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Midrange bog on a 1981 XJ550

Discussion in 'XJ Technical Chat' started by Don Stauffer, May 23, 2020.

  1. Don Stauffer

    Don Stauffer New Member

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    I'm working on this bike for a friend. It's been ridden regularly, but was brought to me with a specific issue. The bike starts and idles nicely, with a little choke. It revs freely in neutral, sounds fine. When attempting to take off from a dead stop, the revs drop and the bike will stall without clutch and throttle. When it's in this state, a slight application of the choke will 'fix' the issue, but leaves the bike with a high idle.

    I've been through the carbs twice, they were actually pretty clean in the first place. The owner or a previous owner had attempted the job, as some of the screw heads were cammed a bit. Jets are 110 and 40, the slides drop freely with no friction, the diaphragms look intact, no cracks or holes. Air screws were at 2 1/2 turns out, which I believe is the factory setting. Airbox is stock. Air filter is new, there is no in-line fuel filter.

    I've removed the jets and the emulsion tubes / nozzles, air screws, everything looked clean, but I used solvent and compressed air on the brass and throttle bodies. I've seen some nasty carbs, but these looked really good, no varnish or corrosion, even the emulsion tubes were clean and untarnished

    The symptoms improve a bit when the bike warms up, but it's never quite right. Since the problem seems to be across all 4 cylinders, I'm not convinced that I have a carb problem. I'll be trying the starting fluid trick on the intake manifolds, visual appearance of them is pretty good, but I need to try something. My second thought is that I have a fuel delivery issue, perhaps the vacuum actuated petcock.

    Any advice would be appreciated.
     
  2. DarthBob

    DarthBob Member

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    If you think the petcock is not delivering fuel via vacuum, switch it to the prime setting and see how it runs.

    The choke is an enrichment circuit that adds fuel, so if a little choke fixes the issue, then it would seem that its not getting enough gas. If its not the petcock, turning the pilot screws out a bit might help this. Have the floats been set and the carbs bench synched?
     
  3. Minimutly

    Minimutly Active Member

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    Unlikely the petcock. More likely low float settings, but I would be tempted to open the mixture screws a quarter turn as well as getting some clear tube to check levels.
     
  4. Don Stauffer

    Don Stauffer New Member

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    Thanks for the quick replies. I'll be checking the float levels, but I don't think they have ever been changed from the factory settings. Just using a tilt and visual check, it looked like they were starting to close the needle when floats were parallel to the carb body. This seems like a specific failure, not just a matter of the carbs being a little out of synch. I was not aware of the 'prime' setting, should have checked the manual. Back to it soon, I appreciate the suggestions.

    One clarification, I had assumed the pilot screws were air screws, so turning them in would be a richer mixture. Do I have that right?

    -dhs
     
  5. Minimutly

    Minimutly Active Member

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    No, they admit fuel. Out is richer:)
     

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