On Thu, 28 Jul 2011 18:00:13 -0500, Zootal
wrote:

I have a Craftsman mower, 6hp, B&S overhead valve engine. This engine has
always started on the first or second pull. Open throttle, close the manual
choke, give one or two lazy pulls and it fires right up. It is the easiest
starting mower I have ever had!

I ran over a stump that stuck up a few inches - it was in tall grass, I
didn't see it, and the blade hit it hard, immediately stopping the engine.
Thunk, engine stopped. The plastic cover over the pull starter popped off,
it hit so hard.

Now the engine won't start. It pops and sputters and backfires out the
carb, but absolutely will not start. Thinking a timing gear (or whatever
valve timing mechanism it has) had skipped a gear/notch, I pulled the valve
cover, pulled the spark plug, and watched to see if the rockers arms would
both rock at TDC, and as far as I can tell they do, though I'm not sure how
accurate or reliable of a test that is.

The engine turns over easily, but it always has - if anything it turns over
easier than it should, which is what made me think the valve timing was
off, though it's hard to tell.

Anyone have enough experience with this engine or type of engine to comment
on what might have happened, and availability/cost of parts should I decide
to tear the engine down? I don't know if sheared a key, skipped a
tooth/cog, bent a valve in the process or what I'll find when I tear it
open. It's been a good mower, and it's worth a bit of time and effort to me
to fix it.


I would start with the shear key. Sounds like it served its purpose.