I have a poulan pro 550 series B&S 158cc push mower. it sounds like a moved the choke when I went under a bush, because its running at a lower rpm. However, I can't find a manual choke.

The first thing I would do is , look at the front of the mower. On your front right side is your air filter. Remove the air filter, I believe that coincidently your air filter got to the point where it couldn't breathe anymore, meaning clogged to the point of choking. You can tell by putting it up to sun light and looking through it. If it's very very dirty , it needs to be changed. Also if there is a choke, it would be just behind the air filter. If it blocked off the end of the carburater then the choke is on. If this is the case then follow the rod that moves the choke. Possibly it was bent when you went under the bush. I believe you will find that your problem lies with a dirty air filter.