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Help with B&S engine electrical issue

Discussion in 'Electronic Repair' started by bob, Jul 26, 2007.

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  1. bob

    bob Guest

    May be off subject if so let me know where else to post
    I have an Ariens ZTR 1742 mower with a 17 HP B&S engine that has been
    afterfiring ( After ingnition is turned off raw gas collects in the muffler
    and explodes) since I got it .This destroys the muffler and I need replace
    another $90 one so I thought I'd look into why its happening.
    When I checked into it there is an afterfire solenoid on the carb that is
    supposed to shut off the fuel supply when the ignition is turned off. Ariens
    didn't do this and as a result the solenoid which is powered from the
    alternator does not disengage until the alternator ( and engine ) has almost
    stopped so I get feul in the muffler.

    When I called Ariens they admited to the problem and sent me a new ignition
    switch that is supposed to fix the problem. When I got it it was clear that
    this would not switch teh solenoid power and when I called again I was told
    that the fix was internal to the switch and did not need any cabling.

    When I pressed for how it was done teh engineer told me that rather than
    switch off the solenoid they simply shorted it to ground when teh ignition
    was turned off.
    While I agree this will fix the afterfire it does so by applying a short
    across the alternator while its still at speed.

    So my question is am I trading a annual muffler replacement for a much more
    expensive Alternator problem down the road
    in Advance
  2. hr(bob)

    hr(bob) Guest

    Is the fuel exploding because there is a spark that causes it, or is
    it just the fuel accumulation in the hot exhaust system. If it is
    anything electrical, you should keep the ignition off/ shorted out
    until the engine stopsrotating.

    H. R. (Bob) Hofmann
  3. N Cook

    N Cook Guest

    I own an ancient Hayterette with a B&S engine.
    Not prone to back/misfires but to stop it I just pull the HT cap off the
    spark plug. As originally a bare metal cap I adapted an old bakelite knob to
    cover it.
    I've always the opposite problem - starting , as usual with these mowers, to
    start - its always take off the airfilter cover and teaspoon of petrol in
    the inlet chmber. Apparently something to do with a carburetor that can be
    taken up and down slopes without playing up.
  4. bob

    bob Guest

    The fuel is exploding because its accumalating in the exhaust after the
    spark is removed. The carbirator continues to feed fuel to the cylinder and
    its just passed through. The solenoid used on the carburator is intended to
    immediately shut off the fuel when you turn the engine off. The problem is
    that the design ties the somenoid to the alternator rather than switching it
    off . The alternator continues to product voltage after the spark is stopped
    which defeats teh purpose of the soleniid.
  5. bob

    bob Guest

    This is a different issue caused by fuel in the muffler. If I pull the plug
    cap it still happens because even without the spark the engine continues to
    pull fuel/Air into the cylinder and then sends it out to th emuffler where
    its ignited by the hot muffler.The issue is that the mechanism to shut down
    teh fuel is delayed by the bad design which I believe just got worst by the
    "fix" which shouts out the alternator output
  6. R!

    R! Guest

    The older B & S engines had a low current self limiting alternator, it
    had only a fuse for circuit protection. Those would not fail if shorted
    with the engine running, they only put out about 2A charging current.

    Some larger engines had a voltage regulator those would fail if shorted
    while the engine was running. Theese put out about 10A charging current.

  7. sparky

    sparky Guest

    Solenoids were not installed until a few years ago and engines have
    been around for much longer than that. On the lawnmower I use the
    book calls it a SAFETY solenoid and it is connect to various limit
    switches (along with the ignition switch) which shut the engine off if
    you do something prohibited.
    I have had several lawnmowers with the same type of engine and no
    solenoid that performed extremely well and had no afterfiring
    problems. Perhaps you should look further into the problem.
  8. hr(bob)

    hr(bob) Guest

    Put a switch on the wire to the solenoid and turn it off that way.

    H. R. (Bob) Hofmann
  9. bob

    bob Guest

    Thanks given this alternator is a 9 Amp unit I'll put a switch ( or Relay )
    directly in the solenoid circuit so I don't damage teh solenoid
  10. James Sweet

    James Sweet Guest

    It's possible that the engine is dieseling, that can happen with some carbon
    buildup in the combustion chamber. It could also be running very rich and
    accumulating excess fuel in the exhaust.
  11. bob

    bob Guest

    I considered that but don't believe thats the case I pulled the heads last
    year and there was minimal carbon which I removed.Also if it was dieseling
    wouldn' the fuel ignite while in teh power stroke with the exhaust valve
    closed ?
    I also considered all the fuel wasn't being burnt but assume that would
    cause afterfire randomly not only when I turned teh ignition off
  12. Dieseling in a low compression small engine also seems less likely.

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  13. Meat Plow

    Meat Plow Guest

    The 13hp Kohler Commander on my John Deer tractor diesels once in a great
    while but I would have to agree. Especially motors with minimal sized
  14. James Sweet

    James Sweet Guest

    It's rare, but it happens, and with a weird problem like this one can't rule
    out the rare just because it's uncommon. This does sound like something else
  15. bob

    bob Guest

    The B&S manual actually descriobes this problem if the afterfire solenoid is
    tied to the alternator which is a 9 amp unit . Their fix is to switch teh
    solenoid . Ariens decided to short out teh alternator instead
    If anyone wants to see the diagram I can post it or email it
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