I have a fairly unused Craftsman 6.5 HP Briggs and Stratton push mower that hit a rock, then ran extremely rough, smoking and without much power for about 10 minutes. Immediately afterwards, it would barely start and only run briefly but with a lot of clanging and with some smoke. It is model that has a has an operator presence bar on the handle that, when released, stops the engine and puts a mechanical brake on the flywheel, if that helps at all.
Some research pointed to a shear pin as a probable problem. So . . . I took it apart and sure enough, it was shorn apart and the flywheel was rotated by the width of the pin at the spindle. I replaced the pin and installed it uneventfully.
Proud of myself for fixing it and getting it back together, I primed it and pulled the cord. It always started easily on the first pull. However, to my disappointment, it smoked and bucked and ran roughly with a noise like someone banging a steel pot - clang - clang - clang and then quit in ten seconds. Subsequent pulls never got much of a kick out of it with it almost sounding like it wants to start, but nothing but a quieter sound - about 5 clangs per pull.
Inspection of the underside - the blade has a ding from the rock, but feels pretty firmly attached and is definitely not hitting anything. Not sure what a bent shaft would feel like, but the blade seems to turn without a wobble. Now that I think about it, should have pulled the spark plug to make it turn easily to help me diagnose, but I did not.
Perhaps the shear pin slipped. But I doubt it. It did seem to go down a bit below the level of the flywheel when I inserted it.
What's the next step?
Thank you for reading!
#2
LawnBoy97
It is probably the bent shaft. If you can get someone to help you, remove the spark plug wire from the spark plug, and then tip it on its side so that the air filter is up. Then have someone slowly pull the rope and look at the shaft. You'll know if it is bent. But remember that you always need to disconnect the spark plug and tip the mower with the air filter up whenever you work on the underside! Otherwise, you might lose some appendages:shocked:.
#3
davbell22602
Sounds like a bent crankshaft, bent mower blade or both.
I pulled the plug and observed the shaft at the blade from straight on. Shaft was bent. Has a wobble of about 5 mm maximum excursion of center of the shaft from front to back.
Is this replaceable or repairable? I don't have a ton of time to work on this sort of stuff but hope there is something to salvage.
Update --> Just watched redneck bent crank shaft repair on youtube - sledge hammer technique. May give it a go. Will report back.
Update --> Did the sledge hammer technique. Took about 20 hits of varying quality to get to about 1mm of wobble and I decided to go with that.
Primed it 3x as I always had per the instructions, pulled the cord and . . . no luck. One tiny puff of smoke on the first pull, but nary a hint of ignition after that. The plug was initially really wet with fuel, so maybe it go flooded with all the turning I did on its side to gauge the wobble. I moved the piston to the bottom and blew compressed air into the plug hole for a while hoping to dry it out, but no luck. So here are a few diagnostic things.
Fuel in tank - check, oil level - OK (was on the side with air filter down, but no leaking that I could see)
Pulls smoothly, no rough spots or clanging as before.
Pulled the spark plug out and pulled the cord and it spins very smoothly.
Spark plug sparks OK when pulled.
Valves seem grossly functional - I can feel compression and then suction with my finger over the spark plug hole, then it sounds like it is exhausting on the next cycle.
I'm going to let it rest and try tomorrow. If no luck, I'll check the shear pin again and make sure nothing has moved.
Update - 12 hrs later rev'd on the first pull, now nothing.
Update - Checked air filter - full of oil - removed it and it started and runs well!! Problem solved! Thanks.
