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Mikuni flatslides?

Discussion in 'Norton Commando Motorcycles (Classic)' started by fiatfan, Jan 7, 2018.

  1. fiatfan

    fiatfan VIP MEMBER

    Joined:
    Aug 9, 2015
    From what I´ve read most single carb conversions is done using the Mikuni VM. Is there any obvious disadvantages using a flatslide instead? Have a good chance to grab a 34mm flatslide at a great price.
     
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  2. madass140

    madass140 VIP MEMBER

    Joined:
    Nov 6, 2011
    is the flatslide set up for your particular intended use?
     
  3. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    I agree with madass ,if it's not set up for your use or close to it then it won't stay a cheap carb, a 40mm TM flatslide is a great improvement on a commando , never tried a 34 but at a guess I'd say it would be better than a VM ,I have tried a 34 and a 36 VM and both gave my 750 lackluster performance I wouldn't bother with a VM again
     
  4. jaydee75

    jaydee75

    Joined:
    Jul 2, 2012
    I've tried both and it feels like the TM flatside is definitely better. I love mine and the reliable first kick starting with no drips is a bonus. Commnoz recomends the 36mm adapter with the TM34 and that's what I have. Love it!
    Jaydee
     
  5. Fullauto

    Fullauto VIP MEMBER

    Joined:
    Apr 13, 2009
    I have a TM34 flatslide waiting in the wings. I have the VM34 pretty much tuned to perfection and when the bike and I return from Melbourne in early Feb, I shall be having the bike dyno'd alongside a pretty standard 850 with a VM34. Then, I shall be fitting the flatslide to see the difference. I found everything I could on this forum about the TM, printed it all out and ordered a bunch of jets based on the users recommendations. So, I should be somewhere in the ballpark to start. After fine tuning with the TM it will be back on the dyno so that we will have a complete apples versus apples comparison. Jim Comstock says that the TM is good for about 10% in the low range and about the same power in the top end. so it should be an interesting comparison.
     
  6. jug

    jug

    Joined:
    Feb 14, 2015
    I have run the vm 34 round slide and the low to mid performance was great. The top end was a let down. I found the carb ran out of fuel up top, perhaps if a larger bowl were available it would make a difference. Bike would starve and start to break down when pushed hard.
    Don't know how a flat side would be any different if it's the same size.
    Jug
     
  7. Momus

    Momus

    Joined:
    Dec 14, 2017
    Jug, if you still have the Mikuni it would probably be worth looking at the float valve size and the float level. With the right settings the VM and TM carbs will comfortably support 50 HP from 500 cc single two and four strokes.
     
  8. jug

    jug

    Joined:
    Feb 14, 2015
    That's one is gone mate. Got a set of JS carbs to go on the new build.
    BTW I tried rejeting to no avail.
    Jug
     
  9. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Fuel starvation as described with a VM is definitely down to the float height in my experience.

    Nevertheless, even with that fixed, even a correctly set up 34 - 36mm VM on a Norton will inhibit performance above about 5k.

    How much is really due to the single carb and how much is due to the torturous manifold (dictated by space) is a question.

    I think Comnoz blamed the manifold for a lot of it, hence the suggestion to use a 36mm manifold on a 34mm carb.
     
  10. fiatfan

    fiatfan VIP MEMBER

    Joined:
    Aug 9, 2015
    Thanks all, the carb is not set up for a Norton, why would it no longer be a cheap carb? Expensive parts or no charts one can read to get the proper parts?
     
  11. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    Because if you have to buy a needle or a slide it will end up dear,it all depends on how cheap your cheap carb is to start with?
     
  12. dorset

    dorset

    Joined:
    Nov 11, 2014
    i use dual VMs on triumphs. two 34mm instruments are good for 117 mph with no issues. dunno how a single 34 would work on a norton, but that seems to be the common mod.

    tuning mikunis are easy. mains are five bucks, needles less than ten, needle jets about fifteen, pilots three or four. a slide is about twenty-five, but is seldom needed, in my experience. if you pay attention to the tuning, you can choose a needle jet and needle in one iteration, mains in a few, pilots in a few more.

    tuning any carb from scratch costs some money, but there's stuff on the net that can help.

    http://www.geckokustoms.com/blog/50-carb-jetting-table.html
    http://www.jba.bc.ca/Bushmans Carb Tuning.html

    one specific suggestion for 34mm mikunis on a norton is this:

    On a 850 Norton for a SINGLE Mikuni VM 34mm dia, use pilot 30-35, throttle slide 3, needle jet type 159, needle jet size P-4, P-6, needle 6DH3 or 4, Main 260, air jet 1.0.
     
