Regarding Single Carb Setups On Commandos

Discussion in 'Norton Commando Classic Motorcycles' started by 1up3down, Mar 13, 2018.

  1. 1up3down

    1up3down

    Joined:
    Jan 12, 2011
    over and over I read people saying that a single carb, normally a Mikuni 34, cannot equal the horsepower performance of, for example, twin Amal Premiers because the single carb "cannot flow enough gas"

    if that were true then simply raising the needle or fitting a larger main jet would richen the mixture, thereby flowing plenty of gas, but that does not work for more power

    isn't the real issue the fact that a single carb set up cannot flow enough "air", not gas?

    and perhaps even more importantly, the single MIkuni's biggest higher rpm comparison drawback is that the air that it can flow is "tortured", disturbed with valuable droplets of mixture falling and sticking rather than being sucked straight into the combustion chambers as a twin carb/twin manifold set up allows?

    After spending a year with new Premiers I have gone back to a single MIkuni 34 on my 850 for the ease of instant starting with a really good choke and no gas tickling, rock solid idle with no both carb balancing, and equal if not better low and midrange performance all at the admitted expense of losing some horsepower above 5000rpm and having the top speed reduced about five mph, frankly I could care less about wanting to ride above 100 mph on the street anyway

    given all this, is there any way to significantly increase a single Mikuni 34's upper rpm performance? would increasing the manifold length permit a straighter mixture path, perhaps having to extend it back and into the stock air cleaner area?
     
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  2. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    Out of interest what was the problems you were having with the premier's ? I have had to go back to twin concentrics simply because my bike is virtually a first kick starter and with a single carb setup it is not.
    But I have to say I can't fault the twin concentrics for starting and reliable tickover etc ,
    They did take a long time to set up properly I had to cable tie the splitter box to the frame .
    I welded a threaded boss in the lh downpipe and fitted a lambda ,and marked the throttle position on the twist grip I spent a long time micro adjusting the carbs but they run so well it was time well spent, cheers
     
  3. 1up3down

    1up3down

    Joined:
    Jan 12, 2011
    ^^^^
    They did take a long time to set up properly I had to cable tie the splitter box to the frame .
    I welded a threaded boss in the lh downpipe and fitted a lambda ,and marked the throttle position on the twist grip I spent a long time micro adjusting the carbs

    ————————————————-

    I didn’t have to do any of that with my Mikuni -
    But my question had to do with any practical way to increase air flow, out of curiosity and not because I felt any desire to add 5mph to top end
     
    Last edited: Mar 13, 2018
  4. lazyeye6

    lazyeye6

    Joined:
    Feb 28, 2014
    I switched a pair of Amals vs a single Mikuni back and forth several times. My conclusion was that there
    isn't an iota of difference in performance up to 5000 rpm, when the dual carbs breath better. However,
    I almost never run my Commando up to 5000 rpms. I just want to have fun. I don't need to have a race bike.
    And, I must say that keeping 2 Amals synchronized requires dedication and patience.
     
  5. XTINCT

    XTINCT VIP MEMBER

    Joined:
    Jan 10, 2012
    +1
    I have had stock dual Amals, dual VM 34's (stock 850 & big bore race bike), and single VM 34.
    Single VM 34 is easy, clean, idles well, transitions smoothly, recommended.
    One more thing, I have never had a Mikuni slide break in half with the broken parts going out through the exhaust when I was 100 miles from home, causing the bike to only run at idle or wide open. It was an interesting ride home. I have still never been stranded by a Norton. If you have a VM 34 that doesn't start well or run well, it isn't tuned right.
    Definition: Carburetor: A French word that means "leave it alone".
     
    Last edited: Mar 13, 2018
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  6. baz

    baz VIP MEMBER

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    May 26, 2010
    Maybe moving the carburetor further back would help I really wouldn't know , I've had a single 32 amal twin 32 amals single mikuni cv32 single vm 34 mikuni single vm 36 and a tm40
    I couldn't tell any difference between the 34 and 36 VMs so boring the 34 carburetor bigger probably won't work?
    The other issue I had with the single VMs was fuel starvation at full throttle
    From memory just above 90mph I never got around to modifying the needle valve seat because I'd moved on to the TM40 that nudges 110 mph sitting upright and never empties the float bowl
    So I would say with the VM 34 make sure it's flowing enough fuel through the float needle seat and maybe fit a long bell mouth?
    It's a bit dodgy commenting on something you haven't actually done yourself maybe someone will chime in that has tuned a vm34 for more top end
     
  7. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    Hi, I remember Jim Comstock , advised fitting a vm36mm, single manifold , with an adaptator ( piece of plastic tube ) to compensate the diameter fitting difference, and said it improved the performance of the vm 34......
     
  8. concours

    concours VIP MEMBER

    Joined:
    Dec 29, 2011

    Set up the cable length once, 3 minutes. Done. Never moved.
     
  9. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    That's good going ,mine always dropped out of balance within a few thousand miles , cable tying the splitter to the frame and careful balancing with vacuum guages has helped with mine a great deal
     
  10. Nater_Potater

    Nater_Potater

    Joined:
    Apr 7, 2013
    'Same here. This is what I found back in '15; https://www.accessnorton.com/NortonCommando/90-of-carby-issues-are-clutch.19549/ As mentioned, it was a combination of a too-small seat (off of a fuel pumped system), plus a failing spring-loaded needle that kept the needle closed long past the point where it should have.

