# How does the B&S autochoke system work?



## junebug1701 (Jun 15, 2009)

I fixed my son's Troy-Bilt mower today, the B&S 126T02-0675-B2 would start, but just sputter along. Just past the warranty, so he's been using my old backup mower till I got time to work on his. At first I thought maybe he hit something, partially shearing the flywheel key because it seemed out of time. After getting the cowling off, the key looked fine, but I found the plastic autochoke air vane was stiff and hard to move. No way would the air from the flywheel vanes move this thing. Checked the carb and of course it was gummed up with varnish. After spraying it with carb cleaner it was moving nice and free. Changed the air filter, plug, and topped it off with fresh oil and gas and it started on the first pull and my son thinks I'm a genius!

But I still don't quite understand how this autochoke system works. I found the thermostat and it has a lever that contacts this choke vane. But why does the choke have an air vane like a governor?


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## 30yearTech (Sep 14, 2006)

When the engine starts, air rushing past the air vane opens the choke. That's what makes it "Auto". When the engine stops running and there is no air pressure from the flywheel a small spring pulls the choke closed. The thermostat holds is partially open if the engine is warm to prevent over choking when restarting a hot engine.


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## junebug1701 (Jun 15, 2009)

30yearTech said:


> When the engine starts, air rushing past the air vane opens the choke. That's what makes it "Auto". When the engine stops running and there is no air pressure from the flywheel a small spring pulls the choke closed. The thermostat holds is partially open if the engine is warm to prevent over choking when restarting a hot engine.


Interesting, I have the same setup on my Craftsman mower, but it's hard to start when the weather is cool. Not a big deal except for the first few weeks of cutting season. Does the thermostat go by ambient air temperature or engine temperature?


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## 30yearTech (Sep 14, 2006)

junebug1701 said:


> Interesting, I have the same setup on my Craftsman mower, but it's hard to start when the weather is cool. Not a big deal except for the first few weeks of cutting season. Does the thermostat go by ambient air temperature or engine temperature?


Engine temperature. Check your choke and make sure it's closing all the way. Debris can build up under the shroud and keep the choke from closing all the way, the return spring can also be broken or missing.


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## junebug1701 (Jun 15, 2009)

30yearTech said:


> Engine temperature. Check your choke and make sure it's closing all the way. Debris can build up under the shroud and keep the choke from closing all the way, the return spring can also be broken or missing.


Thanks, 30year, I'll check it out!


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