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Cleaned and rebuilt carbs, now only runs with enrichment ON

Discussion in 'XJ Technical Chat' started by MinnesotaXJ, Jul 23, 2020.

  1. MinnesotaXJ

    MinnesotaXJ New Member

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    I spent weeks reading and watching every piece I could on rebuilding carbs and educating myself as much as possible before digging in or taking things apart. The bike ('82 XJ 650) ran fine before but had issues at idle, I assumed the pilot jet was probably in need of cleaning among other things. I stripped every piece apart and meticulously took notes and photos as I went to ensure proper reassembly. With the bodies totally stripped I soda blasted them, soaked/sprayed with cleaner and finished up with an ultrasonic cleaning for each carb and its associated smaller pieces. Replaced items below.

    New fuel supply 0-rings
    New throttle shaft seals
    New float valve seat
    New bowl gasket
    New mixture screw o-ring, washer and spring
    New choke plunger rubber cap
    New bowl drain screw

    I recorded previous settings for mixture screws and each was a bit all over the place, I did notice the tamper proof caps were already removed so someone had made adjustments to these previously. I reassembled with all set to 2.75 turns out to get right in the middle of the recommended 2.5-3 turns. I did check the float settings and did the tube method to check float height after assembling. I got them all to be close to 2mm below gasket surface as recommended (at least they looked to be in that ballpark from what I could tell). I did a bench sync and the valves all look pretty spot on.

    Everything looks absolutely stunning with the new shine and I'm so eager to feel the difference with a properly cleaned and tuned carb setup. I installed back in my XJ and used a funnel/tubing for a temporary gas tank while tuning. I purchased a colortune kit and carb sync gauges prior, knowing I would need them for this step. The bike starts up just fine, in fact better than it ever has previously... but it will not stay on unless the enrichment valves are open. The engine responds to throttle and rev's up but only with the choke on. I've tried holding a higher RPM with the throttle and backing off the enrichment valve simultaneously but it dies. I want to get it to idle without the choke on so I can get things synced up and tune the mixture but so far I haven't been able to get the idle operating right.

    This sounds like it's definitely fuel related but I know the main jets are clear, the emulsion tube is clear, the diaphragms all looked to be in good shape with cylinder 1 being just a little sticky with the needle. What else could be causing this and where should I start first before changing too many things, I'm figuring it has to be something relatively simple that I'm just overlooking at this point.
     
  2. Polock

    Polock Well-Known Member

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    ya have to start somewhere so put that sync gauge on there. choke don't matter, try to keep the rpm under 2k. your going to use the idle knob a few times doing the sync, might want to use a glove
    remember [1-2] [3-4] [2-3]
     
  3. JCH

    JCH Member Premium Member

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    That's heartbreaking and disappointing...
    I wouldn't start changing anything especially sense this is a problem that wasn't there before the tear down,its something that you missed.
    You know that it is too lean,no vacuum leaks ?
     
  4. MinnesotaXJ

    MinnesotaXJ New Member

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    I don’t have the YICS tool to block off the passageway but they’re all relatively in sync, nothing is way out of whack to the point that it shouldn’t idle without the choke on. No obvious leaks or manifolds that aren’t properly seated, so I’m not convinced air is entering the system that way.

    I’m still thinking there’s a restriction in fuel somewhere between the carb bowl and the pilot/main jets. If the floats were set incorrectly wouldn’t that also prevent the enrichment valve from sucking up fuel? I’m wondering if I have the float levels set too low and that’s causing my issues.
     
  5. JCH

    JCH Member Premium Member

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    They would have to be extremely low i would think to cause that severe of a issue and you wet sync them correct ?
    Take a small cup and drain the floats and see how much fuel comes out of each bowl for a quick test.
     
  6. Polock

    Polock Well-Known Member

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    check the floats with a clear tube while their still on the bike. forget that yics tool that's for the last 1/10 %, your not there yet.
    i can't see the floats being that low unless their upside down
     
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  7. MinnesotaXJ

    MinnesotaXJ New Member

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    Just found the document on fuel levels and how to set them, with an image of the correct float orientation and upside down. Pretty positive I installed them upside down. Going to rip into that tonight and hopefully have a functioning motorcycle for Saturday!
     
  8. MinnesotaXJ

    MinnesotaXJ New Member

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    Thank you!!! That was definitely the problem and I feel like an idiot on the rookie mistake, but if that was the single issue I had I’m okay with it. Now to tuning this thing...
     
