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Anyone use #300 main jets on a MK3?

Discussion in 'Norton Commando Motorcycles (Classic)' started by jimbo, Jun 3, 2019.

  1. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004

    that's what I thoughto_O
     
  2. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Go up one size with your needle jet and drop the needle to the center notch and try it.

    You have lowered your compression ratio -you will need larger jets but not over 300 mains. Jim
     
  3. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    I will look into it.

    I replaced the busted sprag . Found the timing scale plate in the primary cover misadjusted, after degreeing the rotor . The MK3 scale has slots, screws and is adjustable( moveable too:eek:) .The scale marks are 2* each not 1* , so I was off about 8* then 4* .
     
  4. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    not sure comp ratio is much lower than before the rebuild because of the different pistons, and the shaved rebuilt head.
     
  5. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    Well that much timing will make a difference in plug heat...
     
  6. cliffa

    cliffa

    Joined:
    May 26, 2013

    jimbo, A little tip - Hold down the "Alt" key, then enter 0186 on the numeric keyboard and magically a degree º symbol will appear.


    Cheers,

    cliffa.
     
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Combustion temperatures and pressures are a balance between comp. ratio, ignition advance and fuel/air mixture. If you raise the comp.ratio, you need more fuel. That is the reason you get more power when you raise the comp. ratio. (It gives a slight advantage ). Main jets are only in use when the throttle is more than 3/4 open. If you are using full throttle and close it slightly and the bike goes quicker, you don't have a 'weak main jet', you simply have one that is too rich. Slightly over-rich main jets is good practice because engine damage through lean-ness usually only occurs when you are using full throttle. When you use full throttle, you often lose vacuum and the motor can starve causing detonation and burning of components such as valves and pistons. If your carbs are jetted to give maximum performance, you usually need to feed the throttle on in a controlled manner. If you grab a big handful, the motor gasps. The most important thing when tuning a motor is to get the needles and needle jets right. The mains should be rich enough to give some relief when you are fanging the bike.
     
  8. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    I am still confused, yesterday when I reduced the timing by 2 marks , or 4º it got worse( but still 4º too advanced because of the shifted scale ), blowing out my sprag
     
  9. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    With my bike, I know what size main jets I should use to stay safe, so I never do plug chops - even though if I did, I might get slightly more power. On most race circuits, you are never using the main jets long enough to have a major effect on the result. If you feed the throttle on in a controlled manner, the bike usually accelerates much faster than when you get the bike upright and whack it open. If you whack the throttle open, you lose vacuum. A bit depends on the size of your inlet port.
     
  10. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    bingo! I retimed the ignition to spec after adjusting the timing scale to read correctly. It still backfired, I was going to lose another sprag. So I just backed off the timing until it didn't do it. Now it runs like it did before the needed rebuild ,starts ,idles and does not destroy the starter gear train. I am not even going to look at the timing setting. Might have to change the 280 mains though.
     
    baz likes this.
  11. B+Bogus

    B+Bogus

    Joined:
    Jun 28, 2009
    Just a thought - have you checked your primary case timing marks against a degree disc from true TDC? I've found a few which have been more than a couple of degrees out.
     
  12. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    Yes I did that.The MK3's primary case scale is adjustable and was off. So I adjusted it to read correctly. But when setting the Lucas Rtta to spec it was still too advanced, kicking back wanting to destroy my sprag again. I retarded the timing until the bike was happy with it.
     
    B+Bogus likes this.
  13. jimbo

    jimbo VIP MEMBER

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    Sep 25, 2004
    might try a TriSpark as it is the closest(widest) curve to a oem points ignition. I think the bike was stronger with more advance but I will not sacrifice the consistent starting for that.
     
  14. B+Bogus

    B+Bogus

    Joined:
    Jun 28, 2009
    Trispark seems to have the best rep for low rev stability & not kicking back. Latest units appear to have got through the reliability issues of the earlier version, but I'll stand to be corrected ;)
     
  15. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    I believe elektronik saches has the same low revs stabilization (I could be wrong?) And one of its programmable advance curves is the same as points ignition aar
    It also has separate igniter
    I believe some of trisparks problems derive from living in the timing cover
    Some people swear by them some people swear at them
     
    Last edited: Jun 14, 2019
  16. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    The elektronik saches looks promising ( curve plots against the oem points?) , anybody have the curves for all these different systems for comparison?

    https://www.elektronik-sachse.de/sh...tion-zdg-3-12-for-bsa-norton-triumph-229.html
     
  17. jimbo

    jimbo VIP MEMBER

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    Sep 25, 2004
    bike was running poorly after the float level debacle , so I lowered the needles down,clips to the middle slot, had them at the richest/top position(clip in the lowest), too many backfires and blew out the sprag again!!!!:mad:
     
  18. jimbo

    jimbo VIP MEMBER

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    Sep 25, 2004
  19. htown16

    htown16

    Joined:
    Apr 29, 2009
    It is possible that the timing mark is off if the the outer part of the rotor has loosened on the inner part. I had a rotor that did that. Pull the stator and see if you can twist it back and forth.
     
  20. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    I checked the rotor for a loose center and it was good, I'll look at it again when I pull the primary apart , again . I did find the timing degree plate, not it the right place after I degreed the engine timing. The MK3's have screws and they can be moved a little. I just thought that maybe the rotor for the ignition is slipping. I'll look into that too.
     

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