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XS1100 (BS34s) Guzzles Gas, Runs Rich

Discussion in 'The Garage' started by jetmechmarty, Nov 25, 2019.

  1. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    Rack synced. Vacuum advance tested. Mechanical advance tested.
     
    gggGary likes this.
  2. grizld1

    grizld1 Grumpy old man Top Contributor

    Marty, a single step richer on NJs wouldn't do all that much damage, but you're probably looking at far more wear than that. Two steps (.0004)" becomes significant, and the problem is aggravated by the inverse relationship between NJ size and MJ selection; a look at the year-by-year jetting changes in OE XS650 carbs illustrates that relationship nicely. Adjusting fuel level isn't likely to help; as long as the carbs aren't puking fuel out the float chamber vents, fuel level isn't too high. Severely elevated fuel consumption is typical for bikes with worn NJs and needles.
     
    Jim likes this.
  3. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, I discovered this first hand many years ago when I was refurbing an old BMW. The carbs had sat all gummed up for several years. I cleaned them all out and they seemed to work fine. Bike started right up, idled nice, ran good, but the plugs were black and the gas mileage was terrible. I was getting high 20's to low 30's MPG when it should have been in the low to mid 40's. I tore into the carbs again and this time carefully inspected the needle jets, looking at their insides, the hole through them that the needle works through. This is the metering orifice for the jet and while much larger than main and pilot jet orifices, it is precisely sized. I found the holes all eaten away inside, from the gas varnish sitting in them for several years. They were much larger now than their original size and that was the cause of my problems.

    Now, yours probably never sat all gummed up with varnish, but just the constant movement of the needle in and out of the needle jet metering orifice could wear it away. This wear takes a long time to happen but with 100K, I think you're there. Inspect your needle jet carefully. Look down into the hole that the needle fits through from the top. Inset in that hole, maybe 1/8" down, you'll see a step where the hole gets smaller. This is the actual metering orifice. Often you'll find one side of the step worn off so the hole is now oval shaped.
     
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  4. 5twins

    5twins XS650 Guru Top Contributor

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    I'll also mention that a few years later, I replaced the needles as well because I noticed shiny rub marks on their sides. This made no difference in mileage so I concluded they weren't worn. So, I'd be leaning more towards replacing your needle jets and not the needles. I see that web site you linked to has all this stuff but it ain't cheap. You might be able to get needle jets cheaper from Mikuni. It's a common style but I'm not sure if the size is common. There should be a size number stamped on the jet. I see that web site doesn't give the stock needle jet size or number, just the aftermarket number. That could be a sales ploy so you just can't go somewhere else and buy them cheaper, lol.

    I see they charge a premium price ($7) for their genuine Mikuni mains and pilots. You can source them in 4 packs from most of the big box sellers (Bike Bandit, Dennis Kirk, etc.) for around $12. This is how I usually buy mine. It gives me two to use and two as spares.
     
  5. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    Found this here. Might help in chasing down jets.

    Untitled.png
     
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  6. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    5GL16 needles have a severe step in them near the bottom. They're also shorter than what went in the standard. Jim's post shows the main jets are much smaller than the standard as well. For the time being, I can scrounge my junk and see if I have four X-2 needle jets with fewer miles and maybe even some 5GL16 needles.
     
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  7. 5twins

    5twins XS650 Guru Top Contributor

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    Like I said, look down into the needle jet metering hole from the top. If badly worn, you should be able to see it.
     
  8. fredintoon

    fredintoon Fred Hill, S'toon. Top Contributor

    Hi Marty,
    lower the float bowl fuel level to get better mileage.
    Discussion of any bike at all is welcome here (sez so right at the top) although expert advice on certain obscure machines may be lacking.
    But while the XS11 is a 4 cylinder shaftie with an engine that runs backwards it's amazing how many parts it has that will fit on an XS650.
     
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  9. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    I fully admit my experience with our CV carbs is pretty limited, but I do have a lot of experience with carbs in general. Mostly Strombergs, Marvel Scheibler and the like. Here's what I know about float levels... If the level is too high, you'll be spittin' fuel out the overflow. Dick alluded to that above. If your level is too low, you'll un-port or cavitate the jets that should be immersed in fuel... especially during accel/decel. This gives you an instant lean condition that causes sputtern and popping. In between the two extremes is a correct "enough" fuel level. In other words, lowering the float level will not increase fuel economy or be some magic fix that leans the mixture. We have different size jets to accomplish that. If you're not puking fuel out the overflow and not poppin' and sputterin' during hard acceleration, leave the float levels alone... they're good enough.
    Like others have pointed out, it's pretty likely your needle jets are worn.
     
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  10. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    That makes it easy enough. I'll see what's in my spares. If nothing, then XJ4ever has fresh parts.
     
    Jim likes this.
  11. grizld1

    grizld1 Grumpy old man Top Contributor

    You might also check Niche Cycle Supply. They offer a wide range of genuine Mikuni brass at a very good price (they offer repop brass too, but they're very clear about which is which).
     
  12. ckahleer

    ckahleer XS650 Enthusiast

    Just some random ideas.
    I once bought a xj750 that ran like crap. Plugs were sooty black. the PO had tied rebuilding the carbs and had pilot and main jets reversed.
    Black plugs means to much fuel or to little air. Have you checked for restrictions in the air intake. Air filter, snorkel.
    You could remove the air box cover and ride like that, see if plugs get browner.
    Can a second notch be ground into the top of the needles, allowing them to drop lower.
    As mentioned earlier, ignition timing/advance could cause sooty plugs. Valve timing could too.
     
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  13. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    I cleaned the air filter and made sure all air passages were clear prior to the VYR trip.
     
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  14. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Following you around Marty, VYR 2019' rich gas smell was an improvement over 2018's oil cloud. :boxing: :laugh2:.
     
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  15. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    I hope you noticed my position at the back for Ozark 2019. That was for you.
     
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  16. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    I had four slides that I used for the trip to VYR in a hurry-up swap. A checked all the needle part numbers and put them in the order shown. The washer was under the e clip, costing an additional five mpg. I also pulled 4 matching emulsion tubes. I hope to swap them onto the bike and give it a run next weekend. I should need nothing more than a dry day. Any improvement will show I'm on the right track. If it isn't the cure, I'll drop the fuel level.
     
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  17. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, having the washer stack wrong like that would raise the needle higher than it should be, making it run richer. You do most of your running in the midrange, or "on the needle" so to speak, so it has a big effect on both plug color and fuel mileage.
     
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  18. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    OK, the XS1100 is back together. I rode it 14 miles to top up the tank. I think the exhaust gurgle has changed its tune. I hope to report back tomorrow evening with mpg. I replaced slides, needles, and needle jets with parts from another carburetor.
     
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  19. jetmechmarty

    jetmechmarty XS650 Guru Top Contributor

    Thank you gentlemen, for taking time to help me with this. I rode 140 miles today. I'm up around 40 mpg! :bike:
     
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  20. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Great to hear Marty! Near 40 sounds like a healthy number for an XS1100.
     
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