stroke so... taller?

    I don't have an XJ900 engine to physically measure so I thought I'd ask this question as after a bit of research via the XJCD and the search function I can't seem to find any engine height specs or mount distances.

    Is it possible to fit an XJ900 engine into a XJ750 Seca type frame?

    If anyone knows the
    1. distance between the mount points
    2. height to the top of the valve cover from the top/front mounting hole on the engine.
    3. distance from the top/front mounting hole on the engine to the rear of the engine.
    That'd be great too!

    Thanks.
     
  2. Cooter

    Cooter Member

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    Don't have the measurements you need, but I know it will be close. I know the 750 engine has been swapped into the Seca 650, and the 900 interferes with the fuel petcock. I'm still planning on swapping the 900 into my Seca, and will do whatever is needed to the tank to make it work. I'm pretty sure the mounts are the same, but the height of the engine and the exhaust and induction will be your sticking points. The 900 engine is also geared taller, IIRC.
     
  3. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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  4. cds1984

    cds1984 Active Member

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    A brief musical interlude to be sure, thanks Fitz.

    I'm left with the feeling that it 'might' fit so I'll have to persue it further.

    Thanks for the info and Cooter... Good luck to you... We may both need it.
     
  5. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    The 4 MM extra stroke would need just 2 MM extra deck height. I've measured my 750 against the 900 and can't find any extra height.
    The valve cover seems a little bigger, and the carbs are more bulky.

    People have put the 900 innards in a 650 TURBO.
    I think it's a "go".
     
  6. Kwiski

    Kwiski Member

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    Just did one of these transplant. Put XJ 900 in 82 XJ 650 Maxim frame. Fit right in. Had to build oil plate to hold oil cooler. Had to change choke cable position. No issues with fuel tank. Even hooked up stock to air box. Still finishing the fine tune. Runs like scared rabbit!
     
  7. ol_750

    ol_750 Member

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    [quote="
    People have put the 900 innards in a 650 TURBO.
    I think it's a "go".[/quote]

    Is there a thread or info on this conversion ?
    Cheers
    Andrew B
     
  8. cutlass79500

    cutlass79500 Well-Known Member

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    Thats good to know. Altho the 900 engine is getting kind of hard to find. Did you swap the harness or did the tci plug into the harness on the 650
     
  9. Cooter

    Cooter Member

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    Thanks, Fitz!


    And a big shout out to Chacal- great to see all the work you put into not just parts, but also gathering info for all of us and archiving it. It is very much appreciated!
     
  10. RickCoMatic

    RickCoMatic Well-Known Member

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    I can't find the picture of the 750 Maxium with a 900 Engine in it.

    The Bike looked very strange.
    In order to get the Fuel Tank on without cutting-up the Tank, ... they built a "Riser" and elevated the Tank above the Main Center Frame Tube.

    The job didn't look right. Everything looked too forced rather than designed.
    But, it proved a 900 Mill could be snuggled into the 750 Max Frame and be a runner.
     
  11. cds1984

    cds1984 Active Member

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    Sounds like a real "go" now.
    So I've taken the plunge and have the motor(s) in place for fitti... no! wait I mean, I have the motor(s) ready to be looked at for repair after I weld up the holes in the frame and attend to another hundred odd things... you know the story!
    I'm looking forward to the process, although without studying the manual first, looking at the innards of the gearbox is a bit overwhelming at this point.

    This is my parts bike frame (XJ750P 1984 37H) and 2 * XJ900 Motors (one with a good top end and one with a good gear box)
    [​IMG]
    [​IMG]
     
  12. RickCoMatic

    RickCoMatic Well-Known Member

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    Is this Bike going to have a 750 Rear-end or are you going to fab-up the 900 Rear-end for it?

    Either way, ... you should consider fabricating the SwingArm to accommodate a WIDER Tyre.

    Both the 750 & 900's Swings could possibly be modified to run a wider tire.

    1.) The Shaft Tunnel's Inside Convex curvature can be removed and replaced with a Flat-sided piece of Billet, ... as close to the Drive Shaft as possible, ... leaving merely a very close tolerance to the Drive Shave, ... or Machined down it's Center Line to provide for relief to the Drive Shaft.

    2.) The "Horse Shoe" at the Pivot will need to have the Convex Inside Surface "Cut, Squared, and Reinforced with Solid Alloy or Steel (depending).

    It's a lot of work to do, ... just to gain a little bit more rubber ... But,, if you get that 900 in there, ... you are going to be happy with however many mm's of width you can get on-the-road, at the back-end.
     
  13. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    The 900 swingarm is longer (by 1 1/2" ?, 40 MM)
    This would help control wheelies,
    And you got the 891 CC engines, not the 853 ??
     
  14. cds1984

    cds1984 Active Member

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    Rick, I was going to use the 750 swing(because of the brake torsion bar) and the 900 diff as I have the 750 wheel which is drum brake and don't have either the 900 wheel or disc brake assembly/bits to suit the 900 wheel.
    I don't feel overly confident with re-engineering the swing-arm but that being said... this is going to be a long build.

