As the search for the service bulletins on the Toro site requires the serial number, it's possible that only serial numbers covered by the service bulletin will result in positive response.
The OP has a 21386 rather than a 21387 which is the model covered by the service bulletin. But as the problem of sputtering is the same in both models (albeit different size engines), the service bulletin covers replacing the carburetor, and that corrects the problem in the 21387, the 21387 replacement carb could reveal what change is made (relative to the factory-installed carb) and that might be applicable to the 21386 carb, even though there might not be a corresponding bulletin for the 21386.
Thanks for trying.
The OP has a 21386 rather than a 21387 which is the model covered by the service bulletin. But as the problem of sputtering is the same in both models (albeit different size engines), the service bulletin covers replacing the carburetor, and that corrects the problem in the 21387, the 21387 replacement carb could reveal what change is made (relative to the factory-installed carb) and that might be applicable to the 21386 carb, even though there might not be a corresponding bulletin for the 21386.
Thanks for trying.