I picked up a clean mower with a Briggs 104M02-0121-F1. DOM DEC 2015. The carb was taken apart and the needle was missing as well as the air filter and air filter cover. I bought a new after market carb on ebay for about $12. A new B&S are like $75. The mower started on one pull but it surged and would stall out when you pushed it. I checked all of the usual stuff like the fly wheel key, the air intake, compression is good and valves are in spec. Fresh gas and the oil is clean. It was over filled so I drained some off. As so often what is needs to happen is the jet is too lean on the aftermarket carb. I estimated it was .020 with my micro drills and I opnend to about .024 which is a good amount but it was the closest drill in that range I had. It started on one pull and idled fine. As I push it, again it begins to surge and can stall if I load it up. I let it run a bit and then pulled the plug. It looked a bit rich. That does not bother me. The Head gasket seems ok. I ran it with out the shroud for a bit and sprayed brake fluid around the head gasket and no change in idle. The compression seems fine per the bounce test. I am wondering if there is some other small part missing. I have not worked a lot on these engines as I usually work on older stuff. I looked at the B&S exploded view diagram and it seems all is there. What am I missing?