    Do you think a few extra millimetres of rubber would actually make that much of a difference? Cornering?

    Time, aah well I won't be doing wheelies! I swear!
    They are both 853cc engines.
     
  15. cds1984

    cds1984 Active Member

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    Another question in the same sort of vein...
    I have the,
    XJ750 carbs - Hitachi HSC32 - rebuilt.
    XJ900 carbs - Mikuni BS35 - need major overhaul, water damage, holes in diaphragms, etc.
    FJ1200 carbs - Mikuni BS36 - look new! except holes in diaphragms.

    I can see that the XJ750 Hitachi carbs are a much smaller venturi ID of 32mm compared to 35mm/36mm ID of the Mikunis. They are spaced differently also, so the Hitachis are not suitable I'm thinking for more than one reason.

    I can see that my stock 750 airbox is going to need some sort of boot modification to suit the larger OD on the airbox side of the Mikuni (about 4mm) which is one problem but...

    The main question is based on that I'm looking at these FJ1200 carbs and since they look so good, age-wise and cosmetically, compared to the ones that came with the XJ900 motor... what sort of difference could I expect if I swapped all the jets/needles into the BS36 from the BS35 Mikunis? or would it be a complete farce to even attempt the alteration?
    Any opinion or advice welcome of course!

    Mikuni BS35 condition
    [​IMG]
    [​IMG]
    [​IMG]


    Mikuni BS36 condition
    [​IMG]
    [​IMG]
    [​IMG]


    and, just for laughs, where I'm up to so far with assembly and rebuilding...
    [​IMG]
     
  16. darkfibre

    darkfibre Member

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    Make sure your police wheel fits the 900 diff. My police wheel would not fit my Seca diff. XJ750 and XJ750p diffs were different (the one time I tried it)
     
  17. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    It's got cop tires, cop suspension, cop shocks, a 900 plant. It's a model made before catalytic converters so it'll run good on regular gas. . .

    Oh Yeah ! :twisted:
     
  18. cds1984

    cds1984 Active Member

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    Darkfibre, the XJ750P wheel fit the diff no prob... except the XJ900 Diff has a different shock mount point so, although the diff fits the XJ750P swingarm and the shaft fits the diff the mount points are offset for the shocks!
    So no go on using the XJ900 diff but it looks like the gearing is exactly the same between the 3 diffs I checked.
    XJ750Seca
    XJ750P
    XJ900RK
    and the gearing difference is in the transmission, all in all no loss so I'm sticking to using the XJ750P diff and swingarm.

    Latest pic with Swingarm, Diff and rear wheel fitted.
    [​IMG]

    So a quick question. I have these two 900 motors...
    One that was working till it blew second gear and the other with a good gear box and with the parts for replacing the gears in the first motor.

    After spending a bit of time splitting the gearbox(2nd motor) and looking at the "oil spray nozzle" I suspect the HYVO chain is stretched but in looking at the same components in the other motor the HYVO chain has caused just as much damage to the "oil spray nozzle".

    2nd Motor oil spray nozzle
    [​IMG]

    Main Motor oil spray nozzle
    [​IMG]

    The 2nd motor has an intact HYVO chain guide, although worn, and the Main motor has a broken HYVO chain guide...

    I thought these HYVO chains didn't stretch?
    They only start the bike and run the generator, don't they?

    I'm a bit puzzled but I can see Rockerto had the same worn down "oil spray nozzle" also, so is it a common problem and perhaps the source of the low idle knock, like the XJ550s even though it isn't a primary drive chain in the 750 or 900?


    On a side-note,
    I guess it does pay to have engine guards, I wonder if this had anything to do with the second motor being scrapped... Try getting that to idle!


    [​IMG]

    [​IMG]
     
  19. darkfibre

    darkfibre Member

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    Houston we have a problem... that dowel hole in the crank is probably not worth trying to fix :(

    As for the drive chain, on my rebuild I decided to replace it anyway along with the guide.

    When assessing the pistons and bores for reuse, be critical of ring land wear.
    Evidently in hot climates bikes left idling (police bikes especially) had some extreme temp cycles leading to ring groove wear. Too much and you may get high oil consumption.
     
  20. cds1984

    cds1984 Active Member

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    I decided to shuffle the good chain guide to the main 900 motor and give the oil spray nozzle a bit of a tweak to make it actually squirt oil at the starter clutch again (filing and drilling was undertaken.)

    I was told the top end of the main motor was worked on just before the 2nd gear started playing up and I plan to find out what that means.

    The 2nd gear fault seams to be shift fork 2 and 3 being either bent or mis-aligned but the gear dogs looks like there in good shape, for both the motors, and I have a spare set of good looking shift forks in the 2nd motor so it's looking good.

    I figure no harm in giving it a shot, if I have to pull the motor again, then I'll just do it! So far it will cost me some sealant, a whole bunch of kerosene, some WD40, and time but I believe I have a better understanding of what is going on now inside these motors and am glad I took the time to strip the 2nd motor's lower end completely before going further.

    FYI: (The lower case on the XJ900 has 4H7 stamped on it. I'm starting to see a pattern here.)
     

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