in the fall my gravely walkbehind started acting up. when i turn it on it runs perfectly fine untill i engage the blades. when i do the motor cuts down to about half the normal speed and the smell of gas takes over but still runs. ive changed the fuel filter and airfilter all the time, put seafoam in the gas and before an oil change, ive also checked the choke cable to see if it got hung up or not working properly. everything looks fine, any ideas?