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Engine Knock Help

Discussion in 'Norton Commando Motorcycles (Classic)' started by Jeremy, Jul 25, 2003.

  1. Jeremy


    Jun 19, 2003
    have owned a '71 roadster for about four weeks now and have been going about various basic tasks to familarize myself with the bike. however, there are only so many fluids one can change on the bike and i am now faced with the following problem:

    the right cylinder, to the best of my limited understanding of the subject, knocks. the left side runs smoothly but the exhaust from the right side literally blasts out of the muffler at uneven intervals. machine idles at around 3000 rpm but i'm not experienced enough to know if this is related. went for an extended ride last sunday and the right header started to turn blue. compression test in both cylinders rose to 190 psi after 4 good goes on the kick starter which seems to suggest an abnormally high compression ratio. the previous owner said "it always ran rich" and he addressed the problem with frequent swapping of the coked plugs.

    my most uninformed opinion is that the head has become so filled with deposits that the cylinder volume has been dramatically reduced. and my next plan of attack is to pull the head off and scrape them out.

    is this even a remotely correct response? i apologize in advance if i'm completely missing the obvious answer. kind of new to this whole motorcycle maintenance thing.

    the bike has ~13k miles on it.

    thanks for your input.
  2. Jerry Doe

    Jerry Doe Admin

    May 21, 2003

    If the compression is the same on both sides, I would leave the head on for now and do some more research, to verify a couple of things first.

    What carbs have you got?
    What are the jet sizes in the carbs?
    What exhausts do you run?
    What kind of ignition do you have?
    Have you strobed up ignition on both sides?
    How do the plugs look?

    The knocking could be related to carburation or ignition. Check the ignition is set correctly first, then analyse the carbs.

    Header turning blue, means it is running too hot possibly due to carburation (too lean) or iginition off on the right .

    Have a look down the spark plug hole with a flash light/ torch and see if its coked up. It will also look black around the tailpipes.

    Good luck- Jerry
  3. Anonymous

    Anonymous Guest

    Knock Knock?

    Just a thought, while you are poking around handling all the good stuff outlined above, check to make sure that the old owner of the bike put in matching spark plugs.
    Sounds to me like the guy might have been the type to mis-match plugs and you might find it to be true.
    Just a thought anyways.
    You might want to first set the idle and mixture screws as per the shop manual, an easy process that can yield good results.
    Let us know what you find.
  4. Jeremy


    Jun 19, 2003
    Thanks for the suggestions. Check the simple stuff first, eh? No matter how many times I hear that, I always seem to forget.

    When I get this all figured out, I post back with the solution. I've replaced the plugs already and the left one cokes up fast. The right looks fine but I really haven't used the bike much for fear of exacerbating the problem. I pulled off the right exhaust to add that oil filter conversion kit (it's a 71 remember) and while I was at it, I poked around inside the head. Probably about 1/16 thick layer of crud on the walls. It wasn't greasy crud however.

    I'll do my best to tune the Amals and check the ignition (any suggestions for a good brand of strobe?) and will probably come crying back to the board.

    Oh, and as far as exhaust goes, I've got something quite similar to that pictured on the bike at the top of this page.

    All I really want to do right now is bust out of school and go work on my bike.
  5. Richard


    May 24, 2003
    Jeremy, on a differnt tack to the excellent advice already given, is there a local Norton Owners Blub section you could join over there in sunny 'frisco? Sure would make life easier if you could meet some likeminded owners who could help you with this first service? Good luck.
  6. Anonymous

    Anonymous Guest

    exaust muffler

    To the gentleman with running very rich problem ; The problem might be that your muffler is burned out the way to check is take them off and wieght them it sounds like one has no back pressure .the quick fix is ot drill a little hole at the end of the pea shooter muffler and screw a long screw into it that will creat enough back presure and help your commando to run lean then get new mufflers.I think you have pre- ignition problem an d this will help to fix it. goodluck Andreve.
  7. Roger


    Sep 14, 2003
    Hi I see you have a lot of good items to check for your problem I would like to add I really don't believe you should have an idle of 3000 rpm this is quite high and could be related to throttle settings in your carb or even your cables. It is possible that the needle valve in the carb of the troubled cyl is not seated properly and is as if you were holding the throttle open.I think a good idle is somewhere handy 12 to 1500RPM someone please correct me if I'm wrong.Also if the other cyl runs good check that carb settings and compare to the bad side
  8. Anonymous

    Anonymous Guest


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