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  9. JCH

    JCH Member Premium Member

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    Glad that you found it,you won't ever do that again LOL
    You may want to look up the video on setting the floats wet with the clear plastic tube before you set the floats on these site.
     
  10. MinnesotaXJ

    MinnesotaXJ New Member

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    Got everything pretty well synced and idle is set nicely but I still lingers high sometimes or delays before returning to idle.

    Doing the Colortune on each cylinder, I noticed the first cylinder (left side of bike) is extremely lean with almost no orange or “bunsen blue” just very pale and whitish in color and the idle mixture screw seems to make no difference. This makes me weary that I have more work to do...
     
  11. JCH

    JCH Member Premium Member

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    Did you mess around with the throttle plates when you had the carburetors apart ? if so that could cause the idle to hang up from not seating correctly,not sure why the mixture is off.Did lube the return springs well on the carburetors ?
     
  12. ManBot13

    ManBot13 Well-Known Member Premium Member

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    Don't feel bad! Until you pull a bunch of carbs apart, the direction of the floats is kinda non-intuitive. I know I had to carefully look at the diagrams/orientation and function many times before buttoning up the first set of carbs i did.

    Usually a lean mixture is from an air leak or a clog somewhere in that pilot circuit. Baring that, what order did you do the sync and colortune? You could try another round of sync and colortune, always maintaining a proper idle after each adjustment. There is quite a bit of interaction between the sync and idle screw settings. Hanging idle usually means that some cylinders are helping others along that aren't firing from the idle circuit. When you open the butterflies, the weaker cylinder gets it's gas, and the over fueling/air in the other cylinders tries to hold a new equilibrium, if that makes sense.
     
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  13. JCH

    JCH Member Premium Member

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    Interesting theory ManBot 13,haven't ever heard of that but i'm relativity new to motorcycle carburetors and still have a lot to learn about them,the way you describe it you would almost think that it had a computer management system Lol.This is really interesting to me may i ask how you came about it ? may i ask your background on this subject
     
    Last edited: Jul 27, 2020
  14. ManBot13

    ManBot13 Well-Known Member Premium Member

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    So think of "idle" these 4 cylinder engines as the cylinders generating enough power over 2 full rotations of the crank to just balance the internal resistances and losses. It doesn't matter if one cylinder drops off, or only one cylinder is firing, you could achieve a certain rpm (idle) if there is enough power generated in a cycle. So idle rpm is a function of the fuel/air mixture of all 4 cylinders, and there are many combinations that can result in the same rpm. You want the one that results in all cylinders being equal.

    Using any other combination of throttle plate position and idle mixture will end up with some throttle plates being past the idle/pilot air jet hole, engaging other fuel circuits. Having a throttle plate closed may make it impossible for pilot screw adjustments to have an effect on mixture (because there's not enough airflow through that venturi). Also, the vacuum being generated for a cylinder's intake stroke will be related to the cylinder that is going through a power stroke at the same time. That means that at idle, if one cylinder is not getting fuel, then during its power stroke, another cylinder's intake charge (i.e. vacuum) could be weaker.

    All 4 cylinders are dependent on each other, so you can't just get it right the first time, you need to go through several iterations. All this assuming you have no vacuum leak. I've fully rebuilt a couple of XJs including full carb overhauls, without having air leaks, and the first pass on colortune is always a frustrating one.
     
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  15. JCH

    JCH Member Premium Member

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    Thank you for the detailed response ManBot 13,i'm going to have to read it a few times to get into my slow working brain.
    I had no success with the color tune for some reason,it seem to stay blue not matter how much i turned the mixture screws,i was using it on a sunning day if that matters.
     
  16. Polock

    Polock Well-Known Member

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    i never used one but it seems that touching the choke would change something, then you know what to look for. maybe
     
  17. PJC750

    PJC750 Member

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  18. PJC750

    PJC750 Member

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    I had similar issue...the fuel bowl "enrichment" holes were clogged. Use thinnest guitar string, lots of poking, and carb cleaner. Spray(use red straw) carb cleaner in base of bowl, while turning away from your face!, once you get that 6ft stream of fluid out of the recess for enrichment tube, you should be good. Ck em in the dark with a good flashlight, see pinprick of light. Good to go
     
  19. Peter Perry

    Peter Perry New Member

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    The YICS tool shuts off the passage between the cylinders. If this is not done then you will NOT get a true indication of individual pressure. Hense, even when the carbs are way off, it will look on the gauge as if "Hey things look pretty good!"
     

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