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carb angle

Discussion in 'Other Norton Motorcycles' started by seattle##gs, Mar 20, 2018.

  1. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    does anyone know what the carb angle is on an Atlas motor? I want to re-angle a commando manifold so the Amals sit level. I am sure I am not the only one to try this so if there are complications ahead, please advise. Just eye-balling, I'm guessing it is about 28 degrees. What's the angle of a Commando motor? I made a fixture to hold the manifold and with an angle vice I can be pretty accurate in the mill. I have a few manifolds to play with.
     
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  2. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    If no one chimes in, I will measure my Atlas and PM it to you, but I am one week away from home.

    As I remember, the Atlas manifold front and rear surfaces are square to the bore. Thus the carbs sit at an angle determined by the head.

    Slick
     
  3. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    you are correct about the Atlas manifold surfaces being parallel. The carb angle is quite steep and Norton cured this with the Commando. The commando manifolds are close to working on the Atlas but they are angled a bit too much and I am hoping to correct that in the milling machine.
     
  4. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    As I see it, as an aerodynamicist, the only benefit that might result from levelling the carbs, is to put a curve in the airflow stream. This curve will tend to increase mass flow outwards from the inside radius where the intake port turns sharply to meet the valve. Whether this improves performance, only a dyno will tell for sure.

    Let us know what you find.

    Slick
     
  5. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    Float level is the problem. With the carbs at a steep angle the float level becomes critical....it quickly becomes too much or too little.
     
  6. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    The problem with the Atlas is although Norton used the downdraught head, in the F/B frame the 750 was tilted BACKWARDS in the frame!
     
  7. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    Doesn't look very tilted to me.

    [​IMG]

    Why would that be a problem anyway?
     
  8. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    It looks very tilted to me. Compare to a Commando. Float level is important when balancing dual Amals, it's the first thing to do. A carb at this angle will have the correct level up front near the head but very low in the rear so setting a float level is touchy. I realize it will run well like this.
     
  9. robs ss

    robs ss VIP MEMBER

    Joined:
    Aug 16, 2016
    I just put an inclinometer (an oil damped Suunto) on my 650SS carbs and got 15 degrees
    This would figure because I recall that number for the Works or Manx Nortons.
    Cheers
    Rob
     
  10. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    The Atlas engine does not look tilted backwards to me.

    And why would a tilted back engine cause a problem?
     
  11. robs ss

    robs ss VIP MEMBER

    Joined:
    Aug 16, 2016
    I agree Triton Thrasher - my 650SS top of barrel face is dead level
    BTW - the 60's heavy twin featherbeds all had the same frame (same part no.)
     
  12. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    I agree setting the float level to the correct value is the first thing to do when setting up the carbs, whether dual or single.

    After that, I am in disagreement with the remainder of your statement. The float level that matters to the carb is the vertical distance from the spray tube to the fuel surface. The fuel surface is level, and it does not matter if the airflow is parallel, or not, to the fuel surface.

    The main determinant of float level during engine operation, is the orifice diameter of the needle and seat. The larger the orifice, the less the fuel level drops during high power bursts.

    If you level the carbs, I estimate the vertical distance from the spray tube to the fuel surface will increase by 12% if the carb angle is 28 degrees. (Cosine 28 deg = 0.8828)
    To keep the same air/fuel ratio, the float level should be increased a commensurate amount. After that, it is up to the fuel delivery system, princpally the needle and seat, to maintain that level.

    Fuel flow for various orifice sizes (with a particular fuel tap) is in the following thread. The chart should be taken as relative changes, for a non specific fuel delivery system.

    https://www.accessnorton.com/NortonCommando/flow-measurements-on-ewarts-fuel-tap.21225/

    Slick
     
  13. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    Well, it appears that quite a few of you were NOT AWARE that the Atlas engine is tilted slightly backwards; this info is in one or two books of the Norton bikes. It was done so because the Atlas engine was slightly bigger than the other twins fitted to the f/b frame. You have measured the top of the barrels on a 650ss but have you measured the top of the barrels on an Atlas?
     
