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Timing on mower

Discussion in 'Mechanic and Repair' started by rossco, Apr 25, 2008.

  1. rossco

    rossco LawnSite Member
    Messages: 1

    How do you do the timing on a briggs and stratton vertical shaft 12 HP. Mower has had air, fuel and oil filters all replaced. Carbeurator totally pulled down and cleaned out but motor at rev will not hold constant rev, changing minor amount. Does not backfire, have checked governor spring. The only thing i'm not sure about is how to do the timing on this motor.
  2. jkilov

    jkilov LawnSite Bronze Member
    from MS
    Messages: 1,415

    Hi there, I just moved to LawnSite from another forum,
    perhaps I can help.

    Guess you are referring to the ignition timing. Yes, this can be tuned on most engines by adjusting the Armature air gap rather than moving the ignition coil sideways.

    1. dismantle engine cover to access ignition
    2. rotate engine until magnets on flywheel mate with ignition coil ends
    3. loosen the 2 bolts holding the ignition module in place
    4. place a feeler gauge between the ignition coil ends end the opposing magnets. Armature air gap for an engine should be something like 0.010-0.015 in. or look in your owners manual.
    5. Once you're done just re-tighten the bolts and put the engine cover back on.

    If this won't solve your problem, try adjusting the low-end mixture screw on the carb. And if that doesn't work it's probably briggs' utterly unreliable governor which is not service friendly.

    FIXDISS LawnSite Senior Member
    Messages: 445

    Post model #'s please. Will be most helpful in determining the solution to your problem. If this engine is starting easily and you are only experiencing the "hunting" problem there is a good chance it is a carb adjustment issue. Some older Briggs had 2 adjustment screws one of which was only accessible by removing air cleaner housing.I don't think it is a timing issue unless you have recently fooled with the air gap on the coil.

    FIXDISS LawnSite Senior Member
    Messages: 445

    One other thing I failed to mention. On the rod from the governor arm to the carb there should be a long but hair-like spring that must be attatched at both the governor arm and carb throttle plate. This spring is there specifically to eliminate the variation in rpm by applying tension to compensate for any slack in the linkage. If you are having only slight variation of rpm this could be your problem.
  5. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029


    Could you explain in detail how this engine timing is changed by only moving the ignition coil in or out from the flywheel ?
  6. rclopez1118

    rclopez1118 LawnSite Member
    Messages: 15

    How old is this engine anyways? By what your describing, it`s not a timing issue. it sounds more like it needs a real good carb cleaning. note..good cleaning! Your starving for fuel,.check tank as well. a good cleaning does wonders.:usflag:
  7. jkilov

    jkilov LawnSite Bronze Member
    from MS
    Messages: 1,415

    I expected someone would ask this question.

    Most new ignitions are CDI, they always discharge the same amount once they reach full capacity (unlike the old magnetos). If the gap is smaller, magnets will charge the coil to full capacity faster, the discharge will take place sooner, spark is advanced. Increase the gap and the spark retards.

    Like I said, this adjustment is minor (0.010 - 0.015 in. for such an engine size). Sometimes it's not done correctly at the factory. I have no idea to how much degrees this translates, you got me there.

    I don't think It's the cause of his problems, but the person wanted to know how to adjust the ignition timing.

    I hope I've answered your question in detail. If my understanding is wrong, feel free to correct me.
  8. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    OK, I just wanted to assure there was no misunderstanding for other readers.

    Just to save some typing I dug these up,

    From a Kohler Manual;

    From a Briggs manual;

    So in essence by moving the coil/module closer or further away from the flywheel is affecting the amount of charge induced into the coil/module and has nothing to do with timing. [​IMG]



    A quick test to see if this is a carb or governor related issue is set the throttle to where the engine "hunts" the most then slowly start applying the choke, If it smooths out go to the carb as others have mentioned. If not check the governor link spring as FIXDISS mentioned and governor adjustment.

    Good Luck
  9. jkilov

    jkilov LawnSite Bronze Member
    from MS
    Messages: 1,415

    Looks like I messed up. Only the ones that can be moved sideways are adjustable.

    I'll have to do some more reading next time.

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