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Mikuni maxed out

Discussion in 'The Garage' started by Jthomas, Mar 26, 2019.

  1. Brassneck

    Brassneck XS650 Guru

    Just to be clear, you mean the air adjust screw...vs. idle adjust screw? The idle adjust screw raises/lowers the slide...screwing it in raises the slide, adding more fuel (raising the needle and thus, raising the idle), the air screw manages the air at idle...so turning it in all the way decreases the air flow. It's opposite of the BS carbs.
     
    peanut likes this.
  2. Jthomas

    Jthomas XS650 Enthusiast

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    The one that opens the slide are screwed all the way in. The mixture screws are pretty much backed all the way out.
     
  3. arcticXS

    arcticXS XS650 Junkie

    It has not been mentioned so far in this thread, but have you verified the following before messing with the carbs?
    - Valve clearances
    - Compression
    - Valve (cam) timing
    - Ignition timing
    - Voltage at the coil + terminal. Old OEM harness/ switches/connectors can cause severe voltage drop. Unless you have a magneto CDI ignition (i.e. Powerdynamo) this can be an issue.
     
    Last edited: Mar 28, 2019
    gggGary likes this.
  4. jay760

    jay760 XS650 Junkie

    It might seem like a stupid suggestion but mikuni slides are easily put in back to front as in with the cut out facing closest to the engine, not sure why the made it so you can turn the slides around.
     
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  5. grizld1

    grizld1 Grumpy old man Top Contributor

    Yeah, Jay, I suggested that, maybe he checked it and maybe not. OP, you might want to do a little more research on the incompatibility of most aftermarket electronic ignitions, Pamco included, with battery-free systems. If you just have to run without a battery, consider installing breaker point ignition or a Powerdynamo system. Good luck with the project.
     
  6. Jthomas

    Jthomas XS650 Enthusiast

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    These have a key on them that only let you install them one direction.

    -Ignition timing is good

    Have not checked anything else. It’s on the list though. Wanted to set lash hot, and check compression hot. So I figured get it idling and do so. But it can’t idle.



    For what it’s worth. The bike has 8k miles.
     
    peanut and gggGary like this.
  7. Brassneck

    Brassneck XS650 Guru

    That is true, but that doesn't mean that the left slide is on the right side carb, and vise-versa. Easy enough to double check.

    Ah...ok. With your battery connected, you may want to drop the needle clip to the bottom position...which essentially raises the needle a bit...allowing you to drop the slide back to a more reasonable setting. I would up the pilot jet (try 20), turn the air screw out one turn...and see how it goes.
     
    peanut likes this.
  8. Jthomas

    Jthomas XS650 Enthusiast

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    Sorry for no reply. I had to work really late the past two nights and didn’t make it into the shop.

    Here’s the deal:
    Attempted to start the bike as I left it a few days ago. Would not start. Kicked forever. Checked spark, none at all on either bank.
    Checked connections, all good. No spark.

    Hotwired the ignition off of my truck battery. Fires right up. (Ign switch must have gone bad??)

    Had a dead miss on right bank.
    Tested spark with engine not running. Right bank had very weak spark, or none at all.

    Checked compression- 105 left and 100right

    Installed new plugs- gaped to .035”
    Bike fired right up in like 1/2 kick and ran as good as it ever has so far.

    Bike still running rich, but I was able to warm the bike up.

    Shut off bike. Lashed valves to .004 intake and .006exhaust with engine hot

    Adjusted float levels to 23mm (measured @ point of float contact)

    Verified slides are installed correctly

    Adjusted timing chain tension

    Fired bike back up and corrected ign timing

    Bike runs “better”. But still runs rich

    Have both needles set to the leanest clip positions as suggested previously

    Fired bike up and let it warm back up. Sprayed 1/2 can sea foam into each bank in attempt to clean valves.
    Pulled exhaust pipes to check exhaust valves for carbon. Also checked intake valves.
    Checked compression after sea foam treatment: 120left and 115 right


    Right bank will go dead after a few mins of running from running so rich. Literally blowing black smoke at idle until it goes dead on the right side. It won’t come back on it’s own. I give it a vacuum leak and the cylinder comes right back.

    You can feel the temp difference in the jug between the left and right side. Right is way colder than left.

    Air jets are still removed. Idle speed is still very low. Idle screws are all the way in. Idle mixture screw are still all the way out. Jets are still as listed before.

    When I twist the throttle is blows flames under acceleration. (Doesn’t lean pop, just flames) it also feels like I’m hitting a rev limiter, so I know it’s running rich as hell.


    The fuel consumption is crazy. I can literally watch it flow through the fuel lines as it runs. Sucks carbs dry in a matter of 10seconds or less.

    Scratching my head still.
     
    Last edited: Mar 30, 2019
    peanut likes this.
  9. Brassneck

    Brassneck XS650 Guru

    Wouldn't leaning out the clip, but raising the slides as far as they will go kind of defeating the purpose? (I use drill bits for measuring, maybe start with a 1/4" for each slide opening. I also use the popsicle trick to make sure each slide is raising at the same time.)