  14. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    Ok, but what is the angle of tilt and what running problem does it cause?
     
  15. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    I have put many miles on my Atlas with the concentrics bolted up to the head, no leveling involved. Stock angle. I am working on a theory here...Commandos bothered to level the carbs. Other carb manufactures, Mikuni for one, have gone to great lengths to level the float chambers while retaining a steep downdraft angle...I figure they did it for a reason. So...would leveling the carbs on an Atlas provide positive results? The only thing to do is try it and see. Perhaps the Britts angled the carbs to make them look racey...perhaps it was a space problem. I do not know their motivation. I sure Amal designed (?) their carbs to work dead level. It was up to the bike manufacturer to fit them.
    So, can it be improved? I know that it takes quite some time to get a new pair of Amals working exactly the same. Would it be less touchy if they were level? Would they stay in tune longer? I'm certain they would carburate more reliably going up and down hills if they were level.
    I won't know until I do it. I expect some rejetting and setting the float level. That doesn't bother me. There is very little to tune with anyway.
    The two of us are working on an Atlas project with many modifications. For the start up and the first 200 miles we are going to use a single Mikuni. There is an aftermarket oil tank so there is miles of room for an air cleaner. I came up with an ingenious air filter for a single mikuni on my stock Atlas. The connection between the air cleaner and carb spigot is so close that I use a strip of duct tape to make the connection.
    Did not know the Atlas motor was tilted backwards.
     
  16. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    Hi seattle , when you will found out the angle could you post it please, as my Atlas is running with two amals , but as I want to test it with single mik, and because the cdo one are easier to find, that will help me (and may be others , thanks !
     
  17. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    Will do. I have a feeling that there is too much of a difference in angle between a Commando and Atlas mikuni manifold for shaving and adapting. Try an ad in the "wanted" section of this forum.
     
  18. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    I have the head off my Atlas, so it it easy to measure the angle. Using an adjustable protractor, I measured the angle of the manifold face on the head, to the gasket surface, to be 20 degrees more than perpendicular.

    Regarding the Atlas engine tilt:

    I put a spirit level on the upper frame tubes (the lower tubes being rather inaccessible) and shimmed the bike on a frame lift until the upper tubes were level. Next, I placed two pieces of hex key stock fore and aft of the spigots, a distance of 3.25 inches apart. I then transferred the level to sit on these two pieces of hex stock (this got the level above the spigots). I found the level was low on the rear side of the bike and using a thickness gauge, shimmed the level until the bubble centered.

    It required 0.020 inch shim to bring the the bubble to center. The tangent of the angle thus formed is 0.020/3.25 or 0.00615, and the angle (arctan) is 0.35 degrees. I do not know if the lower frame tubes are parallel to the top. If they are not, the angle may be more or less, referenced to the lower tubes. If the engine is indeed intentionally tilted, the angle should be referenced to the frame in some way. Referencing the angle to the ground is meaningless, as the tire profiles, inflation pressure, and rear shock position would influence the tilt.

    An angle of 0.35 degrees is so small, it could be considered the result of manufacturing tolerances. If someone knows the true angle (parallelism) of the upper and lower frame tubes, the tilt can be re-calculated from my data.

    Slick
     
  19. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
  20. dynodave

    dynodave

    Joined:
    May 28, 2003
    Extrememly long ago project... this one.
    atlas 28 degrees
    edit: ( remeasured... now concede 21 degrees with level barrels)
    , commando 10 degrees at manifold.
    Only one thing I consider...what is the FUEL level at the center of the carb i.e. emulsion/jet tube.
    I have never been convinced the floats are symetrical and have used a fuel sighting tube to verify fuel level entirely in line with the center of the carb. Why would anyone care about fuel or float other than where it matters?

    Atlas engine tilted? 650ss and atlas cases are exact same casting#. Barrel height the same. head casting is the same, only the 4 bolt pattern is wider. engine plates different???? Different head steady bar part# length longer for atlas?
     
    Last edited: Mar 28, 2018
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