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Need help with Carb adjustment (surging)

Discussion in 'XJ Technical Chat' started by SecaRob, May 31, 2009.

  1. SecaRob

    SecaRob Member

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    OK here is my problem. My 750 starts great, runs perfect at idle and pulls good and smooth at wot to redline. The problem I am having is that under a very light throttle around 4k rpm the bike has a mild surging in it. If I hit the throttle hard the surging will go away it will also go away if I hit the choke just a little. My carbs have not been apart since August of 1996 but I have always ran Seafoam or another type of cleaner in it roughly every 4 tanks of gas.

    Last year I had posted some pics of my plugs. At the time I was running BP6ES (hot) and based on feedback from you guys I was definately running lean and hot.

    Below are my plugs from this year. They have roughly 600 miles on them but it appears that I am still lean.

    Is it time to tear into the carbs

    P.S. I have yet to make any adjustments....
     

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  2. SecaRob

    SecaRob Member

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    Any ideas?
     
  3. johno8

    johno8 Member

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    Sounds like the carbs might need some cleaning to me, but I'm not an expert. What about the floats? Surging can sometimes be caused by a misadjusted float. dunno what else to add but I'm sure the Carb King is lurking and he'll set ya straight. = )
     
  4. lowlifexj

    lowlifexj Member

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    Is your exhaust stock? No holes drilled in it or baffles removed.
    Air cleaner?
    Your plugs look scary lean to me.
     
  5. SecaRob

    SecaRob Member

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    Bike is 100% stock
     
  6. Deadulus

    Deadulus Member

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    Holy moly!! That is scary lean....could the surging be from being on just the edge of not enough fuel getting to the engine as evidenced by either giving it more throttle or bumping up the enrichment making the surging go away??
     
  7. SecaRob

    SecaRob Member

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    That is what I am thinking but what puzzles me is how even they are?
     
  8. 85MaximXX

    85MaximXX Member

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    Wow them are some scary lean lookin plugs I thought mine were lean but wow. Seafoam is some good stuff But.... it won't break up hard deposits that have formed over The years. I pull the carb on my snowmobile and bike every preseason of every year. Yes it takes time and they are a pain to get out of the bike but I rarely have an issue during the riding season. If you've made it since 96 you've had a good run but it's time.
     
  9. SecaRob

    SecaRob Member

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    I am hesitant to open the carbs since all four plugs are almost an even match. Could something else be going on here?
     
  10. Polock

    Polock Well-Known Member

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    did the surging start with the new plugs ?
    maybe try taking the hats off and open the drains, count the turns on the mix screws and blow a can of carb cleaner through them catch it on a rag under the carbs might get something out might see a bunch of stuff and decide to take them off
     
  11. SecaRob

    SecaRob Member

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    Polock, the plugs were installed over the winter. It's had the slight surge all spring so I'll take a look at the plugs but would be supprised if they were the culprit
     
  12. Palmer650

    Palmer650 Member

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    I'm with Polock! Clean your pilot circuit well by flushing cleaner from top down and then from the bottom up. Don't be like me and do a half-butt job on them and constantly be wondering why the plugs are too lean. Also, are your pilot screws 2.5 to 3 turns out?

    Check out this post and scroll down to Rick's cleaning advice!
    http://xjbikes.com/Forums/viewtopic/t=1 ... art=0.html
     
  13. MN-Maxims

    MN-Maxims St. Paul Minnesota

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    How does your bike idle? Do you have to really warm it up good before you turn the choke off? Plugs are lean as you well know. Your float levels could be low and there could be some junk in the carbs the Seafoam can't get too.

    MN
     
  14. SecaRob

    SecaRob Member

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    The bike fires right up with the "choke" on. Can take it off after a minute or 2 and it will idle just fine but it still needs to be warmed up before it is rideable without the choke.

    My petcock is an aftermarket unit and it forces the fuel line to travel upward before making the journey down to the carbs. I wonder if this could cause the lean concition in all cylinders>
     

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  15. Palmer650

    Palmer650 Member

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    Rob- The fuel will should flow through that ok, but you never know. That's why I bought a 90 degree inline filter to get the fuel shooting directly down to the carbs. If your really concerned why not turn the carb hat 180 degrees so the hose is contacting the sloped part of the hat. It might bring the hose down a few more degrees and more level. Also to test the fuel flow disconnect it from the carbs and hold the hose at the same angle seen in the photo. Set the petcock to Prime and let the gas run into a little plastic container for 10 seconds. Mark the height on the plastic container and then retest but this time holding the fuel hose level and see how much fuel flows into the container. Just a thought.

    BTW, what was wrong with the original petcock?

    MN-Maxims- I'm really concerned about junk in those carbs now that I see Rob doesn't have an inline filter. And I second a float level test but first would love to see what's resting in the bottom of those bowls. Sometimes Seafoam jogs loose debris that is stuck in the brass emulsion tubes and pilot fuel circuit. However, on a positive note it sounds like your enrichment/choke circuit is somewhat clear if it fires up with choke on!
     
  16. SecaRob

    SecaRob Member

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    I am still stuck on the question as to how all four carbs can be plugged up the same. It makes no sense to me but then my experience is limited...
     
  17. SecaRob

    SecaRob Member

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    What could I expect if I switched frmo the BP7ES plugs to the 1 step cooler BP8ES?
     
  18. SecaRob

    SecaRob Member

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    Before I pull the carbs I think I'll put a new set of plugs in then make sure I have no exhaust leaks.

    What do you think?
     
  19. BlackMax

    BlackMax Member

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    check and see if you have a vacuum leak where the carbs meet the engine...
     
  20. lowlifexj

    lowlifexj Member

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    I agree a propane test would be a good idea, but would that make all the cylinders lean? With the yics I could see 2 cylinders next to each other being lean but for one carb boot to make them all lean? Still a vacum leak test is always a good idea because you don't need to take anything apart to do one.
     
  21. SLKid

    SLKid Active Member

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    I wanna know the same answer that Rob wants to know. What would change if the spark plugs were changed? I got the 8ES in mine
     
  22. 85MaximXX

    85MaximXX Member

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    Rob you are just avoiding the inevitable and trying to put a bandaid on the problem. THey are probablem plugged up evenly because you have had no major debris in the sys. Just the residue left from over the years. Of course you could had leaking butterfly shaft seals.
     
  23. SecaRob

    SecaRob Member

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    Definately :cry:
     

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