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RPM and lean-rich question

Discussion in 'XJ Technical Chat' started by xj650_dude, May 10, 2010.

  1. xj650_dude

    xj650_dude Member

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    I recently purchased my 82 xj650 maxim. It needed a good carb cleaning and I worked up the guts to try it and it went pretty well but now I have a few different issues, which leads me to these questions. The bike has aftermarket exhaust and has a tendency to back fire and pop when I get the RPM's up and let off the throttle. I removed the brass caps covering the mixture screws and backed all four to 3 full turns. This seemed to help a little but it still pops a bit. How far should those mixture screws be backed out? Is 3 enough or should I go more? (I know they are touchy).
    I'm also trying to track down the cause of my throttle sticking. When I get my RPM's up to 4 or 5 grand and pull in the clutch to shift the RPM's linger for a bit before eventually settling back down. Any ideas why it would linger like that when I let off the throttle? Any help would be much appreciated. This is my first bike and I'm having a great time working on it. This website has been a huge help!
     
  2. shnuffy

    shnuffy Member

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    Welcome to the site!

    I'm not 100% about the mixture screws on the 650, but on the 550 I backed them out 2.5 turns and that helped drastically with my idle. So, if anything I wouldn't go more than 3 as a benchmark (before colortune).

    Your hanging rev problem is likely sticking carb pistons. Search for "Carbs need to go CLUNK" thread and do it. It's very easy... essentially if the diaphragms in your carbs don't DROP freely after they are unseated, your revs will hang and your engine breaking will suffer. I sanded the bore evenly with 2000grit paper and some WD40. Worked wonders.
     
  3. yamaman

    yamaman Member

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    +1 on the carb pistions for sure. Wish you didn't touch the mixture screws, did you record the original positions by any chance? the popping is probably from being out of sync. did you split the rack for the cleaning? if so what method of bench syncying did you use?
     
  4. xj650_dude

    xj650_dude Member

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    Thanks shnuffy! That was a very informative article and I plan to pull the carbs again and give it a look in the next couple days.
    Yamaman, before I backed the screws out to 3 full turns they were each at about 2 1/2. I'm still very new to the whole xj scene since this is my first bike. I've heard of syncing your carbs but I'm not a 100% sure what that means. Is there an article that your aware of that might help me out? Obviously I didn't sync anything when I attempted to clean the carbs so that could very well be one of my problems. Let me know what you think and thanks for the advice
     
  5. shnuffy

    shnuffy Member

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    Be sure to perform a bench sync when you take off your carbs. Again, it's very easy. Search for "bench sync."

    The idea behind syncing the carbs is so that the throttle opens each of the carbs exactly equally. The bench sync gets you close enough to perform a true vacuum syncing, which requires the YICS tool. Search for "two bottle sync tool" to see what that's all about.

    The bench sync will get you in the right direction.
     
  6. Polock

    Polock Well-Known Member

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    synchronizing the carbs will most likely take care of the rpm's dropping
    the backfire with a aftermarket pipe is pointing to a lean condition
    the screws can only do so much, you went the right way but might have to go up a size in jets, 1 size bigger low and 2 or 3 bigger mains isn't out of the question for a 4-1 pipe
    you need to see what's in there now
    first thing is check for vacuum leaks and look at the plugs
     
  7. Zyggy

    Zyggy Member

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    This will mean you need to take the carbs off the bike....again....if you don't bench synch when you have it off for the CLUNK test. Don't worry, that's exactly what I did. I'm almost a pro at mounting and unmounting carbs since owning this bike!

    Since you have the carbs off you might as well do the spray test too, which can be found here:

    http://www.xjbikes.com/Forums/viewtopic ... +test.html
     
  8. RickCoMatic

    RickCoMatic Well-Known Member

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    You NEED the Carbs Rehabilitated.
    Not just Cleaned.
    A little more than Cleaned.

    You got Carb Bodies that have 28 Years of exposure to the environment on them, ... at least.
    You got Carb Body Bores that are Oxidized.
    That stuff has to go.

    You need the Bike to IDLE with the Throttles Closed.
    The Engine needs to be able to have the Mixture it needs to run; supplied by the Pilot Circuit.
    So when the Throttles get closed, ... they actually do CLOSE ... shutting OFF the Air Supply and making the Bike dependent upon only what you regulate by Tuning.
     

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