Take the diaphram out and inspect it. I had a engine I could not keep in tune. When I held the diaphrame up to the sunlight I noticed a very tiny crack in it. Thats when you need a new one. If it has more than one rubber part that does not look good... replace all. Capt,n P.S. when I store a carb...I run silicone spray in it with red tube. Works Great!