Killed a B&S Polan Lawnmower

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  #1  
Old 02-26-12, 06:25 PM
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Killed a B&S Polan Lawnmower

Lawnmower was running fine till I hit the piece of 2X4 and it died. Now it won't restart. It feels okay and sounds okay when I pull the rope but it doesn't even try to catch. I even tried a bit of gas straight into the carburettor but no go. Can't say if I have spark because I'm still trying to figure out how to stand behind it to pull the rope while kneeling in front to check the spark.

I know I haven't given you much to go on but was asking for a list of likely possibilities.
 
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Old 02-26-12, 06:53 PM
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I haven't worked on newer mowers, but the older ones used to have a zinc flywheel key that fit into a keyway on the top side of the crankshaft (under the shroud). If you ever hit something hard, that key would shear, and then the timing would be thrown off, causing no start. The key was a 20 cent part, getting the flywheel off was the hardest part, since it takes a flywheel puller.

Not sure if ur mower is so equipped or not.
 
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Old 02-26-12, 07:11 PM
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Good chance you sheared the flywheel key.

Take off the shroud from the flywheel. Remove the center fixing nut. Then carefully pry up on the flywheel with a piece of wood while you tap the outside of flywheel with a hammer. Don't hit it too hard, just go completely around the flywheel with the pressure from the wood on it. It will pop off and you can check the key.
 
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Old 02-26-12, 08:36 PM
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Thanks. I'll pull the shroud in the morning.
 
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Old 03-07-12, 06:03 PM
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I was able to see the key just by removing the nut and it wasn't sheared and the slot in the fly wheel was still in line with the slot in the shaft. Any other thoughts what the problem might be? As I said earlier the engine turns freely till compression builds up with no unusual noise. Thanks.
 
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Old 03-07-12, 06:13 PM
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Huh! The rest of this seems very unlikely if the key isn't sheared. Since that's made to be the weakest link.

Pull the plug and ensure the piston is still moving up and down. I would doubt it broke the connecting rod or piston pin but it's easy to check.

With the plug out (and still connected to the wire) you should be able to spin the flywheel by hand and figure out if there is any spark by holding onto the contact with one hand while you spin the flywheel with the other. It should give you a pretty good zap. You could also ground the end of the plug against the frame and then give it a spin. Should be a hot blue spark.

If the piston is still moving... and you have spark... the next thing I'd wonder about is the valves... that maybe the gears where the crank and cam mesh have broken some teeth, and that as a result, the cam is not working the valves properly. To check the valve operation, you could either remove the head and watch the valve heads as they go up and down through their 4 cycles, or take off the breather cover to expose the valve springs, and watch for each of the springs to compress and expand as you spin the flywheel by hand.

It could be that its just very flooded. A little starting fluid in the cylinder and on the spark plug followed by a good amount of compressed air would dry it out.

There's also a keyway on the blade clutch but that shouldn't affect whether or not the motor starts... unless the loss of centrifugal force is making it harder to start. Might be worth checking. The blade clutch is the part that the blade mounts to, so you would have to take the blade off, remove the bolt into the crankshaft and then use a puller to remove the clutch from the keyway on the bottom of the crankshaft.
 
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Old 03-07-12, 07:37 PM
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You can't really tell if the flywheel key is sheared without removing the flywheel, I may look fine until you remove the flywheel. Have a good one. Geo
 
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Old 03-07-12, 07:42 PM
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Thanks guys. I will take another look.
 
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Old 03-07-12, 07:57 PM
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the slot in the fly wheel was still in line with the slot in the shaft
reading that, I would also assume the key is fine... but he's right, it could be fractionally off, which affects the timing, and you can't look at the key without actually popping off the flywheel. Sometimes the key is just knicked, but any amount of shearing is bad.
 
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Old 03-08-12, 09:24 AM
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This for flywheel key reference to show although it may look good when you look down on it, it may still be bad, this is a Tecumseh key. Have a good one. Geo

Looks good here:


However in reality it isn't:
 
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Old 03-08-12, 11:03 AM
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Crap, now I'm going to have to take the flywheel off instead of leaving it at the curb for heavy trash. LOL Seriously thanks for explaining the matter more clearly for me.
 
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Old 03-08-12, 11:50 AM
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If it looks like the key slot in the crank and flywheel are aligned I'll bet they are. It's pretty easy spot the misalignment.

Here is what offset keys look like and you can pretty easily see the offset. A partially sheered key may look similar. They are used in racing engines to adjust the timing.



Is the engine making any sounds like it's trying to fire? You can usually feel or hear many internal engine failures so I'm thinking it might be an ignition problem. Maybe the grounding shutoff wire got knocked loose. I would insure you are getting spark at the plug when you pull the cord.

Unfortunately I have a lot of experience with this problem. My mother has a habit of finding cast iron water meter enclosures and iron survey markers with the push mower. One season she hit three. I've even gotten good at straightening crank shafts.
 
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Old 03-08-12, 12:35 PM
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Unfortunately I have a lot of experience with this problem. My mother has a habit of finding cast iron water meter enclosures and iron survey markers with the push mower. One season she hit three. I've even gotten good at straightening crank shafts.
After living here 40 years I am convinced it is night elves that carry stuff in while I sleep. After forty years you would think I would have found everything to be found.
 
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