I have a Fradan leaf vac with a 18hp brigs engine. Just got it ready this past weekend and have noticed that it has a stumble when running. When you first start it up i turn off the choke and it will want to stall. I'll have to chycle the choke on and off a few times till it stays running. then once running it has a stumble no matter what throttle position i am in. I have replaced the air and fuel filters and sprayed carb cleaner into the carb with the air filter removed. It still does the same thing, I have looked at the plugs and they look normal, not covered in fuel. Any ideas?