On newer model front engine tractors, mufflers are located at the furthest position from the operator. Depending on wind and operation, occasionally engine exhaust may travel towards the operator.
Operators are advised to try and mow with the direction of or perpendicular to winds in order to reduce exhaust exposure, instead of traveling into the wind whenever possible.
Bear in mind that all engines used, meet or exceed all applicable EPA compliance regulations at the time of manufacture.
If the emissions seem excessive, then a tune-up or carburetor adjustment may be necessary to correct this.
The brand and type of fuel being used, a dirty air filter or old oil in the crankcase can also aggravate these exhaust odors.