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Cannot start my newly-rebuilt Mk3

Discussion in 'Norton Commando Motorcycles (Classic)' started by rick in seattle, Jun 10, 2009.

  1. rick in seattle

    rick in seattle

    Jan 1, 2009
    Hi Guys,

    My mk3 is finally complete after a complete frame-off rebuild. It looks great but, after two days of trying, I cannot get it to start. When engaging the starter, the bike fires, snorts, and backfires, occasionally through the carb. The engine is a total rebuild; new rings, valves, and guides. Cam showed no wear. The choke on the carb (single 34mm Mikuni) is always engaged when starting. Here's the state of the bike:

    -The Pazon SureFire is hooked up correctly (I checked the magnets and confirmed that the trigger wires are properly connected internally and to the trigger plate). The static timing is dead-nuts on.
    -Both plugs show a robust spark,
    -the battery is a new Yuasa 18 A-H showing 13.4 v at the beginning of each frustrating starting session.
    -The valve lash is set to spec. I've checked it twice.
    -The Mikuni carb has been rebuilt, all new soft parts, needle, jets, etc.; I pulled off the bowl and found it filled with gas to within 0.5 inches of the top of the bowl and both floats functioning. When I pull the plugs and turn the engine over, I can smell gas vapor being ejected from the plug holes.
    -The plugs show a light oil film which wipes off. There is certainly some residual oil on the cylinder walls left to protect them during the rebuild.

    I have not yet pulled off the timing cover, but was completely anal during assembly to allign the timing marks on the pinion and intermediate gear, and between the timing marks on the intermediate and cam sprocket, and to insure 10 rollers between the marks. With a rod in the plug hole, I checked the opening and closing of the valves vs. the piston position and found them grossly correct, although this was not an accurate test.

    Something is grossly wrong, likely with the timing, but I'm out of ideas and short on experience. You guys have always come through for me during this project; just one more time.
  2. 79x100


    May 19, 2006
    It really does sound like ignition timing.

    You've not got anything to lose by retarding the ignition to see if it helps. You can set it with a strobe when it's running.

    Reversing the pick-up wires is usually worth a try on the basis of 'see what happens'.

    It sounds as if you've checked everything but connecting up a spare car battery might help show up any problems with your battery and a direct feed from battery to ignition unit would help rule out a problem there.

    Chin Up !
  3. grandpaul

    grandpaul VIP MEMBER

    Jan 15, 2008
    If you have the e-start, you could try jump-starting it while connected to a car (LEAVE THE CAR TURNED OFF), just for the cold cranking amps and no drain from bike's battery to bike's ignition.

    Have you given it a shot of starting spray (ether)?

    79's suggestions are good ones.
  4. seaguy


    May 19, 2009
    Sounds like induction. Like GrandPaul says. Try some ether or an unlit propane torch. If it starts don't run it much on either. This will tell you if it is induction or spark( timming) The 2 or 3 degrees retard will help by eliminating the poping ( which esually means too lean. Also like 79x100 suggested might be the pazon. aaI remember reading a post several days ago about someone buying a united that was marked backwards on the pickups or some such.
  5. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004
    What happens when you use the kickstarter?

    Are you certain you are using the correct alternator timing mark? = Turn the crank in the normal direction to TDC, (with either cylinder on compression), then turn the crank backwards until the first rotor mark that comes into view in the primary case timing aperture lines up with 31 degrees on the timing scale, the Pazon rotor paint mark should then be visible through the upper ("anti-clock timing") hole in the Pazon pickup plate.

    The White/Purple ballast resistor bypass wire originally connected between the starter solenoid and the ignition coils must be disconnected when using any type of electronic ignition.
  6. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008
    To check the timing you can cheat a bit.

    Pull the plugs, but make sure they are plugged into their cables, and firmly grounded and pointed away from the plug holes

    Shut off the fuel and empty the float bowel if you can, you don't want fuel vapors surrounding the sparking plugs.

    Attache an inductive strobiscopic timing light to an ignition cable.

    Do jumper in a large 12 volt battery.

