I have a Briggs and Stratton 16 horse on an older Montgomery Ward riding lawn mower. The engine won't stay running unless the carb is choked a little. If I push the choke all the way in the engine will bog down and die. It runs ok with it choked a little, but lacks power and bogs down when put under load. It won't start either unless it is choked also, regardless of how long it has been running. I may have put some 92 octane gas in accidentally, would that have an effect on the performance? It does backfire after I shut it off now since I put in the 92 octane. Any ideas? I have a snowblower on the front and don't feel like shoveling, so please give me some ideas.