My old Briggs & Stratton 45 is "surging" (ie: revs cycling up and down rhythmically about once per second). As the mower is self-propelled, this causes it to go forward in jerks. I thought it might be "hunting" but choke is going off properly and it is starting easily. All tubes are clear and air filter new. What should I check next, please?

James Ellis, May 2011