help please

Discussion in 'Mechanic and Repair' started by mrlee410, Jul 25, 2007.

  1. mrlee410

    mrlee410 LawnSite Member
    Posts: 3

    repaired a 12 hp briggs stratton 281707 4001 motor now i am breaking keys in the fly wheel belt driven mower connecting rod was broken
    i have broken down the motor 6 times yet cant find what is causeing the fly wheel key to break after a few minutes of running
    any ideas would be helpful
    thank lee
     
  2. subs1000w

    subs1000w LawnSite Member
    Posts: 160

    are you making sure the bolt is tight i had that happen a few time on a machine and it turned out i wasnt tightning the bolt enough now i use an impact wrench which does the trick every time
     
  3. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Make sure there is no oil/grease on the crankshaft or mating surface in the flywheel, Clean with carb & choke spray cleaner then torque the bolt/nut to 65 ft.lbs., Do not put anything on these surfaces.

    Also, This flywheel does not fit down on the crank flush so make sure the key is pushed all the way down into the key slot in the crank.
     
  4. John Stiles

    John Stiles LawnSite Member
    Posts: 42

    Be sure you have the big spring-washer on the shaft before the nut...but even before that, inspect the coils and the magnet in the flywheel to see if there is a foreign object, or any unnatural contact has been made!~john>>>P/S if the compression is way over spec...you may be damaging it[key] while starting.
     
  5. mrlee410

    mrlee410 LawnSite Member
    Posts: 3

    THANKS SO MUCH! TO BE HONEST I THINK THIS OLD FRIEND HAS ME BUFFALOED. CRANK AND FLYWHEEL TURN EASY ENOUGH, BUT THE MACHINE WHEN RUNNING A FEW MINUTES THEN START BACK FIREING. WHEN I REPLACED THE CONNECTING ROD. I NOTICED THAT ON TDC THE EXHAUST VALVE STARTED OPENING, IS IT POSSIBLE THAT THE CRANK IS BENT? IT APPEARS OK, THE KEY ON THE BOTTON OF THE CRANK DID NOT SHEAR WHEN THE ROD BROKE. SORRY, BUT I AM AT A LOSS I APOLOGIZE FOR BOTHERING YOU
    BUT IT IS ONE OF THOSE THINGS THAT YOU DO NOT WANT TO CONQURE YOU, YOU KNOW WHAT I MEAN? HA HA THANKS A LOT I FEEL LIKE I HAVE A FRIEND IN MY CORNER NOW . LEE ALEXANDER mrlee410@bwave.com
     
  6. John Stiles

    John Stiles LawnSite Member
    Posts: 42

    I would say that the crank is apt to twist or skew anytime abnormally halted...like hitting a stump, or breaking a connecting rod then jamming. But even if it did "twist" a little[enought to be outta time], there would almost hafta be a discernable wobble/vibration. But anything is possible. I have a 12hp engine out here that the owner says is knocking...I will refresh my reccollection on it. Meanwhile, you might check the top of the piston for hits or chunks of imbedded metal. I really don't think it'd even start if it was outta time.~john
     
  7. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Can you tell me how far down the cylinder (from the top)the piston was when the valve just started to open while coming up the compression stroke ?

    How far did the valve open and did it close again before completing another stroke ?
     

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