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Help Mikuni VM 26 on GSX250

Discussion in 'Suzuki 250cc Twins' started by Evo, Jan 20, 2016.

  1. Evo

    Evo Well-Known Member

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    image.jpeg image.jpeg image.jpeg So just before Christmas I bit the bullet and order a pair of VM26.
    Picked them up last night and had a play around this arvo.

    They came with a 60 pilot and a 190 main

    Chucked them on this arvo and fired up no dramas, much to my relief. I then started to play around with the air screw and idle screw. Was able to get it started, and idling but only just. Was 1/2 to 3/4 turn out on the air screw but then had the idle screw all the way in.
    Any other combo,and I tried quite a few didn't seem to run with out choke. Had a few other strange scenarios such as much higher RPM with very small changes in the air screw. Will have to spend a bit more time on this.

    Happy for any and all ideas, suggestion, recommendations in terms of jet size and the like.

    Will post updates on my progress. Not expecting this to be quick process.
     
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  2. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Hooray! Someone bought some VMs
    Sounds like starting with bigger pilot jets would be a good idea. The air screw does not make a great deal of difference. It is nothing like the fuel screw on the CV carbs.
    You will notice in the manual where they suggest a bias towards the rich side (better to choose 1.5 turns out rather than 2 turns out) it indicates that your screw should be somewhere near that position. You cannot go more than 3 or the screw may fall out.
    Don't change the float level.
    Do you have the manual http://www.mikuni.com/pdf/vmmanual.pdf ?

    Pity you could not get a left and a right. Are they 4-stroke versions?
     
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    Last edited: Jan 20, 2016
  3. Evo

    Evo Well-Known Member

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    Thought this would make you a happy man @maelstrom

    Was thinking the same re pilot jet. Any suggestions on size?

    Yes. I do have that manual. It's very easy to read and has cleared up so many things for me.

    As for the idle screw, how should this de set? Should you aim for the screw having no effect on the slide, a bit, lots?

    I am lead to believe that they are the 4stroke version as it was discussed with the shop I bought them from as to what bike they were going on. Is it just the jets that change?

    I'm very new to this so please feel free to educate me.
     
  4. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Cable synch et al
    • Ensure that there is an even amount of slack in both cables and test the handlebar turn from one side to the other to make sure that there is still cable slack. You can set that the synch is identical by opening to full throttle and feeling the slide is flush in the throat on both carbs at the same time.
    • The idle screws should be very responsive once they start to lift the slide.
    • The slide cutaway is very, very important. If it is too large (lean) the carbs will make a gasping sound as you lift off idle. It almost sounds like they are about to backfire. Sorry, it is very difficult to describe, but once you know what it sounds like you will know forever.
    • When you go from a lean slide to the correct one that sound disappears completely and the response is instant.
    • I would buy a range of sizes of the pilot jets. Perhaps 65, 70, 75, 80, 85 & 90. I saw them on the web for $3.21 each so better safe than sorry. VM22/210 Series I believe.
    • With the pilot and slide cutaway done I would jump straight to main jet.
    Soldier on you will succeed :)
     
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  5. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    You may be the first to have a GSX run properly without the airbox.
     
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  6. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    I forgot to ask. Does the engine have good compression?
     
  7. Evo

    Evo Well-Known Member

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    Both cylinders at 155psi

    Thanks for the input. Feel free to continue.
     
  8. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Well make sure you get filters/pods that will flow enough air to feed the engine. Something for a 750cc engine should be suitable.
    I wonder how many people have put those tiny little pods on and found that they could not get over 60Km/h and started fiddling with the jetting?????
     
  9. Clancy1

    Clancy1 Well-Known Member

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    Why did you only do 26mm? Aren't the stock CV carbs 30mm? or do these flow better?
     
  10. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    I have never found the article again but remember reading about someone having 'success' with elbowed pods.

    Something like this

    pic31_7.jpg
     
  11. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Those filters are WAY too small for anything but a lawnmower.
     
  12. Evo

    Evo Well-Known Member

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    This is what was recomended and if you look at "fig11" on page 7 of the link below it looks correct. GSX 250 hp is quoted at 45hp or 22.5 per cylinder

    http://www.mikuni.com/pdf/vmmanual.pdf
     
  13. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Obviously .... its just to show the elbow concept
     
  14. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Yes, I seem to recall reading somewhere that CV carbs are usually oversize to compensate for their poor flow characteristics. If Sudco recommend the 26's then that should be a solid place to start. They have been in the carb business for an eternity.
     
  15. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Intake Length:
    Looking at your carbs and mounts I have to say that I think your intake length is too short which will exacerbate or create a flat spot in your midrange. You can find some rule of thumb calculations on the web for calculating intake tract length.

    Edit:
    David Vizard's rule of thumb for intake length.
    The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.
    So 26cm for the 8000rpm torque peak of the GSX. That sounds good to me. In your case it is from the inlet valve face to the end of the bell mouth.



    Anyway the two solutions are to lengthen the intake manifold and/or add velocity stacks. I do not know of any over the counter stacks for the Mikuni VM26 but you need something that looks like this.
    [​IMG]

    I found these on eBay
    http://www.ebay.co.uk/itm/YAMAHA-MIKUNI-VM-CARBURETTOR-VELOCITY-STACK-OPEN-BELL-MOUTH-/171046706115
    The flush fit to the throat, small taper and well formed ring on the end look perfect to me but it does not show the opening for the pilot feed etc. Those openings would need to be external. Here is an article about the shape
    https://www.highpowermedia.com/RET-Monitor/3780/the-air-trumpet

    The book "Four-Stroke Performance Tuning" by A. Graham Bell has a lot of useful information and a tuning section on Mikuni carbs. He has a handy hint for choosing needle jet (emulsion tube) size that I think is a good idea.

    Fitting the air filters will be another can of worms.
     
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    Last edited: Jan 22, 2016
  16. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    May have to fit foam socks over the bellmouths.
     
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  17. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    + what Murdo said.
    But it would be good to have enough clearance between the end of the bellmouth and the wall of the sock. You could create a backing plate assembly and then have one large oblong filter. Anyway first things first :)
     
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  18. Evo

    Evo Well-Known Member

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    Rrrrrrrriiiiiiiggggttttttttttttth. Starting to think I have bitten off more than I can chew.

    Was I naive to think that I could bolt them on, whack on k&n pod filters, play around with the pilot and main jet then when I feel that I have it running well possibly get it dyno tuned?
    Or am I way off the mark?

    Please be honest
     
  19. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Not at all. Do as you have planned and it will be good.
    Bellmouths and big airfilters will help to get you the last tenth of performance from your new carbs. Definatly get it dynoed to get it all spot on and go enjoy the ride.
     
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  20. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    +1 Murdo
    I don't think you have been naive. I think you have taken a considered and logical approach to the task. Of course there is a learning curve but that is part of the fun. It is not too difficult, and you have well priced carbs for which a complete range of tuning parts are available. Dyno tuning will be good if they have the parts that are needed to tune the carbs and the operator has a good understanding of tuning carburettors.
    Consider my advice the extreme case and what will work will most probably be not so bad :)
     
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