1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

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
    On my newly rebuilt engine, and with twin 32MM amals, I seem to have a lean condition on wide open throttle. I started with 260's and now have 280s. Ordered 300's . Has anyone used #300 main jets on a MK3? Seems huuuge!
     
  2. htown16

    htown16 VIP MEMBER

    Joined:
    Apr 29, 2009
    Fuel starvation problem? Remove the carb drain plugs, put a cup underneath, open the taps. After one minute you should have about 300 cc's from each carb. Fuel level in the float bowls? Check with utube.
     
  3. gortnipper

    gortnipper VIP MEMBER

    Joined:
    Nov 11, 2013
    I have a mk2 with a stage one type motor and 32mm Premiers. On the previously lean side am running a 280, and am in the process of installing one of the Maney reproduction 2EX1 exhausts, so will be running a 300 on that side to start with.

    This guy seems to have run 300s on his with that same exhaust, and they were too small!

    https://www.accessnorton.com/Norton...tw-tour-850-barrels-needed.25460/#post-379279
     
  4. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    When I tried a Maney type exhaust I had to WEAKEN the needle on my FCRs compared to running stock pipes and peashooters, main jet stayed the same.

    You’re probably safest trying the 300s, just don’t be surprised if that changes.
     
  5. gortnipper

    gortnipper VIP MEMBER

    Joined:
    Nov 11, 2013
    Yeah, I know wherever I start out probably wont be where I end up!
     
  6. gtiller

    gtiller VIP MEMBER

    Joined:
    Nov 5, 2012
    220 mains, 106 needle jet and number 19 pilot jets over here.

    The only thing non-standard is the pre-mk3 ham can instead of the plastic airbox

    (with the plastic airbox they were jetted at 200/105)


    300s seem waay over the top!
     
  7. Danno

    Danno

    Joined:
    Feb 7, 2010
    I too, would suspect fuel delivery. Have you made the cut in the fuel inlet boss of the float bowls?
     
  8. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    What is that?:confused:
     
  9. tricatcent

    tricatcent

    Joined:
    Jul 30, 2009
    I agree with Danno. Nortons need the modified float bowls. The huge main jet won't properly correct this problem
     
  10. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    Its the first I have heard of that, (been around a while)please explain.
     
  11. Deets55

    Deets55 VIP MEMBER

    Joined:
    Oct 3, 2013
    903E4FE0-D28F-455B-826E-3A07CF3423C8.jpeg
    There is a BSA service note with the specs. I can’t find it at the moment
    Pete
     
    nortriubuell likes this.
  12. Deets55

    Deets55 VIP MEMBER

    Joined:
    Oct 3, 2013
    6BCBA9BE-A646-4A66-9CEE-B495E67D09C5.jpeg
    Got it
     
    nortriubuell likes this.
  13. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    I can do that, but I never seemed to need it for decades
     
    baz likes this.
  14. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    I'd be interested to know if anyone has done this on a commando and the difference it made?
    Cheers
     
  15. illf8ed

    illf8ed

    Joined:
    Dec 30, 2003
    Hi Jimbo

    Before cutting the fuel bowls I’d follow htowns suggestion to see if the fuel is flowing to the bowl. What petcocks are you using? The Taiwan copies have a problem with the rubber seal inserts rotating and restricting gas flow.
     
    baz likes this.
  16. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    I checked the pingel(s) they flow fine. tank cap breather is clear. Just wondered if 300's were unusual
     
  17. Deets55

    Deets55 VIP MEMBER

    Joined:
    Oct 3, 2013
    Baz,
    I my case I was chasing a problem something like Jimbo. Everything indicated I was having a fuel flow problem at high RPM, high load. The slot did not solve the problem. The issue wound up being in my Boyer MKIII ignition box. Bike ran great except for top gear, high RPM acceleration.
    Pete
     
  18. jimbo

    jimbo VIP MEMBER

    Joined:
    Sep 25, 2004
    wow, how do you explain that?
    When I had the bike WOT ,then closed the throttle just a little , it seemed to pull harder, that's why I think I need more jet
     
    nortriubuell likes this.
  19. Deets55

    Deets55 VIP MEMBER

    Joined:
    Oct 3, 2013
    Jimbo,
    Maybe the transistors were breaking down under load? Intermittent internal connection?
    I have no idea. Only found the problem about a month later. Bike stopped running when I pulled into a parking spot. I ripped everything apart in the parking lot and couldn’t get it to go. I was getting ready to call for the hook and decided to tap the ignition box with a screwdriver handle. Bike started and I drove home. Replaced the box with a MKIV. Bike ran fine and my “fueling” problem was gone. That’s also when I found out cheap DVM are terrible for resistance readings.....but that’s another story
    Pete
     
    nortriubuell likes this.
  20. cash

    cash

    Joined:
    Nov 10, 2006
    An over advanced ignition can give the same symptoms.
     

Share This Page