Help! suspect bent crankshaft, how do I confirm?

Discussion in 'Mechanic and Repair' started by wellynton, Jun 24, 2004.

  1. wellynton

    wellynton LawnSite Member
    Posts: 3

    Have B&S Sprint 3.75 hp mower, I cannot pull the starter rope. It acts as if something is binding inside engine. Blade can be turned by hand(with spark plug out, of course,) so I don't believe engine is siezed. At first thought safety switch was rubbing against flywheel, but that is not the trouble. Pull-cord works okay when not mounted. Engine was overheating before it quit, oil level was okay. I changed the oil and it still overheated but was running. I turned it off, let it sit over night and could not start it. Has anyone had same thing happen? Shaft does not appear visibly bent? How do I confirm that it is or is not the problem ? Thanks.
     
  2. fixer67

    fixer67 LawnSite Silver Member
    Posts: 2,098

    What makes you thank the crank is bent in the first place? Did you hit something with it? If you hit something the first thing you need to do is check the flywheel key to see if it is sheared. If it is sheared replace it only with the same type B&S key. DO NOT use a steel key to replace a flywheel key because the next time it will be the crank the brakes in to instead of the flywheel key. To check for a bent crank is easy. Take the spark plug out and hold the blade brake handle back and have some one pull the starter rope to spin the motor over as it spins look at the bolt that holds the blade on the shaft. If the crank is bent you will see the bolt move from side to side as the shaft turns. As for overheating what makes you thank it was overheating if I may ask. What type of oil (weight and brand name please) are you using? And remember this is an air cool engine so be sure there is not trash blocking the cooling fins. Good luck
     
  3. beransfixitinc

    beransfixitinc LawnSite Senior Member
    Posts: 592

    My response to this would be as such.. if it is a 3.75 HP Sprint, chances are, the mower that it came on didn't cost you over $110 when new. Depending on the age of it, you'd be better suited to disposing of it and purchasing another. This is like what happens when somebody comes in with a totally trashed Wal-Mart $99 Murray with an engine code of 950530 something. Wheels are all starting to fall off and the blade is torn to heck. Why can't people get it through their heads that the $99 mowers are not built to be worn out and then repaired, they are built to be used maybe 3 or 4 good years, and then junked and rebought. It is just not worth spenind money to fix all four wheels, overhaul the engine and replace a blade, when you've already only got 2 mounting bolts holding the engine on because the cheap deck metal is already coming apart.

    Note to Murray: Please do not feel that we are singling out your brand. This also includes the cheap little MTDs, and other "like built" homeowner mowers.
     
  4. wellynton

    wellynton LawnSite Member
    Posts: 3

    I've hit lots of stuff : rocks, tree roots, ground post for an electric meter ( got zapped that time) etc. I try to avoid these things, but sometimes, especially in high grass, you don't see them until it's too late. Don't Know of anything, that last evening though. As for overheating- I could smell the oil and there was also a frying or sizzling noise in it after it ran for maybe 45 minutes to an hour. Shear pin under the flywheel is okay. As for the other one on the bottom of the shaft I can't tell as I can't get the blade mount off. I did take the blade off, but could not budge the mount. Thanks for the tip about the bolt wobbling, I'll check it in the morning. This is the most frustrating week I've had in a long time. I'd like to not have to drag this week's trouble into next week, if I can avoid it. Oh, about the oil, it's 10W-30.
     
  5. fixer67

    fixer67 LawnSite Silver Member
    Posts: 2,098

    Well as for the oil you should use SAE 30 ,10W-30 is ok if it is snowing but these engines run so hot that 10w-30 thins out too much. Two causes of overheating in small are cooled engines are one- clogged cooling fins or cooling intake. two- lean carb. You can run a carb so lean you will melt the engine. From what you are saying you may have already done that. And if it is a "throw-a-way" mower like beransfixitinc says just throw it a way and go get a new one. We have people bring these $49 trimmers in the shop all the time and they get mad when we tell them the coil will cost US more that. Or they want to put a cylinder and pistion on a $79 blower. Now days any mower, trimmer or what ever that can be replaced for $150 or less is not ment to be fixed.
     
  6. beransfixitinc

    beransfixitinc LawnSite Senior Member
    Posts: 592

    You know fixer67, what gets me is we just became a Kohler dealer, and Kohler oil is 10w-30. However, Briggs oil is straight 30w. So, who hasn't caught up to the brain train? Don't even want to think about what Honda, Tecumseh, or Kawasaki oil is going to be.
     
  7. fixer67

    fixer67 LawnSite Silver Member
    Posts: 2,098

    We are a Kohler dealer as well. Kohler oil comes in SAE 30 for engines with standard valves ("L" heads) and OHVs. They also have 10w-30 for engines with hydraulic lifters. If you use SAE 30 in a Kohler with hydraulic lifters the valves "knock" and sound like they are going to jump out of the engine. If you get a Kohler with hydraulic lifters that is making a knocking sound 9 times out of 10 it is because of the wrong weight oil.
     
  8. shopbytch

    shopbytch LawnSite Member
    Posts: 37

    I bent a crank once, and just saw the dented blade of another one (5-6 hp toros). You'll know that the crank is bent if it vibrates/shakes right into your fingerbones. They'll run for another month or two, become hard to start, foul the plug too much get kicked and lost in a corner somewhere. Not worth fixing if it might fail a crew in the field; so use them for parts.
     
  9. wellynton

    wellynton LawnSite Member
    Posts: 3

    Hi guys,
    Thanks for your help, but we were all headed in the wrong direction. Turns out that someone had put a twig about 2 1/2 inches long, 1/4 inch in diameter inside the cylinder. It must have been in there for a while, before it finally got into the right position to cause mischief, since it was charred on both ends. All they had to do was unscrew the spark-plug and drop it in. If you look at the design of these engines, it's physically impossible for something like that to get in there without human help.Oh, well, I guess in spite of being angry I'm grateful they used a twig and not a piece of steel or something like that! Thanks again.:angry:
     
  10. fixer67

    fixer67 LawnSite Silver Member
    Posts: 2,098

    The mystery is how did it run for so long before it caused a problem. I do not thank it was in there as long as you thank. For one thing it would have been burned up instand of just charred at the ends. I do not thank it would have ever even started at all my self.
     

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