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650 -> 820cc conversion

Discussion in 'XJ Technical Chat' started by dkavanagh, May 13, 2020.

  1. MrSeca

    MrSeca Active Member

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    I will. I have a starter clutch issue. It slips ONLY after the bike has warmed up. Being that I just acquired it, I'm going to ride it for a year and enjoy it before I perform open hear surgery. Is there anything in particular that stood out about the XJ motor that I should be aware of?
     
  2. Mezzmo

    Mezzmo Active Member

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    Not really. Check my full build video to get some ideas. Only thing I'd recommend is replacing the primary chain and guide if you're going for a full build. Other than that a good carby clean and you should be good.
     
  3. MrSeca

    MrSeca Active Member

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    Great vid. I saw it a couple of times. How long did the actual engine build take you?
     
  4. Mezzmo

    Mezzmo Active Member

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    The engine assembly in that video took me one night, around 6 or 7 hours.
     
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  5. nablats

    nablats Member

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    3 things have got my build back on track - this forum, a covid-assisted early retirement, and the Mezzmo outstanding xj build came at just the right time.....
     
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  6. Dalpha22

    Dalpha22 Member

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    Using the compression ratio, bore, and stroke of a 650, I calculated an estimated combustion chamber volume. Then used that volume with the 750 stroke and @Mezzmo 's bore of 68 and it looks like his hybrid engine should have a compression ratio of around 11 (could be wrong without knowing piston dimensions). Mezzmo, do you need to use higher octane fuel in your build? Also, have you measured the power output of your engine? I would be curious to see how much more power per cc you are getting with that higher compression.
     
  7. Simmy

    Simmy Well-Known Member

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    @Mezzmo may chime in but I believe he used the 750 barrels & head.
    Aftermarket big bore piston kits are typically engineered to account for the increase in compression, meaning the piston deck height is less.
     
    Last edited: Apr 6, 2021
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  8. Mezzmo

    Mezzmo Active Member

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    Hi guys, I use standard unleaded fuel. I used the 750 barrels and head. The pistons are built to big bore a 750. I tested compression at the first start and it was around 170psi.
    Definitely more power than before but not measured so don’t know for sure. Still need to do some fine tuning on the carbs to get mid range right.
     
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  9. Dalpha22

    Dalpha22 Member

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    I am considering trying something similar on my 700. Did you see any significant differences between the 650 and 750 barrels or head?
     
  10. Mezzmo

    Mezzmo Active Member

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    650 and 750 barrels will interchange on the engine casings. But the barrels are different between YICS and Non-YICS heads. 750 head has a different bolt pattern for the carby boots and different cams. Other than that pretty much the same. Of course there's also the YICS and Non YICS heads.
    I started with an XJ650 non yics and converted it to an 820 with YICS head. Not sure about the 700 and if its the same as the 650 or 750....
     
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  11. Dalpha22

    Dalpha22 Member

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    I watched your build and that was some really solid work with a beautiful result. You really put some impressive ingenuity, detail work, and a lot of elbow grease into that! I love the way the xj900 wheels look on your 650. Everything all painted and polished looks beautiful.
     
  12. Mezzmo

    Mezzmo Active Member

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    Thanks mate, I appreciate the feedback
     
  13. Strannik191

    Strannik191 New Member

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    Hi everyone. Five years ago I converted 750 cc to 819 cc by installing the Japanese repair kit with big bore pistons. This repair kit was rather cheap (about 160 bucks) and consisted of: pistons, piston rings, piston pins, circlips and head gasket.
     

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