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fuel/air mixture screws next to enrichment plungers

Discussion in 'XJ Technical Chat' started by jpacman, Mar 16, 2015.

  1. jpacman

    jpacman Member

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    While rebuilding the carbs on an 82 XJ750, I received new fuel/air mixture screws in the carb rebuild kits. I removed the first one then read in the Haynes manual that they DO NOT recommend removing these at all; they are set at the factory.

    So I installed the first one and tightened it down about the same distance into the carb body. What is the effect of these screws being misadjusted? How do I adjust them properly?

    Someone said that 1/16 of a turn can make a big difference.
     
  2. Stumplifter

    Stumplifter Well-Known Member

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    Well . . . .
    I believe the manual says that 'just because'.
    You DO want to remove them and replace the o- ring and washer.

    A good rule of thumb is that prior to removal you turn the screw in until it gently bottoms out (counting the number of turns as you go along). This gives you a reference of where you were at from the factory/ PO. When putting new screws/ o-ring/ washer in . . . Gently bottom them out and then back them off 2.5 turns.

    That may not be close to where the original settings were but it will give you a good jumping off point prior to doing a colortune/ running synch setting (after making sure your valve clearances are happy!)
     
  3. k-moe

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

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    The Haynes reccomendation is because of EPA air quality rules. There was a TSB that told dealerships to adjust the idle air screws if customers complained about hard starting in cold weather or some such.

    To get the idle mixture close you will softly seat the idle screws, and then back them out 2 to 2 1/2 turns. To get them spot on you will want to get a Gunnison Colortune (14mm), which allows you to see the color of the fuel/air mixture as it burns. You can do a plug reading, but in all honesty that takes too long to do for my liking, besides: new tools :)

    Proper carb cleaning and bench setup: http://www.xjbikes.com/forums/index.php?threads/in-the-church-of-clean.14692/

    Fuel level setting: http://www.xj4ever.com/setting fuel levels.pdf

    Carb setup and running tune information begins about 1/4 of the way down this page: http://www.xjbikes.com/forums/index.php?threads/the-information-overload-hour.27544/

    Have you checked valve clearances yet?
     
  4. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    The TSB was actually about "lean surge" associated issues, since the carbs were set up pretty lean at the factory in order to meet EPA emissions testing in that era. Almost all of these bikes can benefit from tweaking of the mixture screws (richer) but I would use a Colortune see-thru plug in order to do so. It never hurts to bump up the pilot fuel jets by one size (stock on most 650/750 Hitachi carbs is #40, almost all other non-US markets used #41 or even #43 pilot jets in the same engines). You'll get less fuel mileage but better performance.
     
  5. jpacman

    jpacman Member

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    This is my second XJ750 build so, yes, the valves are checked and adjusted to be in spec. Of course that doesn't mean that I did everything right but it is my second time around. The carbs are "zestfully clean", even the enrichment circuit. I have followed all of the herein recommended bench tuning tips (paper clips setting to #3, floats set with my pvc carb holder and clear tubing, etc.).

    Playing with the carbs always makes everything else that needs to be fixed seem easy. Patience is a virtue and all that. I know that when I got the first one properly dialed in, it ran great.

    Guess it's time to invest in a Colortune plug and learn how to use it.
     

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