Bad Thermostat choke on Briggs 550EX???

[6.5GRENDEL]

My neighbor has a Yard Machines push mower with a Briggs 550EX (model 09P702-0145-F1) with that stupid ReadyStart auto-choke feature. He brought it down saying it wasn't running right so the fastest thing to get him back in operation was to just throw a new carb on there (since they're only $22) and clean the old one at my leisure and keep for back up. The plan was going swell until he returned with the same symptoms; after a short time of use the mower starts surging and coughing out black smoke. So I pop off the air cleaner to have a look and the choke butterfly is closed. That explains the black smoke and running rich and rough. The thermostat, when hot should hold the choke open. As best I can tell the thermostat and linkage that controls the choke butterfly is intermittently working. Does that sound right or even possible? I've looked for a replacement thermostat and can't find them available separately but only with a muffler for about $40. I've disabled the automatic choke and the mower runs fine with no possibility of the choke closing now on a hot engine. Is that the answer to these goofy auto-choke engines???

 

[Joe Kuhn]

Just had the same problem on my own mower and found the part separately for less that $20. Hardest step was finding the part number on the Briggs site. With a model number shown like this:



I discovered typing in the first part before the first dash brings something up on the Briggs site so you can see the formatting they use. I ended up using this:

128M02-0943-F1

to get the parts diagram and part number so I could search further. Your details may vary.