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Runs well, plugs foul....

Discussion in 'Norton Commando Motorcycles (Classic)' started by Lineslinger, Mar 30, 2019.

  1. acadian

    acadian VIP MEMBER

    Joined:
    Mar 5, 2010
    Approx 5mm fuel level on one of mine, this puts it in the middle of the fuel level range given by Burlen, the float sits fairly parallel with the bowl top and about 1-2mm proud

    IMG_2831.JPG
     
  2. Onder

    Onder

    Joined:
    May 11, 2010
    The carbs don't sit level in service on the bike and I think you measure at the forward edge of the bowel.
    When I got my premmies from Burlen the float tops were level with the top edge of the bowel. I lowered
    them a bit.
    I could be wrong about this so Ill await correction by those who know.
     
  3. acadian

    acadian VIP MEMBER

    Joined:
    Mar 5, 2010
    I queried Burlen about this a few years back, here's the response I got:

    "
    Hi,

    Although we describe setting the fuel level by referring to the top edge of the float bowl, the place where the fuel height matters is in the jets in the centre of the carburetter. As the carburetter is tipped into a down draught position, the fuel rises in the front of the bowl, falls at the rear but stays the same in the jets.

    Kind Regards

    Phil Beresford"
     
  4. Danno

    Danno

    Joined:
    Feb 7, 2010
    That right there looks good to me.
     
  5. dave M

    dave M

    Joined:
    Oct 19, 2005
    The filter issues previously reported were NOT to do with poor air flow through the filter, but something to do with where the rubber interfaced with the carbs for mounting, or disturbing air flow into the carb itself, it will take you all of 5 minutes to run the bike without the filter unit on to see if this is the problem.

    An Amal is a fairly 'blunt' instrument and quite amenable to running considerably out of adjustment without the symptoms you describe.
     
  6. DevonNorton

    DevonNorton VIP MEMBER

    Joined:
    May 7, 2015
    Hello Lineslinger
    Just a thought but I wondered if the new throttle slides are too tight in the carb bodies.
    This will cause rich running.
    I had this problem with refurbished slides which the carb reconditioner had fitted with not enough clearance.
    The references I have seen, e.g. extract from John Healy doc attached, indicate a 3.5 to 4.0 thou clearance between throttle slide and carb body.
    Probably worth comparing with your old slides and measuring slide OD and body ID or gap if poss. You can get an indication of the gap using a feeler gauge (I used 0.1mm).
    Andy
     

    Attached Files:

    ludwig likes this.
  7. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    6D8AEEEF-E862-49B9-8EFC-52A378B97161.jpeg View attachment 9577
    Dave....Crossover pipe...snug connections...no leaks.
    Again a hotter plug is mentioned.
    New floats showed up yesterday. I am setting them .04 from top edge of float bowl.
     
    Last edited: Apr 5, 2019
  8. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Entire choke assembly removed during original carb rebuild.
     
  9. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Now have a clearer understanding of what you were advocating....easy enough to check.
    Thanks.
     
  10. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Thanks Andy.
     
  11. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Thank you to all of you offering up insight/help.

    New floats going in today and will head out to the airfield tomorrow for some testing with clean fuel and a fistfull of plugs to see what’s in store....
     
  12. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    Compared to the non-ethanol pump gas available here , the 116 oct. race gas mix I use improves the Norton in all areas of performance ... I strive for a 100 oct. mix and with no other operational changes the engine is transformed , single 34vm .... wishing you all the best results in your testing ... hope the floats are your fix ...
     
  13. acotrel

    acotrel

    Joined:
    Jun 30, 2012

    I usually set the ignition timing with a degree disc and once it is set I don't change it, unless I am prepared to start jetting again. If you add a degree to the ignition advance, that usually has the same effect as leaning off the jetting quite substantially. In the end the knock-rating of the fuel determines how much advance you can use and how lean the jetting may be. With methanol fuel, you have unlimited antiknock, so you burn a piston or get a seizure before you get detonation. I never race using petrol. With methanol and low comp., I usually set the ignition advance 4 degrees ahead of standard, then jet against that.
    With float levels, the actual fuel level is not usually critical as long as it is the same in both carbs or you actually care about what the recommended standard jetting is. With petrol as fuel, getting it really right is much more difficult than it is with methanol because the jets are much smaller. With methanol, you have twice the flow, so a small error in the jets dors not do much. And it hides up the tuning errors anyway.
     
  14. Danno

    Danno

    Joined:
    Feb 7, 2010
    I read years ago that the reason Nortons were timed at 28 degrees btdc was the erratic control of the twin points plate with bobweights and springs and using electronic ignition, with it's programmed advance curve, you could set the fully advanced timing at 32 degees btdc as long as you use decent fuel.
     
  15. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    Just my opinion here Danno but I think once the mechanical AAU is fully advanced, the only source of erratic would be from the cam chain and pinion drive in the timing chest. Beyond say 1,500-2,500 the AAU is pinned full advance.

    Sophistication of a stone ax.

    Jim Comstock once stated that the Commando (maybe race Commando) responded well to a little bit less ignition advance at higher rpm; this is relative to mid range advance.
     
  16. htown16

    htown16 VIP MEMBER

    Joined:
    Apr 29, 2009
    With a Pazon EI you set the advance at 31 degrees at 4000 rpm. They state in the instructions that it might go up a degree or so at higher rpms with no harm. Using 93 octane. Plugs are clean except for full ring of carbon on the base of threads. Pipes are blue. I'd say it is running as lean as you would dare go.
     
  17. acotrel

    acotrel

    Joined:
    Jun 30, 2012

    When you are operating at full throttle, there is less vacuum, so the motor tends to run leaner. With the Harley Dynatech ignition system, there is purple lead which is sometimes connected to a vacuum switch in the inlet tract. In other models, it is simply left floating. It's function is to switch the ignition to a less aggressive advance curve when there is loss of vacuum. With a programmable ignition system, you need two inputs - one is a measure of the revs, the other is a throttle position sensor, and if the motor has fuel injection, there is even more control. In Commando engines, the inlet port size is a factor. If the port size is large, the motor is more likely to suffer loss of vacuum. With petrol as fuel, you might get detonation.
     
  18. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Two-stroke ignitions are an extreme case where loss of vacuum causes problems. With fixed timed ignition systems in a 54mm stroke RD350 road bike, the normal advance is about 3mm before TDC. In a TZ 350 road racer, it is less than 2mm before TDC.
    When I tune my 850 Commando engine, I treat it in the same way I would any two-stroke motor. Max power is achieved slightly before destruction. With my bike, you need to feed the throttle on, you cannot just whack it open.
     
  19. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    Alan,

    It may more likely has to do with combustion chamber turbulence obviating the need for "as much advance".
     
  20. Danno

    Danno

    Joined:
    Feb 7, 2010
    More like two stone axes whirling in attempted synch.:cool: Once you get both sets of points breaking at 28 at full advance, then all you have to be concerned about is the equaliity of the springs and the friction on the bobweight pivots both being equal at all times. Now I'm hearing nightmare tales about cam runout a the seal surface influencing the points. It's all too much when you can plug in a box.

    I can see where a hi-perf engine being run up to redline in each gear repeatedly would like a little less advance.
     

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