It is normal for a brand new two-stage snow thrower engine to run in this manner. The engines on these snow throwers are set to run with a slightly rich fuel mixture. This is to prevent the engine from bogging if the unit were to experience a heavy load of snow throwing. Any engine that is operating as a new engine will most likely experience this.

If a single or two-stage unit is not brand new or is not operating as a new engine should, then probable causes to this issues are fuel or fuel line related. A unit with this issue may also experience the engine surging, troubled starting, stalling or the engine may only run properly with the choke partially open.