    With all this set up get an assistant to punch the starter button while you shoot the timing marks, it won't take but a few seconds to see where you are with the timing. You won't be at full advance but you should be on the scale. I'd adjust the Pazon until you are within 10 degrees of 28 BTDC, that should get you started if your malady is out of wack timing.

    I do this while running oil into a new engine, before I put fuel in the tank.

    You did an excellent job of describing your circumstances, best wishes on your start up, I do mine on a Mk III I did similar work on this coming Saturday at 1:00 EST with an audience! Go Red Sox!


  7. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004

    A Pazon would normally be set to 31 degrees BTDC, same as for a Boyer.

  8. rick in seattle

    rick in seattle

    Jan 1, 2009
    Thanks, everyone. Lots of info to sort through.

    With new rings I can barely kick it through. I've got Dave's starter and a new sprag and sprocket, so I've got a lot of starting power available. I've also got a spare fresh battery which I will jumper in tonight just to drive the starter.

    I'm setting the timing on the alternator rotor mark that corresponds to the pistons rising and 5 mm below TDC. I'm using the anti-clock timing hole and am setting the timing a few degrees retarded, at 26 degrees advance of TDC, just to get the damn thing started. There are no ballast resistors nor capacitors on the bike. The loom is new.

    What does it mean when the bike backfires through the carburettor?

    Pazon sent me a static electical test to confirm their internal wiring, which I successfully used (I read the post as well complaining about the trigger wires). I also checked the polarity of their pair of magnets in the timing rotor.

    I've never used ether. How do I use it?

    I do like the idea of shooting the rotor while cranking with the plugs out. I'll try it tonight. The monthly meeting of the Northwest Norton Owners Club is tomorrow. I'd sure like to show off my shiny Mk3.
  9. seaguy


    May 19, 2009
    Common starting fluid comes in a spray can and is usually ether. Have evrything ready to go...open the throttle and give a one second spray into the carb throat close throttle about half and kick. With two people the propane is safer but it takes a lot for 750 + engine. At idle throttle you can keep it running with the propane bottle valve. This proves your spark and rough timming. If it will run ok with propane them the carb is leaking air or not enough gas is comming through the Idle ckt. The popping back thru the carb is: valve not closing all the way, too lean a mixture,or spark too advanced. All it takes is the basic 3. Spark at the correct time... good fuel /air mix ...correct mechanical timming.
    ps. one thing I forgot to mention. There are certain carb cleaners that when in contact with a sparkplug's "electrode end" insulator will cause it to become conductive and practically ground out the plug when under pressure. ie: they fire improperly when in the head. SO if you have used any around the plugs try a new set.
  10. Cookie


    Oct 5, 2008
    Same crank as before? If you still have your old ignition try that to test.
  11. bigstu


    Jan 2, 2009
    Something is wrong...it should start easy...remember that...it's so easy to think everything is right...but it CANT be.
    set your ign at 31 dbtdc, not 26, if you want to retard a bit set at 30.
    If your bike backfires into the carb, it suggests the bike fired before tdc and reversed the engine rotation the wrong way, or fired when an inlet valve was open.
    If I was you, I would whip the timing cover off, and double check the valve timing.
    Then whip the rocker covers off and examine the movement of the valves in reference to the pison position.
    Then set your timing at 30 dbtdc
    Spray easystart into the carbs with the throttle wide open. tickle the carbs.
    Shut the throttle , drop the choke in, take up the slack on the throttle cable, get the pots at top dead, turn on ign, and leap on the bugger.
    Good look
  12. GavinJuice


    Jul 24, 2007
    Whats the reasoning behind turning the engine backwards?
  13. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004
    Because that is (in my opinion) the easiest/quickest method to identify the correct BTDC timing mark on the alternator rotor to set the ignition timing with.

    As there are often two timing marks placed 180 degrees apart on Lucas rotors, if the other (wrong) mark were to be used by mistake, then that would result in the sparks occurring 180 crank degrees out (31 deg. BBDC F/ADV instead of 31 deg. BTDC F/ADV) as a Boyer (or any other wasted spark EI system) only fires both plugs simultaneously once every 360 degrees, or = one full turn of the crank.

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