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Discussion Starter #1
Hello,
I'm trying to get the Cub running again. It quit a while back, and ever since it will only backfire(frontfire?) through the carb. A new set of points didn't help.
Any help will be greatly appreciated. The push mower is a drag.
Thanks,
Mike
 

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I agree with Kbietz. If it just haoppened like stated the key is the first thing I would look into. As I am a beliver in the KISS solution.
 

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Aparently it has been backfireing alot so I'm with Kabitze on this one it's fly wheel key. kiss , after you get the new key in - check the carb and see if it's running lean and the plug wire under the sheet metal is not chafed and for good measure change the spark plug .
 

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If you set your points with a timing light or if you have a magneto ignition, then it could be the flywheel key. Otherwise that’s probably not it (the points on a K-301 get their spark from the coil not the flywheel).

If it were mine, I think I would probably pull the cylinder head and see what’s going on in there, if the head hasn’t been pulled in awhile you will probably want to de-carbon it anyway. Turn the motor and make sure the valves are operating properly.
 

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Old Guy With Old Toys
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What kind of compression do you have?
 

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Discussion Starter #8
Thanks guys

She was running at full speed under load when she quit. Cranks really hard now too. I think the flywheel is on the driveshaft end so the motor will have to go back on the bench. Makes perfect since, it's acting like a motor with a jumped timing chain. Never gave the key a thought.
Thanks again
 

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Like CCH said, They're not timed off the flywheel. I would also pull the head first (much easier) carbon or a broken cam or gear. check your valve timing. Could also be a broken valve spring. Good luck and keep us posted.
 

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The K301s if original were timed off the flywheel. Sitting on the tractor the timing hole will be on the right side.
 

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draddogs, you are correct.

If you time the engine with a timing light, you use the "S" mark on the flywheel...

Upon installing new points, you set them at or about .020 and there the motor should run, and can then be timed with a light.

In my earlier post I said "the points on a K-301 get their spark from the coil not the flywheel" meaning that there is no magneto and no magnets on the flywheel to create a spark.

If the point gap is .020, placement of the flywheel is irrelevant

I will however agree that if the flywheel key is completely sheared and the crankshaft is spinning inside the flywheel, this situation could cause the engine to "backfire" through the carburetor.

I have to agree with Ironmower, I would be looking for carbon under the intake valve. With it being hard to crank though, kinda sounds like the exhaust valve isn't opening...

Mike, in any event, would you be so kind as to let us know what you find out.
 

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Mike,
I think you have an intake valve problem. Pull the head or get a compression reading. I have never seen myself a broken flywheel key. Hard cranking could be a broken acr spring. Worst case is a broken cam stuck on intake stroke. This I have seen. But a compession test will tell all.
Mike
 

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Discussion Starter #13
I'll put it on the bench and see what I find. Depending on when the sleeve puller arrives I may end up doing the 8N first.
Regardless, I will post my discoveries.
 
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