    Nathan
     
  11. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    In general, increasing the length of an intake manifold runner increases midrange and reduces top end. BUT that totally depends on the particular situation. For example, if the length of the runner is totally wrong for the engine, lengthening it might improve top end. Keep in mind that tuning intake runners (length) and exhaust headers for a street engine is basically about balancing the power output. IOW, you might adjust intake to provide the best power at, say 4000 RPM and the exhaust to do the same at 6000. On a non-computer-controlled engine, that concept is/was fairly typical If you set them up to provide max efficiency at the same point, you will have more power but very peaky delivery with no midrange...or...ONLY midrange, depending on the selected RPM points for the header/intake.

    Nowadays, many performance engines have variable length (actually, variable volume) intake/exhaust systems and, along with variable cam timing, can generate "race engine" power or be comfortable in the grocery-getter mode.
     
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  12. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    It depends on how you look at it. A single carb on a branch manifold is subject to twice as much vacuum as a carb in a dual system, so the main jet needs to be smaller than it is in twin carbs. Also the mixing is probably better in the single carb set-up. However in twin carbs, you have better ram effect. So it depends on whether you think in terms of flow or resonance and whether you believe a richer mixture delivers more horsepower. In the end best power is achieved when the mixture and ignition advance are set to operate just below the stage at which detonation occurs. So if the single carb is not jetted leaner, the bike will be sluggish. All of this including port dimensions, affects the torque achieved in the motor, so gearing is a factor which determines whether the bike will be faster or slower. If you get more torque you can run higher gearing, - if you get more top end, you can run lower gearing and get more speed by revving higher. Comparing the effect on top speed or acceleration, of changing between single and dual carbs without taking the rest into consideration, is not too bright. The way I would do it, would be to set the ignition advance, and the exhaust system - then jet the carbs to suit the new configuration. You will note that on Harleys which use the Dynatek programmable ignition system, the purple wire attaches to a vacuum switch in the inlet port, which goes to earth when there is no vacuum. It switches the system to a less aggressive advance curve to stop detonation. Their carbs must be jetted very lean - probably to get better fuel consumption while getting best power. Many Harleys have the purple wire permanently grounded.
     
  13. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Regarding the original quaetion; I believe the answer is yes in that the manifold is the main culprit to power loss. Therefore, a longer, straighter manifold would certainly help. This has been the case on other bikes I’ve tried, and I believe others on here have done similar on Cdos.
    The real root cause of the issue is the frame design. The frame does not allow longer, straighter manifold without cutting the central gusset away.
     
  14. Triton Thrasher

    Triton Thrasher

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    Feb 10, 2009
    People's standards vary. I wouldn't call that a problem.
     
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  15. baz

    baz VIP MEMBER

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    May 26, 2010
    Yes I know what you mean I think back in the day I would have just accepted a little bit of rough running where as now I expect the bike to have a perfect tickover at traffic lights and rev out without a hiccup,
    I spent a long time with the lambda fitted setting the carbs up just how I wanted them ,
    I had always blamed the amals for being worn out before but it was me not tuning them correctly
     
  16. gripper

    gripper

    Joined:
    Dec 3, 2010
    In the early days of single carb Commandos the single MK1 Concentric was always going to run short of fuel without some mod to the float needle jet. Most people who were going down this route were probably looking for an easy life/tickover/start. Running with a single VM 34 My exhaust had a noticeable crack to the exhaust compered with the twin 32mm concentrics and ran out of puff at about 85. It would be interesting to see a VM 34 with a velocity stack grafted on and a large foam box air filter (just for piece of mind)
     
  17. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I've had single carb and dual carbs on the one motor at different times and could not tell the difference. However I've also fitted a single carb to a twin motor in a race bike in a desperate attempt to get better mid-range power - it didn't help. The type of exhaust system and the cam timing have much more effect. I think most guys err on the side of caution when jetting their carbs. The slightest bit too rich, can make the motor sluggish. But it is always wise to keep the main jets slightly rich. - Get the needle and needle jet right. The motor might be slightly slower up high - but it will be safe.
     
  18. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    I mean if the carbs go out of balance after a few thousand miles, I balance them.
     
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  19. Craig

    Craig

    Joined:
    Dec 20, 2005
    I think maybe over these last several years with many threads on this topic we can agree that there are prolly 3 or 4 distinct groups with opinions .... what appears to be lacking is the understanding that we all have different wants , likes and wishes ... let’s just acknowledge that some prefer the original twin carb setup while others prefer the simplicity of a single , still another group warms to the flat slide in single and twin setup while at least one individual swears by FI .... it great to trade ideas and theories but in the end like most things , to each his/her own and I say more power to them ..... in my case I swear by the single vm34 Mikuni .... if I need more speed I ride a different bike
     
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  20. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    sorry i see what you mean
    Yep I guess you are right, people opting for the 34 VM are not doing it in search of speed ,when I was comuting 74 miles a day I had the 34 fitted ,but we are getting well away from the original question as to what could be done to improve the 34s top speed
     

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