Small Engine Lookup

Lawn mower engine surging or hunting: causes and checks

Reviewed 4 primary sourcesSee sources

Surging, or hunting, is when the engine speed rises and falls on its own. Kohler’s carburetor tables list it as a symptom of the engine running too rich or too lean; the governor that holds speed steady is the other usual suspect.

Checks in order

  1. Check the air filter

    A clogged air cleaner makes the engine run rich. Clean or replace the element.[2][4]

  2. Check the fuel

    Kohler’s owner’s manual lists improper fuel and dirt in the fuel line as causes of erratic running.[1]

  3. Check the debris screen and cooling fins

    Kohler lists a dirty debris screen. Briggs & Stratton explains that air-vane governors need clean cooling fins and air intake to hold speed.[1][3]

  4. Check the choke

    A choke left partly closed makes the engine run rich, which Kohler lists among the causes of hunting.[2]

What the symptom can point to

Running rich

Besides the air cleaner and choke, Kohler lists dirt under the fuel inlet needle, a plugged bowl vent or air bleeds, and a leaky float.[2]

Running lean

Kohler lists plugged idle holes and dirt in the fuel channels. Both mean cleaning the carburetor passages.[2]

When to stop and book a repair

Related

Sources

Facts on this page are restated from the documents below and linked to the publisher. Check the document for your exact model before relying on a value.

  1. XT650, XT675, XT775 owner’s manualRehlko (Kohler engines) · 14 590 36 · Safety p. 2, high altitude p. 4, oil chart and maintenance p. 5, fuel p. 6, spark plug and air cleaner p. 7, troubleshooting and specifications p. 8 · retrieved
  2. XT series service manualRehlko (Kohler engines) · 14 690 01 Rev. G · Identification p. 5, troubleshooting guide pp. 13–14, carburetor causes p. 20, spark plug table p. 27 · retrieved
  3. How does an engine governor work?Briggs & Stratton · How pneumatic governors work · retrieved
  4. 5 mower troubleshooting tipsBriggs & Stratton · “Inspect the air filter” and deck-clearing steps · retrieved

Primary publishers: Rehlko (Kohler engines), Briggs & Stratton. Spotted an error? Email the address in the footer with the model and the document you are using.