Lawn mower starts and stalls

I have an unknown brand lawn mower with at Tecumsah engine, probably 3 or 3.5 hp. It starts every time, but often runs rough and often stalls, sometimes backfiring before stalling. Sometimes after it stalls, it runs well and strong! One day it was running strong until I bumped it into the mailbox post. Sometimes when runnign rough it sounds like something metal is rattling arouind. Obviously something is moving around. My guess is either it has a sticking valve or the timing gear is jumping due to a loose bearing. I can't find any troubleshooting guides that describe this situation. Any other ideas on hte possible cause? Thanks.
Mike D.
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My guess - the flywheel key is sheared.
Bob
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or
runs
the
metal
loose
Nope.
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Let's look for simpler problems::
1. Spark plug -- clean and regap, or replace.
2. Air filter -- clean or replace
3. Carburetor -- clean, check float for sticking.
4. Water in fuel tank from winter? Drain, put in fresh fuel.
A valve problem (4 cycle only) would possibly cause backfire. However, carbon buildup could also be a culprit. Pull head, and clean, but you will likely need to replace the head gasket, and you need a torque wrench to put it back on correctly. Torque sequence of bolts is important..
If there is a gear problem, it shouldn't run at all, I would think.
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?????????????????? A bump will affect this? I doubt it!

Done first thing I got it. Also tried removing the filter. No change.

A possibility, especially if the previous owner used ethanol.

Already done and how would this explain the sudden rough running after bumping the mower?

Wouldn't it always run bad then? That doesn't explain how a bump would cause it to suddenly run rough and stall and then start up again and run fine.

Yea, but it explains the backfiring and it would still run if it was only off 1 tooth.
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Sounds very much like a valve which occasionally sticks open. Squirting a little oil into the carburetor while running may clear it up. If not, it may be necessary to remove the head and clean the carbon from the valves and combustion chamber.
Don Young
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