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highly modded 650 Seca

Discussion in 'XJ Modifications' started by Simmy, Mar 25, 2016.

  1. Simmy

    Simmy Well-Known Member

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    Hello to this forum. This is my 1st post here although I used to be subscribed to an XJ Forum back in the 90's, not sure if this has evolved from that. Here are some pics of my highly modified 650 Seca which I now call a Seca-X. I've been riding it in this final configuration since about 2001 and it has been solidly dependable. It has an XJ750-X engine with the XJ650 Seca gear change mechanism to keep the gear change appropriate for the Seca's more rear mounted footpegs. Cylinder block and pistons are 87/88 FZR1000 which combined with the X750-X stroke results in a displacement of 911cc. Front forks and wheel are FZ600, rear wheel from XJ700 Maxim. I can answer any questions if someone is contemplating similar mods. P1030797web.jpg P1030798web.jpg P1030799web.jpg Dave's Collection 2012 frontweb.jpg Dave's Collection 2012 rearweb.jpg
     
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  2. Simmy

    Simmy Well-Known Member

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    a few more pics P1030800web.jpg P1030801web.jpg P1030803web.jpg P1030804web.jpg
     

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  3. a100man

    a100man Well-Known Member

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    Love it. Well, perhaps not the colour.;). I have an XJ550 with a Radian engine installed and I may end up building another 550 with a 600 'top end'. Also have a standard-is FZ750 so I know that engine a bit which is superb.

    Thanks for sharing..
     
  4. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    Drooling.....

    You are the second person that I know of who has transplanted a Genesis engine into a non-X frame (the other was a transplant into an airhead Maxim frame).
    I am curious about the second radiator between the bike and the hack.
     
  5. Simmy

    Simmy Well-Known Member

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    The main rad is from a 750 Sabre. I added the 2nd rad mainly to add a fan. It used to boil over when caught in traffic so I added the 2nd unit with an FZR1000 fan. I have a manual switch if I've been sitting awhile but really with the extra volume of coolant the fan is rarely needed. The frame rails coming down in front of the motor are too closely spaced to fit a fan as that was never an intention on the XJ650. The colour is from a 55 Ford T-Bird, not to everyone's taste but you can't miss it.
     
  6. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    It's such a good looking machine that I will forgive the use of a Honda radiator :)
     
  7. lostboy

    lostboy Active Member

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    HPIM0564.jpg HPIM0565.jpg HPIM0562.jpg Here is some more pictures.
     

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  8. Alan63

    Alan63 Active Member

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  9. Simmy

    Simmy Well-Known Member

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    Hey that's me at the Paris Vintage Rally! Thanks for posting this Lostboy.
     
  10. lostboy

    lostboy Active Member

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    If you look in the first picture the white Seca beside yours is my bike.
     
  11. Simmy

    Simmy Well-Known Member

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    Increasing my 750-X motor to 911cc's I always suspected could use larger carbs, so last year I acquired a set of 900 Seca carbs. They are 35 mm versus 34 mm for the 750-X. I cleaned them up, replaced the fuel stop plungers and installed a commercially available Stage-3 jet kit. The only problem I expected to encounter was the insertion of the larger carb bodies into the Maxim-X carb boots. This year I finally got around to this project only to encounter an abrupt dead-end. The addition of a water pump on the X necessitates longer carb boots than the air-cooled XJ's, there is no other donor for carb boots, however that was not the problem. It turns out the Maxim-X has custom Mikuni carb bodies made shorter to clear the water pump. If you look at the pics it is obvious. Disappointed, I remounted the old 750-X carbs and played around with the adjustments. They are now bang on, the carbeuration is perfect, even the typical flat spot between 5 and 6 grand is gone, smooth acceleration all the way. Really, the larger carb might have helped the top end but for sidecar duty the smaller ones probably deliver better bottom to mid torque anyway. I would say this bike now accelerates as smoothly or better than some OEM set ups. I never did find a commercially available stage-3 jet kit for the Maxim-X but my current set up with help from a local dyno tuner is spot on. 900 Seca carbs.jpg 900 Seca carbs.jpg 750X MaximX Carbs.jpg 750X MaximX Carbs.jpg 900 Seca vs 750X carbs.jpg
     
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  12. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Nice ride!

