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Xj750 seca with big bore kit 818cc rejeting

Discussion in 'XJ Modifications' started by Mathieu, Aug 3, 2015.

  1. Mathieu

    Mathieu New Member

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    Hi guys, i just finished completely rebuilding a 1982 seca 750 in a cafe racer style.

    Mods i made are:
    -big bore kit, 818cc
    -air pods
    -stock exhaust

    I bought a jet kit on ebay that was specialy made for those mods... After rebuilding it, i turned the engine on and when i try to accelerate at full or half throttle its not doing well, i think its too rich. It starts with barely using choke, it burned almost 10 liters of gas in the 100km i rode it... Im not an expert on carbs but anyone has had this set up tuned correctly before? What jet size, kit? I believe the big bore kit makes it harder to tune.. Does 10% more displacement means jets 10% bigger?

    I need advice from a carb rejeting guru!


    Thanks
     

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    Mathieu, Aug 3, 2015
    #1
  2. Toomanybikes

    Toomanybikes Well-Known Member

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    Ok what size jets are in there now and do the needles have groves for clip adjustment
    base line on 5 clip needle is middle clip position
    I see pods and no 10% bigger might be too much
    Stock mains are 120 for your bike and 43 or 45 pilots depending on US or canadian bike.
    Mixture start with 2 1/2 turns not a lot more for big bore ,your nose or burning eyes will tell you if its way fat(rich) at idle
    Start with colour tune for idle mix
    It might be worth finding a dynotune shop to get it right. Looks like you spent plenty of time doing this bike( nice xs750 tank)
    And stock exhaust will limit how much more you bike will flow
    Engine is a big air pump lots of intake and extra cc will be slowed by stock exhaust.
    Have you done a plug chop ? I know the engine is new so no full power yet but look at plugs in general, back or tan
    So many things to check do one change at a time and keep track of every change in jet or mixture and you will get it right in time .
     
    Last edited: Aug 3, 2015
    Toomanybikes, Aug 3, 2015
    #2
  3. k-moe

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

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    Stock pilot jets vary some by what year of Seca, not just by which country of sale, and none of them use a 45.
    From The Church:

    XJ750 Models:

    Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca USA models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 15R00 (Maxim, Midnight Maxim)
    Carb Model ID: 5G200 (Seca)
    Main FUEL Jet Size: #120
    Pilot FUEL Jet Size: #40
    Main AIR Jet Size: #80
    Pilot AIR Jet Size: #225
    AIR COMPENSATOR Jet Size: #36
    Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-13
    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: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca Canadian models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 15T00 (Maxim, Midnight Maxim)
    Carb Model ID: 5H200 (Seca)
    Main FUEL Jet Size: #120
    Pilot FUEL Jet Size: #43
    Main AIR Jet Size: #80
    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-14
    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: 1981-4 XJ750 11M UK/Europe Seca models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 5N100
    Main FUEL Jet Size: #120
    Pilot FUEL Jet Size: #43
    Main AIR Jet Size: #80
    Pilot AIR Jet Size: #195
    AIR COMPENSATOR Jet Size: unknown if used.
    Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-14
    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: 1983 XJ750E-II models
    Carb Manufacturer: Hitachi
    Carb Series: HSC33
    Carb Model ID: 29R00
    Main FUEL Jet Size: #105 (cylinders #1 & #4)
    Main FUEL Jet Size: #102 (cylinders #2 & #3)
    Pilot FUEL Jet Size: #40
    Main AIR Jet Size: #70
    Pilot AIR Jet Size: #225
    AIR COMPENSATOR Jet Size: not used.
    Starter FUEL Jet Size: #43 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-17
    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: 5mm +/- 1mm (.197" +/- .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: 1984 XJ750RL Seca models
    Carb Manufacturer: Hitachi
    Carb Series: HSC33
    Carb Model ID: 41Y00
    Main FUEL Jet Size: #106
    Pilot FUEL Jet Size: #40 or #41 (unsure which is correct)
    Main AIR Jet Size: #70
    Pilot AIR Jet Size: #225
    AIR COMPENSATOR Jet Size: not used.
    Starter FUEL Jet Size: #43 (non-replaceable, in float bowl)
    Main jet NEEDLE ID: #Y-18
    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: 1mm +/- 1mm (.039" +/- .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")


    I would say to split the difference between the 750 and the 900, but the 900 used Mikuni carbs and the measurements for jet sizes are not exactly the same between the two manufacturers.
     
    Last edited: Aug 3, 2015
    k-moe, Aug 3, 2015
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  4. Toomanybikes

    Toomanybikes Well-Known Member

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    I stand corrected Moe US bikes 40
    All the canadian bikes I've worked on 43 pilots 650 seca and 750 SECA
     
    Toomanybikes, Aug 3, 2015
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  5. Mathieu

    Mathieu New Member

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    I d like to find the right jets by myself, i assume the yamaha dealer can get me all jet sizes and help me with that.. Hopefully they wont empty my walet. Ill repost once its disassemble with the jet size they gave me plus the new size i try when i get some.
     
    Mathieu, Aug 4, 2015
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  6. k-moe

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

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    Your Yamaha dealer is unlikely to have jets for Hitachi carbs (and will have to order them if they do have a supplier). Hitachi stopped making carbs quite some time ago. Chacal has the correct jets, and there are other suppliers of the correct jets.
     
    k-moe, Aug 4, 2015
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