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

Runs well, plugs foul....

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

  1. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004

    Then you certainly misunderstood what I said.
    As Bob Rowley states below, the fitting of stepped spray tubes was required in order to pass a roll-on acceleration test when mutes were fitted (as mutes were normally included in the crates and fitted by the dealer if local noise regulations required) therefore, it appears all bikes had to comply regardless of whether mutes were eventually fitted or not.

    The test did not, however, fail when mutes were NOT fitted as Bob R says "...this was fine without the mutes fitted,.." so = no mutes-no problem.


    "A new acceleration test was introduced for USA and from memory the test was 40 mph snap open throttle in top gear, this was fine without the mutes fitted, but with the mutes
    ( small disk fitted to the end of the exhaust silencer ) , this caused a [????] the muted condition, no matter what you say our suggest the final cure was to install a cut away spray tube, this allowed a much smaller main jet to be used this in turn stopped the eight stroking when trying to pass the USA acceleration test, think 40 to 60 mph in a certain time.

    It is not difficult to see how this worked, FOR THE MUTED CONDITION

    Most times with the mutes fitted we would snap at the 1/4 mile point at 30 and 40 mph and be doing the same speed, 30 or 40 mph at the timing lights, after 1/4 mile, of course you could feather the throttle back and judicially pick up speed and improve, but the idea was that a person when accelerating passed a long vehicle could not be expected to intervene and should expect a solid reliable acceleration.

    You may not be aware that AMAL did not set up the Carburettors for the trade, BSA, TRIUMPH, NORTON but development worked closely with Amal, and the responsibility for final sign off and payment was down to the Test and Development department of each of the firms, as Deputy group test rider under Bob Mann's we carried such responsibility.

    It's quite wrong to suggest that we would have changed or introduced things for no good reason.

    One final thing that would vindicate the modification was the fact that we had over 150 bikes held up and not allowed in California test Authority and after the modification they and all subsequent Norton's passed straight in.

    I am not sure if Alan Lines? of Amal is still alive but he would no doubt give you chapter and verse on how and why the cutaway spray tube was beneficial to the MUTED CONDITION.

    One day I may find my notes on this mod but in the meanwhile could I ask you to at least give some benefit of the doubt, the cutaway spray tube was a category 'A' modification for all machines that required Mutes.


    Bob Rowley "

  2. dynodave


    May 28, 2003
    The passing test seems to be written a bit confusing. Is this what he's claiming?
    You are driving down the road 4th gear 30-40 mph (1500/2000RPM), then snap open the throttle (all the way) and have only 1/4 mile to gain 20MPH up to 60MPH. This would simulate a 1/4 mile pass?

    Their original test results would cause 8 stroking (super rich?) and would not get up and go??? and only way to accelerate is you close the throttle? Which is not allowed by the test rules?
    I don't believe it!
    I can see a failure to pick up (minimal) fuel at all (almost complete loss of vacuum at fuel pick up), and would sporadically keep running while coasting. This would be lean fade out, bogging, not 8 stroking rich.

    Muting would restrict the air flow through the whole system and make it even harder to get the engine to pick up/atomize fuel/air due to the mute flow restriction. If it can't go out, it can't get in ... A richer spray tube would give a little shot more of fuel and make a few HP to push the air out of the mute and go down the road for the 1/4 mile roll-on up to 60.
    Last edited: Apr 9, 2019
  3. acotrel


    Jun 30, 2012
    The ability to whack the throttle open, might be the reason some bikes have pumper carbs. My experience has been, that if the carbs are jetted for best performance, you usually need to feed the throttle on - in one strong steady squirt if you want max acceleration. If you do that with a properly jetted motor, the effect is probably better than you get with pumper carbs unless the pumps are very well controlled in the amount of fuel they deliver. The problem is the loss of vacuum which is caused by large throttle openings combined with low revs. It causes the mixture to be too lean, so the motor simply gasps.
    There is also another problem which can occur. Sometimes the mixture is fully controlled by the needles, so the main jets never operate. If you do plug chops to set the mains, when you fit a bigger main, you should get a change in the plugs. If you don't, it might be necessary to recess the needle jets and meter lower down, so the tip of the needle always remains trapped and does not hang-up the slide.
  4. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004
    To go back to:

