I noticed on my 158cc briggs and stratton that when you turn the crank by hand and when it starts its compress stroke the exhaust valve opens a bit and then closes before it hits top dead center for the spark. There is something below the cam gear that hits the valve stem when it does this. Is it supposed to do this? I don't know what else the thing below the gear does? Sorry I can't send a pic., but here is the part number for the cam gear,691998. Any help would be helpful. Thanks.