I have a 16.5 OHV Briggs that was running great two days ago and I went to mow and it was surging at high throttle. I can back it off to half throttle and it will run. I took of the carb and sprayed with carb cleaner and blew off passages and reassembled but I didn't take off bowl or carb solenoid because I couldn't get it off. Once I reassembled it, it worked great for about 45 minutes of mowing and then started surging again at high throttle. What can I check?