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Hello,

I purchased a used snowblower last year with a Tecumseh engine. It starts on the first pull and seemly runs fine at first. After about 20 min of use it starts running rough and sparks start to fly out of the exhaust. The muffler is glowing red. The snowblower then stalls and will not start until it cools down.

Here is a video of it running:
https://photos.app.goo.gl/1oJewheLVYXDpuWz9

I replaced the ignition coil last summer and it didn't make a difference. I just cleaned the carb as the logical next step.

Any other tips? I read a few posts where there could be issues with exhaust valve lash or head gasket issues. Rather not dig that far if I don't have to.

Thanks,

--Ryan
 

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Check the valve clearance. They are known for hammering the valve seats down, and the clearance gets to tight.
 

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Valve clearances being too small were my first thought. If the clearance is too tight, the valve may not close fully, once the engine heats up. If it's the exhaust valve, this causes a glowing muffler, etc, since flames from combustion are actually getting into the muffler, since the cylinder area isn't fully closed-up.

I would at least start by *measuring* your clearances, which requires a set of flat feeler gauges. There are Tecumseh manuals in the Stickied threads at the top of the Repairs forum. This is the Tecumseh 4-stroke flathead engine service manual:
http://www.barrettsmallengine.com/manual/tecumsehlheadmanual.pdf

Unfortunately, this looks like a flathead engine, not an OHV engine, so adjusting valve clearances is more difficult.

One method is apparently to remove a small cover on the side of the engine, and use a small file to take some material off the back of the valve stem, for whichever valve's clearance is too small. There are other ways as well. The forum search function isn't working great at the moment, but fixing valve clearances on flathead (L-head) engines is a fairly common topic, if you need to go down this path.
 

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To find out, you could add partial choke, and see if it smooths out. You did say you cleaned the carb, but several things could contribute to running lean, including an air leak at the gasket between the carb & engine.
 
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