    Unless I'm misunderstanding how you have your carbs set up...I think your pilot is too lean, based on the fact that you're compensating to get the bike to run by raising the idle....have you changed your jets out yet?
     
  10. Jthomas

    Jthomas XS650 Enthusiast

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    We could be miss understanding each other. I’ll explain my thoughts, and you correct me.

    I think the bike is running super rich.

    Raising the slide adds air to the engine and increases engine speed. But it also raises the needle, allowing more fuel into the system. So I lean the needle out as much as possible, so the needle doesn’t come out of the seat as far when the slides are raised up.

    I originally tried to set the slides about 2 turns out (.040” slide opening) and about 1-1.5 turns on the mixture screws. Couldn’t get engine to start. Twist throttle open a bit, and engine will fire up. Let off throttle, it stalls right out.
     
    peanut likes this.
  11. Brassneck

    Brassneck XS650 Guru

    Right, ok, thanks for clarifying.

    Your pilot jet is too lean. Get a bigger pilot, drop the slide back to your settings you originally tried leave the needle on the bottom or 2nd from bottom clip, and air mix out 1-1 5 turns.
     
    peanut likes this.
  12. Jthomas

    Jthomas XS650 Enthusiast

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    Will do. Be right back
     
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  13. Jthomas

    Jthomas XS650 Enthusiast

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    2nd from bottom on needle.
    17.5 pilots (biggest I have)
    2 turns out on idle screws
    1.5 out on mixture screws

    Bike fires, stumbles then stalls out.

    I think I need a bigger pilot.

    Going to max the needle in rich position and try again.
     
    peanut likes this.
  14. Brassneck

    Brassneck XS650 Guru

    Yep, I'd go min of 20 for pilot...
     
  15. Jthomas

    Jthomas XS650 Enthusiast

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    Maxed needle out in rich position.
    Stumbles- won’t idle
    Installed the air jets (2.0)
    Stumbles a little more- won’t idle

    I’ll go pick some more pilots up tomorrow.
     
    peanut likes this.
  16. Brassneck

    Brassneck XS650 Guru

    There's a thread on here somewhere that shows a bunch of VM set ups that have worked (all based on each person's location, bike configuration, etc...) which boils down to each bike is different and what works for some may not work for others...that being said, there are a few common traits that seem to be consistent. The biggest one I saw was that most people ended up with pilot jet's being closer to 25... I copied this summary for my own purposes when I went to VM carbs this year... it helped me get a general starting point and what size jets to get in anticipation of tuning needs. Here's the list I came up with (Again, stolen from a thread which actually has a lot more info, but condensed for my purposes). Might help you as well:

    Mikuni VM combos

    MJ = Main Jet
    PJ = Pilot Jet
    NJ = Needle Jet (emulsion tube)
    JN = Jet Needle
    Slide = Cutaway number
    _________________

    Roy Fisk
    Carb: VM34
    MJ: 190-200
    PJ: 22.5
    NJ: P6
    JN: 6DHS, 3rd slot
    Slide: 2.0
    Air jet: 2.0?
    Air screw: 1.25- 1.75
    Float height: 25mm
    _________________

    Richard, from the Aussie site
    Carb: VM34
    MJ: 180-190
    PJ: 20-25
    NJ: P5, P6
    JN: 6F9, 3rd slot
    Slide: 2.5
    Air jet: Removed

    Dwayne, from the Aussie site
    Carb: VM34
    MJ: 180
    PJ: 25
    NJ: P6
    JN: 6F9
    Slide: 2.5
    Air jet: 2.0?

    Bob Hart, from the Aussie site
    Carb: VM34
    MJ: 180
    PJ: 27.5 - 32.5
    NJ: P5, P6
    JN: 6F9
    Slide: 2.5
    Air jet: Removed

    Yamaman
    Carb: VM34 with Ramair pods, Triumph silencers ,Halco 1 1/2" pipes.
    MJ: 185 - 190
    PJ: 25
    NJ: P6
    JN: 6DH4, 3rd slot
    Slide: 2.0
    Air jet:

    gggGary's Pristine, never had gas in them yet.
    Carb: VM34
    MJ: 180
    PJ: 25
    NJ: 159-P6
    JN: 6F9, 3rd slot
    Slide: 2.5
    Air jet: ?
    Float valve 3.3

    gggGary's Unknown
    Carb: VM34
    MJ: 260
    PJ: 40
    NJ: 247-Q2
    JN: 6DH4, Top slot
    Slide: 2.5
    Air jet: ?
    float valve 1.5
     
    TwoManyXS1Bs likes this.
  17. 650Skull

    650Skull Dinosaur Lives Top Contributor

    Brass, got a link to the thread.
     
  18. Brassneck

    Brassneck XS650 Guru

    Hmmm....let me see if I can find it.
     
  19. 650Skull

    650Skull Dinosaur Lives Top Contributor

    Thanks, I as looking through my bookmarks the other day looking for this info.......thought i had bookmarked it...........you know what thought, thought
     
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