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.
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
If there is a gear problem, it shouldn't run at all, I would think.
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
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