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  13. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    Don't forget he is talking about a flatslide carb IE a TM I'd guess for example a slide won't be as cheap as a VM one,but things like jets would be the same price, a quick look showed slides about $42 on eBay there's not that much info out there about setting up a TM compared to all the info on VMS
     
  14. dorset

    dorset

    Joined:
    Nov 11, 2014
    i forgot we were talking about flatslides, and you're right about cost. the TMs are more expensive new. but they share mains and pilot jets with the VM carbs. different needle jets and odd needle tapers. still, the circuits are the same, and i know people that race them on triumphs. i'd start with a 300 main and 35 pilot, put the same needle in the middle with the same needle jet and test from there. nichecycle.com sells stuff for them.

    http://www.nichecycle.com/ncs/categ...ikuni-carburetors/flat-slide-carburetors.html

    i run 35mm keihin flatslide FCRs on a 650 triumph, and they have phenomenal throttle response, which is the whole reason for a flat slide. but they have accelerator pumps, too, so i dunno which is really responsible for the improvement.
     
    Last edited: Jan 8, 2018
  15. fiatfan

    fiatfan VIP MEMBER

    Joined:
    Aug 9, 2015
    The ones I´ve looked at (the guy sells two) seems to be a simpler model, no accelerator pump and no push/pull cable. So the price is probably not as good as I thought....around $180 or £135 for one carb.
     
  16. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    What are you planning on doing with the bike? I only ask because there's nothing wrong with the VM if you don't want to use the top end of the bikes performance,it's good for town riding you get a reliable idle, good economy etc, I have used many configurations, single amal , mikuni cv34 VM 34 a VM 36 and TM 40 the TM 40 is light year's away from all the others but you have to cut a section of gusset out of the frame to fit, nothing wrong with twin amals as long as they are not completely worn out, if you want performance stick with the twin set up or a single 40mm, cheers
     
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  17. fiatfan

    fiatfan VIP MEMBER

    Joined:
    Aug 9, 2015
    The plan is to recreate, dare I say it??? a copy, or at least as close as I can get, of my chopper (I said it!) that I built and rode for 10 years in my youth. Pure nostalgia. So I won´t be racing....I really like the idea of a single carb, easier to adjust and less stuff that can mess up. I did use an Amal MKII for a while back then, still have it in a box somewhere, and I also had a big SU or possibly a Stromberg? Don´t remember, don´t even remember how they worked, long time ago, RAM memory is almost full......
     
  18. robs ss

    robs ss VIP MEMBER

    Joined:
    Aug 16, 2016
    I put a TM32 on my 500 single which performs very well!
    Relatively easy to tune - although needle selection was time consuming.
    I even (foolishly?) went to the extent of using a dremel to polished out the Japanese branding so as not to overtly admit the clash of cultures.
    Still, as I said, it works very well on this head with virtually no downdraught.
    108mph with a 19 tooth front sprocket - have a 20 tooth to try when I get around to it.
    Cheers
    Rob
    IMG_1130.JPG
    PS - the white pushrod tube o-rings are VW items. I've found them to work better than Norton ones
     
    fiatfan likes this.
  19. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    That sounds good,if I were you I'd just buy a single manifold and fit a single concentric MK1 easy to tune,I have an almost unused SU phoenix kit on the shelf I may be tempted to part with,my bike ran really well with the CV mikuni fitted but didn't like going above 90mph without emptying the float bowl,this carb was one out of a bank of 4 from an fj1200 Yamaha I had to go up on the main jet and shim the needle , cheers
     
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  20. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    Are there any actual dyno tests (or, better yet 1/4 mile trap speeds) comparing a pair of Amal Concentrics vs the various Mikuni setups on the same stock Commando engine with no other changes?
     
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