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Runs well, plugs foul....

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

  1. pommie john

    pommie john

    Joined:
    Nov 18, 2005
    I spent ages chasing a problem like this, it was burning a little oil which was blacking the plugs. I had to change the valve guides.
     
  2. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Just overhauled the head....new guides and seals, thanks for the attention.
     
  3. PeterJoe

    PeterJoe

    Joined:
    Dec 8, 2011
    Does your carburetor have the stepped spray tubes that the 850's originally came with? If so, the needles that you replaced need to be the type that have the 4 inscribed identifying circles at the top of each needle. If your replacement needles only have 2 inscribed identifying circles on the top,that would explain an overly rich mixture no matter what position you place the needles.


    Peter Joe
     
  4. Danno

    Danno

    Joined:
    Feb 7, 2010
    Sounds like a deal. The Stay-ups are easily adjustable, having brass bracketry as opposed to the solid plastic stock floats, which require raising or lowering he needle seats in the bowls. In the old days,before the Viton-tipped float needles, those were white plastic (nylon?)
    also, and they only seated well when new.

    Too low
    [​IMG]

    Too high. bumps the bottom of the carb and won't quite shut the fuel off.
    [​IMG]

    Just right. with the gaskets on, the floats are barely visible looking at the bowls from the side.
    [​IMG]
     
  5. Danno

    Danno

    Joined:
    Feb 7, 2010
    My only concern would be that once you get the float level down a bit, you may miss those choke slides. Both of mine need choke to start and warm up a bit, the Titanic more than the SS, which has other mods.
     
  6. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Thank you for the photos and guidance Dan. Nothing beats a visual reference.
    Well, some things do I guess.
    Returning the chokes, if needed, is not a big deal.

    I’ll PM you and so we can find a time to get together.
     
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Start from the beginning. Check your float levels. Fit the richest recommended main jets you have (220 ? ). Get the motor idling with both slides at bottom. If the idle screws adjust the fuel, wind each one in until you get the miss, then back them off slightly. If they adjust the air, do the reverse. You should get smooth idling. From that point on, the rest is slide cutaway (usually number 3 s), needle jet and needles. Fit the clips in the centre groove of the needles. If you lower them both one notch, you should induce a cough as you open the throttles. If you get the cough, raise the needles back to where they were. If you don't get the cough, keep lowering the needles stepwise until the clips are in the top grooves. If you reach that point, you need a thou smaller needle jets, then repeat the lowering needles exercise. You should not need to fool around with the slide cutaway or any slow running jets. However the needles and needle jets are critical.
    The size of the main jets is irrelevant unless you are using more than 3/4 throttle. So use the biggest recommended mains unless you intend to fang the bike. Then you need to do plug chops to get the mains right. You should not need to fool around with the slide cutaway or any slow running jets. However the needles and needle jets are critical. When doing plug chps to set the mains, it is wise to use the next hottest heat range spark plugs to the ones you normally use. That way you stay safe.
     
    Dances with Shrapnel likes this.
  8. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If your bike is jetted properly, under normal circumstances, you should probably need the chokes until the motor warms up.
     
  9. napanorton

    napanorton VIP MEMBER

    Joined:
    Apr 19, 2009
    acrotrel - that might be the best explanation I've seen about how to dial in your needles and jets. Thanks! I might have to frame it.
     
    Dances with Shrapnel likes this.
  10. Lineslinger

    Lineslinger VIP MEMBER

    Joined:
    Feb 26, 2017
    Thanks for taking the time to offer the technique....it is appreciated.
     
  11. Onder

    Onder

    Joined:
    May 11, 2010
    There are those who believe idling eats the cam and followers. If you have no choke you will have to keep the engine speed up so a plus.
    Chokes arent really needed unless you live in a cold area or use the bike as transport, rather rare these days.
    Burlen offers plugs for the carb top if you delete the chokes.
     
  12. jbruney

    jbruney

    Joined:
    Jan 5, 2019
    I have never had call to use the choke, but then the coldest I've ever cranked in is perhaps 45F. Always used throttle.
     
  13. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    It is not good practice to rev a cold motor, When I start my 850, I use choke - kick it over and let it idle until it is warm. For me, it is probably worse than it is with you guys, because I use methanol fuel. But even if I was using petrol, I would not rev a cold motor. It is probably possible to crack a piston. That might be worse than wearing the cams and followers ?
     
  14. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I'm glad you liked the tuning sequence I posted. If you think about it, it is logical. There are two things - the jets meter and the vacuum varies as the throttle opens. That taper on the needles is about the variation in vacuum as the throttle opens at various revs, so it is a compromise for various types of usage. In some bikes, the ignition systems have advance curves which help adjust for the same circumstances. Normally your fuel octane rating is fixed, as is the comp. ratio - most of us use fixed ignition advance and adjust the carburation as the only variable. However there is probably something to be gained by playing with advance curves and different taper needles.

    One thing I have not said, is that with my bike I have never bothered to get the main jets right. I know they are probably a bit too rich and that is all that matters. On a race circuit, you are not on full throttle for much of the time, and with the methanol fuel I use, there is a much greater margin for error than you have with petrol.
     
    Last edited: Apr 1, 2019
  15. chris plant

    chris plant

    Joined:
    Nov 6, 2008
    can you pls tell me where the fuel level should be on concentrics by using this tool,ive done it on monoblocs with the pip on the float chamber cover but theres no pip on concentrics, at the moment its only going to tell me that they are both the same or ones higher than the other. thx in advance chris
     
  16. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    Al I agree with all of the above apart from your comment that you shouldn't need to fool around with the slide cutaway
    My commando 750 definitely prefers a 3 1/2 to a 3 cutaway
    My engine and exhaust system are stock,also stock carburetors but a k&n air filter
     
    ludwig likes this.
  17. jbruney

    jbruney

    Joined:
    Jan 5, 2019
    Don't really have to rev but that's another story and away from the OP's problem. I sincerely hope to see him come back with : Got it!
     
    baz likes this.
  18. CanukNortonNut

    CanukNortonNut

    Joined:
    Aug 8, 2005
    Chris,
    I install and try to get the tube held in a vertical position just as far forward to the front of the carb bowl. I use safety wire to hold it at the tickler. I try to get the top of the fuel level meniscus shape just below the gasket. Both carbs should be even and target for 1.5 turns out or close to that.
    I use a colourtune system to get my bunsun blue idle. I also never drop my idle below 1100 rpm. I want to feed my cam with oil and I am not trying to Put Put Put like a 6-1 Listor.
    Cheers,
    Thomas
     
    Last edited: Apr 1, 2019
    baz likes this.
  19. Danno

    Danno

    Joined:
    Feb 7, 2010
    Alan, is the "cough" what kills the motor as it appears to be idling ok, then sort of spits and dies? If so, I may have to raise the needles on the Titanic.
     
  20. Danno

    Danno

    Joined:
    Feb 7, 2010
    I don't know if either of these would start without choke, but with, both are one or two-kick starters. Crank the chokes up on either one too soon, and they both die. The SS clone, which has a little more compression, seems to warm up quicker.
     

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