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I have a push type lawn mower with a 3.5 hp Briggs & Stratton engine that idles very rough especially when I fill the tank. It smooths out over time but idles rough again after a fill up. I replaced the plug, the air filter, the oil and I use fresh regular unleaded gasoline. I took off the top covers, checked for anything looking broken or out of place but everything seems ok. The other day, I was using it and it backfired just before shutting down on empty gas. There are no speed adjustments - the only items visible are the throttle with the linkage spring and the safety "dead man" connection.
What could be causing the rough idle?
It sounds like your cap vent may be partially plugged. Clean the cap and it's vent to see if that helps. You could test it by putting the cap on but not screwing it down quite tight. If it runs ok then, it is probably the cap. If cleaning it doesn't help replace it.
Bert