    When I bought my 850 Mk3 it had non-standard 932 carbs with flat top spray tubes, 2-ID needles, 3.0 slides, separate pipes and peashooters.
    Later, a balanced system was fitted (after one pipe fractured) which made no apparent difference to performance or carburation so I'm sceptical that the difference between separate and balanced pipes has all that much effect on breathing.
  5. ludwig


    Aug 30, 2006
    "..IMO " ?

    Not good enough . Proove it ..
  6. dynodave


    May 28, 2003
    Rowley/Norton/Amal They don't give proof either!
    Did individual pipe 750 pass the "overtake/passing test" yet fail with mutes?

    As I've said on occasion the 32mm 750 carb (configuration) can wholesale be swapped with the balance carb setup since it is rich enough across the band. I base this on the balance pipe set up being lean and usually idle warm up benefit from the choke use. The balance pipe setup is noticeably leanish in comparison.
    The 750 conventional set up is very often stripped of the chokes (not that I recommend it).
    I'm pretty confident an exhaust analyzer would show a difference at the low rpm balance pipe operation. That would of course have to take into account the exhaust note level.
    Balance pipe has been used and proven for decades in nascar and in my own dual exhaust truck from 73 until 94 for 160,000 miles. New it got 8mpg (dealer said I would never see 10MPG) I got it up to just under 16mpg. I believe balance pipe physics is not a myth...
    It's only science and instrument measurements will ultimately tell the truth. That has been my whole life career's work and sometime even now in retirement.
  7. Lineslinger

    Lineslinger VIP MEMBER

    Feb 26, 2017
    Thanks for taking the time to share your experience on my carb issue
    If I ask the question I never ignore an answer whether I like it or not.

    I finally got my carburetion quirks figured out.

    It was a combination of components/settings adding up to my fouled plugs.

    Danno advised I change from fixed nylon bowls to adjustable bowls, and how to adjust, good change.

    I attained a clearer insight as to how important float bowl adjustment can be.

    I switched back to the 3 groove needles from the four groove...
    and here is where I was getting ambushed. The circlips supplied for holding the 4 groove needles in place were slipping up past their seated position and getting hung up on the spring. I was never getting an accurate setting because the damn things kept moving within the slide and needle jet.
    So sometimes they would hang up during a run and sometimes would drop back down into proper position. Now, I should have more carefully inspected these when I installed them, but I trusted the vendor who advertised they were applicable to 932's.

    I figured this out after a day of frustration by pulling the bowls and then the jets while in place on the engine...and saw that the needles were not equal length.

    Once again I pulled the slides and this time I found a needle/circlip hung up on the internal slide spring.

    So as o0norton0o, Acotrel and others recommended I returned to all the original "book" settings I had started out with including a return to original the 3 groove needles with the circlips mounted in the #2 groove position.

    I have properly set floats, properly adjusted needles and air screws and am running a hotter plug.

    Now, after some tweaking of idle adjustment and air screw the engine starts on the first or second kick, idles wonderfully and was still pulling strong in excess of 90 MPH. I got caught too. The police officer let me go. He also rides a motorcycle.

