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1981 XJ650 Jetting confusion - what to order.

Discussion in 'XJ Technical Chat' started by Dirty Harry, Apr 12, 2016.

  1. Dirty Harry

    Dirty Harry New Member

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    Hi guys, cleaning out the carbs on a project bike I picked up and found the wrong sized fuel jets. The whole bike is a messy hack job with messed wiring, bad spraypaint, wrong plugs... It's got a 107 main and a 45 pilot. The bike also has a K and N air filter with stock exhaust.

    The stock jetting is supposed to be 110 and 40 pilot. From the Church of clean thread.

    #110: all XJ650 North American models (except 1982-84 Canadian models), all 1980-81 XJ650 UK/European (4K0 and 4K1) models, and all XJ650 Police (37G) models.
    #40: all 1980-83 XJ650 USA models, 1980-81 XJ650 Canadian models, 1982 XJ650RJ and RJC Seca Canadian models, 1980-82 XJ650 UK/European (4K0) models, 1981-83 all XJ750 USA models, XJ750E-II models, and XJ750RL models.

    So I'm guessing the previous owner went in the complete wrong direction with the main jets (lean)? And possibly a little too rich with the pilot? The plugs are pretty black but I haven't been able to take it for a good drive yet. I'd like to at least get the stock 110s for it or maybe 112s for the K and N filter and then go from there. What do you all think? Or did the person go the right way and I am missing something? Thanks!
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    information overload speaks to this issue factors include altitude and filters aftermarket exhaust pods
    are you talking about stock style K&N air filter? you may want to go back to oem style.


    XJ650 Models:
    Years & Models: 1980-1 XJ650 Maxim, Midnight Maxim, and XJ650RJ Seca USA models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 4H700
    Main FUEL Jet Size: #110
    Pilot FUEL Jet Size: #40
    Main AIR Jet Size: #50
    Pilot AIR Jet Size: #195
    AIR COMPENSATOR Jet Size: #36
    Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-10
    Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
    Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
    Idle RPM's: 1,050 rpms

    Although not part of your carbs, your valve clearances should be considered as part of your "intake system", so here are those specs, too. Note that valve clearances should be measured with the engine "cold", meaning 70-degrees Fahrenheit or less:

    Valve Clearance Intake: 0.11 - 0.15mm ( = 0.004 - 0.006")
    Valve Clearance Exhaust: 0.16 - 0.20mm (= 0.006 - 0.008")

    Years & Models: 1980-1 XJ650 Maxim, Midnight Maxim, and XJ650RJ and RJC Seca Canadian models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 4H800
    Main FUEL Jet Size: #110
    Pilot FUEL Jet Size: #40
    Main AIR Jet Size: #50
    Pilot AIR Jet Size: #195
    AIR COMPENSATOR Jet Size: #36
    Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-12
    Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
    Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
    Idle RPM's: 1,050 rpms

    Although not part of your carbs, your valve clearances should be considered as part of your "intake system", so here are those specs, too. Note that valve clearances should be measured with the engine "cold", meaning 70-degrees Fahrenheit or less:

    Valve Clearance Intake: 0.11 - 0.15mm ( = 0.004 - 0.006")
    Valve Clearance Exhaust: 0.16 - 0.20mm (= 0.006 - 0.008")
     
    Last edited: Apr 12, 2016
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    are these hatachi carbs air jets are shown incorrectly in the manuals and should be installed like this. also notice main needle jet sizes
    jets.jpg
     
  4. Polock

    Polock Well-Known Member

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    just go back to stock jets and air filter,then you know that's right. Once you get it running good then play with jets
     
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  5. Dirty Harry

    Dirty Harry New Member

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    Thanks guys. XJ550 yea it's a stock style K and N, the airbox came without the top cover as well so I guess we'll see it's a bit less restrictive then stock lol. I'll be checking the valves next. I got it out for a little rip today and it ran pretty well. All the plug are the same color, on the lighter side (lookin like a lean condition) now that the carbon is burned off. Pulls good to about 7 grand and then started to fall off so I didn't go much past that. Should it pull good past 7 or is that normal? It's an old bike after all. Elevation is only 1650'. I will order stock main and 112s since they're cheap and see what happens...
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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    get the air box cover.
    you should be able to redline that baby around 10K rpm
    open air box and incorrect jets will do that
    valves out of spec unsynced carbs and un tuned carbs will do that too
     
  7. Dirty Harry

    Dirty Harry New Member

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    Thanks! Yea that makes me think to small jets as well because things go downhill fast past 7 grand. I'll have to fab up a home made cover I guess, I'm on a budget and plan on turning this into a scrambler. But overall wouldn't having the top cover help it breathe better and make more power (once the valves are confirmed in spec) in combination with the right jets? The stock intake hole looks so tiny! Here's a picture of the plug, all 4 are the same. Cheers.
     

    Attached Files:

  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    no or pods would work. on the constant velocity carbs it is about the balance of the 4 carbs and the "restricted" balanced air flow
     
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  9. Dirty Harry

    Dirty Harry New Member

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    Man, you were so right! I made a cover for the box and this thing pulls great all the way to 9000+ It's like a brand new bike now, whoever threw the cover away needs a slap :) Thanks a lot!!!
     
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  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    the carbs work off of vacuum, those diaphrams in the hats pull the carb slides up opening the main jet needle by the vacuum caused from the pistons sucking the air out of the box (drawing air out of the kidney shaped orifice). unsteady/insufficient vacuum is a bad thing, thats why they are called Constant Velocity carbs.

    always nice when it is a simple fix
     
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  11. k-moe

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

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    And also get a paper filter. The K&N and the UNI airbox filters both make the bike run too lean on stock jets.
     
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  12. Dirty Harry

    Dirty Harry New Member

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    It all makes perfect sense to me now! I'm more used to automotive engines where you wanna get the most unrestricted airflow as possible, but that's apples and oranges because of how these carbs work. Learn something new every day.
     
  13. Polock

    Polock Well-Known Member

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    put stock jets in it anyway. They were as lean as possible from the factory, don't go leaner.
    Lean motors run really good until they don't run any more
     
  14. Dirty Harry

    Dirty Harry New Member

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    That's my plan, order 110s and 112s for how cheap they are, thinking i'll try the 112s with the K and N.
     
  15. Biffed It

    Biffed It New Member

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    PXL_20221108_194508981~2.jpg I enjoy reviving old threads and this one has helped me at least find the air jet sizes and order. But what about these jets on the front side?

    Are these the AIR COMPENSATOR Jet Size: #36 ?

    Where can I order air jets and the compensator jets?
     
  16. hogfiddles

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

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    Yes, those are 36’s.
    I have a bunch, Len ChacAl at XJ4ever has a bunch......
     

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