My 1999 model Briggs Twin II 42E707 model on Murray lawn tractor has had an issue where it surges at idle for at least 3 yrs. I took the carb apart 3 times and rebuilt the fuel pump twice in an effort to get it to stop but it persists. Engine runs pretty well under load but will issue a single backfire at shutdown after running it an hour or two. I recently found a new factory carb at a reasonable price so replaced it this Spring. First time out of the box the engine ran great, smooth idle. Second time, right back to surging again. I was using the round white gas filters on it as they were listed as the ones to use with the fuel pump engines. Looking through owners book on the engine, it specifies the 5018 filter even though package says non fuel pump engine. So installed a new one of those. Compression is 100 and 110 on the 2 cylinders. Air filter is new, ignition is new, spark plugs changed in DEC. Fresh 90 octane ethanol free fuel, changed fuel lines sometime in last 3 yrs trying to sort out the problem. Fuel tank looks clean. Any ideas?

Re: push mower with a B&S 3.5 hp engine, Model: 95902, Type: 3107 01, Code: 96050451 When priming, gas leaks from the carburetor below breather connection. There are two small holes there, obviously by design, both plugged with what look like a tiny stainless balls. The one just below the breather leaks when pushing the primer bulb. When one plugs the hole with a finger, the carburetor can be primed just fine and the engine starts without any problems. What are those holes for and how to fix the leaking one?