Snapper w/ Briggs 286707 12.5hp surges when riding
So I've replaced a lot of parts on this Briggs 12.5hp since I got this Snapper from a friend. Where it sat outside for a couple years. I've done all the maintenance including cleaning the cylinder head which wasn't really too bad. I replaced the carburetor, filters, cleaned the fuel tank, put a new filter, plug, starter clutch, rewind spring, throttle cable, adjusted the throttle control lever, coil. It actually will run and mow without cutting off anymore but it surges every couple seconds. It seems to be more noticeable at fast speed. It starts up real easy, usually with just a single pull. It feels strong, just has a hiccup every couple seconds that you can hear. I'm guessing it's considered a miss. I thought maybe the flywheel key was sheared but it wasn't. I replaced it when I changed the starter clutch. I will say that I thought maybe just the mixture on the carburetor is off. But when I go to adjust the mixture screw and it's suppose to stumble when you turn it all the way in it never does. Actually turning the mixture screw doesn't seem to affect it either way, at least in idle. It's like it has no affect, so it's hard to adjust. Any ideas?