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Why small-engine carburetors gum up in storage

Tiny jets and passages plus old fuel is the recipe for a mower, trimmer or generator that won’t start in spring. Here’s why it happens and how to head it off.

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Small-engine carburetors have very small fuel passages. When gasoline sits, the lighter parts evaporate and the fuel oxidizes, leaving sticky gum and hard varnish behind. A small amount of residue is enough to partly block a jet, which shows up as hard starting, surging or an engine that only runs with the choke on.

What causes gum and varnish

  • Time. Gasoline begins to degrade within weeks to months, faster in heat.
  • Evaporation. Fuel left in a float bowl evaporates and concentrates the residue.
  • Ethanol and moisture. Ethanol-blended fuel can absorb water, which adds to corrosion and deposits in fuel systems.
  • Partial tanks. More air space in the tank means more room for moisture and oxidation.

Two ways to store a fuel system

Stabilize and run: add a fuel stabilizer to fresh fuel, then run the engine long enough for treated fuel to reach the carburetor. Run dry: some manuals call for running the engine out of fuel or draining the float bowl. Use the method your owner’s manual recommends; some equipment makers specify one or the other.

Storage checklist to prevent gum

  1. Start with fresh fuel from a clean, approved container.
  2. Add stabilizer at the label rate before the last fill-up of the season.
  3. Run the engine several minutes so treated fuel reaches the carburetor.
  4. Follow the manual on whether to leave the tank full, drain the bowl or run dry.
  5. Shut the fuel valve if the machine has one.
  6. Store spare gas treated and use it within the stabilizer’s stated window.
  7. In spring, if it starts hard, try fresh fuel and a carburetor cleaner before replacing parts.

Frequently asked questions

Will stabilizer fix a carburetor that is already gummed up?

A stabilizer is meant to prevent the problem. For existing deposits, a cleaner designed for carburetors may help; heavy varnish may still need the carburetor cleaned or serviced.

Should I drain the tank or leave it full?

Follow your manual. Many recommend a full tank of stabilized fuel to limit air space; some call for running dry.

Is two-stroke mix affected too?

Yes. Premix sitting in a handheld tool can leave deposits as well. Use fresh mix or treated mix, and follow the tool maker’s storage guidance.

AMSOIL products for this job

  • AMSOIL Gasoline Stabilizer. AMSOIL says: “Helps keep fuel fresh up to 12 months”; “Protects against varnish and gum buildup”; “Fights ethanol corrosion”; “Helps eliminate the need to clean or replace carburetors after long-term storage”.
  • AMSOIL Quickshot. AMSOIL says: “Cleans gummed/dirty injectors and carburetors”; “Stabilizes fuel during short-term storage”.
  • AMSOIL Power Foam. AMSOIL says: “Removes gum and varnish”; “Restores startability”; “Especially effective in powersports and small-engine applications that run, then sit for extended periods”.

Product wording above is quoted or summarized from amsoil.com. Read each product’s label and data sheet for directions.

Use AMSOIL’s official lookup guide to see what is listed for your vehicle or equipment, then order online whenever it suits you.