    DETAILS on the engine mods, engine mounting issues, and front fork conversions would be great. LOTS of details............ :)
     
  13. Simmy

    Simmy Well-Known Member

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    engine mods;
    to fit the 87/88 FZR1000 cylinder block I had to carve the 750-X lower cases larger to fit the cylinder sleeves as they stick into the lower cases.
    I went from 68 mm bore to 75 mm!
    The water pipe mount between cylinders 2&3 is different on the FZR vs 750-X. The FZR block was welded up, machined down and new threads tapped to accept the water pipe.
    I trial fit the motor with piston 1 at top dead centre, greasing the rings and filling the combustion chamber with WD40. Adding the amount of WD40 used to the swept volume of the stroke & bore I was able to determine the compression ratio. I then machined what I needed off the FZR1000 block to bring the CR back to 11:1. The FZR 1000 pistons were actually shorter than the 750-X pistons. Note, this conversion applies to the 750-X (Canadian model) and not the 700-X the USA got. The 700 had the same 68 mm bore but had a shorter throw crank, requiring even taller pistons than the 750-X. If I remember correctly the 700-X had only a 48 mm stroke so combining it with the FZR1000 piston might require more shaved off the block, at some point you might need to worry about cam chain tension.
    So my larger 911-X is actually physically smaller than the 750-X.
    The 650 Seca gear change mechanism bolted right on to get the gear change to match the rear set pegs of the Seca.
    The engine mounts at the rear of the motor required 10 minutes with a file to fit. The front mounts fit.
    The base of the gas tank had to be pushed in to clear the taller valve cover.
    TCI unit from the X installed.
    Cam sprockets slotted and intake and exhaust cams positioned for best mid-range.
    There you have it!
    The front forks, wheel, calipers and fender are right out of the YX/FZ600 and slip right into the Seca triple tree.
     
  14. Brian MacRae

    Brian MacRae New Member

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  15. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    His came that way from Yamaha. There are threads on here about changing to a round headlight. Do a search .
     
  16. a100man

    a100man Well-Known Member

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    I'm curious. Did you ever try the standard 750 motor and decide not enough power? I'm massively impressed by my FZ750, but then again I'm not hauling a chair..

    J
     
  17. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    This bit of your question confuses me. The Maxim-X is essentially a FZ750 top-end on XJ900 cases.
    Are you asking if he considered using the airhead XJ750 engine, or the FZ750 engine?
     
  18. a100man

    a100man Well-Known Member

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    My bad. thought thought the X engine was the same as an FZ750, which to me is pretty racey (Maxims are import only here).. I wasn't thinking of the implications of shaft drive etc..
     
  19. Simmy

    Simmy Well-Known Member

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    as k-moe stated, The Maxim-X is essentially a FZ750 top-end on XJ900 cases.

    I've thought about playing with FZ or FZR camshafts, but if there is a difference I probably already have the best cams in the Maxim-X for sidecar duty anyway.
    I'm stuck with the X head as the FZ/FZRs had the inclined motor and downdraft carbs. I haven't done anything to my head, the shape of the combustion chamber is not ideal as there is a step where the 75 mm diameter block meets the 68 mm diameter combustion chamber. This could have been smoothed out with a Dremel.
    As I learned this spring I'm also stuck with the X carbs, however really happy the way it's currently running.
    I put the sidecar on a standard 650 Seca in 1989. In 92 I put the 750-X motor in. In 2000 I put the FZR1000 block in it.
    I used to have a Supertrap 4-1 but once I put 16" wheels front and back I lost about an inch of ground clearance and ended up smashing the pipe flat on the bottom over speed bumps. Since then I put an exhaust together with XJ750 head pipes, XJ650 turbo collector, then a custom elbow to mate the Supertrap silencer. I got at least an inch or more of clearance back.
    A lot of people ask, Seca-X, when did they make them? I'm never sure where to start.
     
  20. Quixote

    Quixote Active Member

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