    Thanks guys for letting me in on what you have learned and sharing it.
    Now I have go figure out how to keep the chain guard from rubbing the chain.
    Last edited: Apr 10, 2019
    Dances with Shrapnel, Craig and baz like this.
  8. baz

    baz VIP MEMBER

    May 26, 2010
    Wow this post is getting way off track !!
  9. dynodave


    May 28, 2003
    Unbeknown to you (I guess) except for acotrel blab on pumper carbs, that will not advance the amal carb 74 850 with balance pipes solution.
    How the nonstandard amal jetting will affect the more or less standard balance pipe 850 does seem relevant.
    Feel free to add something useful.
  10. bill


    Jun 1, 2003
    most of this is very confusing. first is how you describe the needle ID. both 4 ring and 2 ring needles have 3 adjustment grooves. second you NEVER said which spray tubes are in your carbs as this is what dictates which needles are to be used. the problem you had with the needles not staying put on the clip sounds like junk aftermarket parts and i dont see how the needle was getting hung up on the spring.

  11. o0norton0o


    Apr 27, 2015
    Hey Great! Your persistence paid off. FYI, the circlip getting caught in the spring rather than getting seated below the spring does happen sometimes upon reassembling the carb bodies. I've done it myself before.
  12. DevonNorton

    DevonNorton VIP MEMBER

    May 7, 2015
    Well done!
    I have also seen the problem.
    If it occurs again I have a couple of suggestions for prevention.
    Norton Owners Club supply a clip which greatly reduces the likelihood of the clip getting caught by the spring.
    It is difficult to fit compared with the standard one as it has the throttle cable passing through it. I found it took 4 hands to fit!
    Also a shim can be made to fit between the standard clip and the spring. I'll try and dig out the details. [edit - M10x16mm 0.5mm thick shim washer. ID opened up with Dremel to 12mm] This means there is a circular face above the standard clip and below the spring so no chance of 'catching'.
    I was struggling to match this problem to the symptoms but this is a known problem - maybe wider than generally thought!
    Last edited: Apr 10, 2019
  13. jbruney


    Jan 5, 2019
    Great use of foglights Lineslinger to make your way through all of this and arrive at a solution. I got so twisted that I forgot what the issue was in the first place.
    So much experience bouncing around is confusing and difficult to collate.

    I've got those. They weren't a great pleasure, but did work.
  14. Lineslinger

    Lineslinger VIP MEMBER

    Feb 26, 2017
    In regards to needle hang up please see the photos posted by DevonNorton.
    I made the jet and needle purchase from a reliable supplier I used throughout my restoration. If you want to tell them about your “junk aftermarket” opinion I can PM their contact info to you.
    Sometimes $#)t happens.
    I did my best in trying to explain issues via the written word...very often two people can both read the same sentence and come away with two entirely different understandings of its intent.
    I assumed that when I made note in the beginning of this novel “Standard Amal 932’s” that would be explanation enough in regards spray tubes.
    Hope this helps.
  15. bill


    Jun 1, 2003
    yes please PM me the supplier you used as i have never had that problem. as to standard 932s there is NO STANDARD 932. the 72 750 combat and 73 750 had 32s with a flat spray tube. than we have the 850 with 32s and the cutaway spray tube. it sure makes hard to diagnose with incomplete info.
    you still have not confirmed what needles you have as to 4 ring or 2 ring NOT groove.

  16. Lineslinger

    Lineslinger VIP MEMBER

    Feb 26, 2017
    I’m in a pretty good place with it now.
    You be well and thanks for the attention.
    Dances with Shrapnel and baz like this.
  17. jbruney


    Jan 5, 2019
    Good fortune with it and log some riding time. I believe you've earned some stress free miles.
    baz likes this.
  18. baz

    baz VIP MEMBER

    May 26, 2010
    Good to see you got it sorted
  19. htown16


    Apr 29, 2009
    With your carb issue solved you might try going back to the original plugs and see what if anything happens. Running a hotter plug is probably something to avoid if possible.
  20. eskasteve

    eskasteve VIP MEMBER

    Sep 24, 2012
    Glad that you got it sorted. Wish that I'd read of your issue earlier as I had the exact same problem a few years back. Between the new stay up floats being too high and switching over to the 4 position needles and faulty too small retaining clip, my sweet running 850 became a testy beast. All is good now

Share This Page