I have a 5hp Briggs on a Leaf Blower. Engine will start cold on second pull with choke, it will run at full power for 30-45 minutes then loses power. At that point it will run at idle fine and will run a bit better at higher RPMs with the choke half open. It will lope up and down though and occassionally backfire through the carb (air cleaner removed). When it dies it is very hard to get to restart. I checked fuel tank cap for vent Running without air cleaner New Spark Plug New oil (since I was working on it) Any thoughts?