Small engine repair advice

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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.
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-snip-
-snip-
Plus one on that--- Also be sure the blade hasn't come loose. That'll do some odd stuff.
Jim
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On Thu, 28 Jul 2011 18:00:13 -0500, Zootal

Almost definite diagnosis - sheared key. A couple of bucks and half an hour's work for a half-handy guy with a few tools
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So the general consensus is a sheared key - where is this sheared key that it can be fixed in a half hour? I thought I'd have to take the engine apart to get to it? I worked as a mechanic, but I retired my tools in 1984, and haven't been into a lawn mower engine since the early 1970's...(yeah, I know, I'm dating myself....)...
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On Thu, 28 Jul 2011 21:10:03 -0500, Zootal

You pull off the top tin- a 5 minute job for an ex-mechanic with his eyes closed, and remove the flywheel. The key is right there between the tapered shaft and the flywheel. An ex-mechanic knows how to remove the flywheel in about 2 minutes - including finding the hammer. Puting it back together is the reverse - without needing the hammer.
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snipped-for-privacy@snyder.on.ca wrote:

And here's a picture to prove that there's one under the nut:
http://www.kichline.com/chuck/fixit/mowers/key1.jpg
Jon
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Since you hit a stump, I agree that the flywheel key should be replaced whether it is bad or not. But! FIRST check the blade to see if it is bent and also the crankshaft to see if it is bent. If the crank (shaft) is bent, throw out the mower and go no further.
Hank
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I used to have an el-cheapo made-in-China piece-of-crap Wal-Mart lawnmower, 3hp engine. I hit something, and it bent the crank, bad. After pricing parts and finding that they cost more than the mower did, I tossed the entire thing in the trash.
As for this mower - it's an old Craftsman, 6hp engine, built a bit more solid than the made-in-China piece-of-crap mowers Wal-Mart sells.
This picture pretty much says it all:
http://zootal.no-ip.info/stuff/2011/2011 _dad_fixes_lawnmower/images/SDC14346.JPG
My daughter took pics of the entire operation:
http://zootal.no-ip.info/stuff/2011/2011_dad_fixes_lawnmower /
So, yeah, not only did it shear, the flywheel turned almost 180! A quick trip to Ace Hardware, $2.99 buys a 5-pack of flywheel keys that "fit all B&S engines". Had to use a wheel puller on the flywheel - it was very tight. Did a bit of cleaning while I was there. Put it together. Close manual choke, one lazy pull on starter cord, and it fires right up.
So thanks to everyone here - I had not idea the flywheel would shear the key and turn, I was thinking I was going to have tear motor down and good luck finding parts for it.
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6HP Briggs in a Craftsman will NOT have points. Briggs stopped using points long before craftsman started using Briggs instead of "eager 1" branded Tecumseh engines.
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But if there was a problem there, the OP would have experienced hard starting before hitting the stump. And "magneto contacts" were mentioned - which do NOT exist on that engine.
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You are just digging yourself deeper.
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No issue with the gap - and as you and I both said - if it ain't broke, don't fix it. But you mentioned contacts. If you mean the bround connections, if it ran before, they are OK - don't touch them. Loosening the attachment screws, in most cases, means losening the coil mounts - which can screw up the magnetic air-gap between the coil and flywheel magnet - which we have pretty well established should not be screwed around with if the engine was starting and running fine before. I've seen too many cases where someone played with the air-gap, without knowing EXACTLY what they were doing, and ended up with the coil catching the magnet, damaging the flywheel, coil, or both, within a short time after re-assebming the assembly.
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On Sun, 31 Jul 2011 23:06:27 -0400, clare wrote:

Yeah, small engines don't seem too picky about the gap I've always found - too wide, and the spark tails off quickly (and is obviously wrong), too close and the flywheel hits the magneto and trashes it, but anything inbetween gives good results (always rotate the engine by hand after changing it to test that it's clear).
cheers
Jules
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I finished mowing the back yard today - it is a happy mower. If anything, it runs better now than before, which doesn't really make much sense unless the flywheel had already rotated just a degree or two...I didn't touch anything else (knocks on wood...)...
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That's possible. I lived in a rented house in MI and the yard was a disaster of stumps and roots and rocks, and as much as I tried to maintain it, eventually I'd always forget and hit something with the mower - which also belonged to my landlord. I had a stash of flywheel keys in the garage for just that purpose, because said landlord wasn't handy at all and if a mower stopped running he'd just give it away and buy a new one which seemed like an unnecessary expense. I could usually get 2 or 3 hits out a flywheel key before it'd stop running, but it would run noticeably better right after replacement and slowly get worse over time. Then eventually I'd hit something hard and do it in and spend half an hour replacing the key again.
nate
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I didn't bother with the magnets, and left the gap where it was. Is this critical? The engine fires up with the first pull and AFAICT runs just fine.
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On Sun, 31 Jul 2011 17:42:38 -0400, "Stormin Mormon"

Rephrase that to:" it is important the gap between the flywheel and ignition coil is correct". If it started and ran properly before the key was sheared, the adjustment is correct. DON"T SCREW WITH IT.
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Depends on age of the mower. I bent the shaft on a fairly new 6hp Snapper a few years ago and got it fixed for less than $200. Beats 2-3 times that for a new replacement.
RonB
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Not necessarily. How old is the mower? I hit a stump with a 6hp Snapper about 6 years ago and bent the crank. A local mower shop fixed it for less than $200. That was a lot better than 2-3 times that for a new Snapper and we are still using it.
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It was easy to do - remove a couple of bolts, lift off top cover, air wrench to remove flywheel nut (big old 1/2" impact wrench, never get to use it anymore, why use hand tools when you have huge impact wrench that yeah might be a bit overkill but bzzziippp and it's off in two seconds), wheel puller pulls flywheel right off. Replace key. Clean here and there. Torque flywheel nut. Put everything back together. Started right up.
Reminded me how much I loved working as a mechanic way back when. Kids today couldn't set a point gap to set their lives. And time an engine? Set idle screws? CO2 meter? wtf? I could have a carb off, tanked, cleaned, reassembled and purring like a kitten in nothing flat. Kids today hardly know what a carb is.....ask a so-called mechanic today what a dashpot is, or what a choke pull-off diaphragm is, and watch them give you blank looks.

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