It is a 1999 MTD Yard Machine. Model 13AQ670G372. It has a Briggs and Stratton 18 HP Twin 2 engine. It starts and will do up to a higher than idle speed. When I put the blades down to mow it will mow about 5 to 10 mins and start to bog and die. If I put it in slow, it will stay running and I have to keep it like this for awhile. I've cleaned the carb, tank, and fuel lines. I replaced the fuel filter also. According to the repair manual, 1 spring is missing in the fuel pump. It doesn't seem to want to stay running. Like maybe the governor is not functioning right or something. I even gapped the plugs to .030 like the manual said. I've been working on it for 4 days. Getting no where with it. Thought it had a battery charging problem but I checked it via the procedures in the manual and it's good. I really need some help with this thing. Thanks for any info or help in advance.