This is what I've seen can happen to old carbs.
How Micro-Cracks Form in a Carburetor
Carburetors are usually made of die-cast aluminum or zinc alloy (pot metal). Over time, several things can create microscopic cracks or porosity in the casting:
⚗️ a. Corrosion from Ethanol Fuel or Water
Ethanol attracts water from the air.
That water, combined with fuel residues, forms acids that attack aluminum or zinc.
The corrosion pits and weakens the metal, especially around thin internal walls and galleries.
With time, these pits can connect, forming tiny internal cracks or porous spots between adjacent fuel and air passages.
🔥 b. Thermal Cycling
Lawn mowers go from cold to hot repeatedly.
The carb body expands and contracts each time, stressing thin casting areas (like around the emulsion tube or fuel well).
This mechanical fatigue can form hairline cracks over years of use.
🔩 c. Overtightening Screws or Fittings
If the fuel inlet fitting or the bowl screws are overtightened, the metal can distort or crack, especially around threaded holes or sealing surfaces.
Once the crack starts, vibration can slowly extend it further into the body.
🧴 d. Manufacturing Porosity
Some carb castings have tiny air bubbles or inclusions left over from the die-casting process.
With time and corrosion, those pores open up and act like tiny leaks.
⚙️ 2. What Micro-Cracks Do to Carb Function
Carbs rely on precise pressure differentials to meter fuel — a few inches of water column is all it takes. Even a microscopic leak can upset this delicate balance.
Here’s how cracks cause trouble:
💨 a. Internal Air Leak
A crack between an air passage and a fuel passage lets air bleed into the fuel stream.
That leans the mixture — so the engine surges or “hunts” as the governor constantly tries to compensate.
⛽ b. Fuel Leakage Between Passages
If a crack connects the main jet well to the float bowl vent or to the atmosphere, fuel pressure can’t build properly.
The carb can’t pull enough fuel up through the emulsion tube → lean running or stalling at throttle transitions.
💧 c. External Fuel Seepage
A visible hairline crack in the casting (often near the bowl threads or inlet) can allow fuel to weep out slowly.
You might see dampness or fuel smell even if it doesn’t drip.
🧭 3. Diagnosing a Suspected Crack or Porosity
You can try a few tests:
Vacuum Test:
Plug all outlets and apply light suction to one circuit (like the idle jet port). If it won’t hold vacuum, there’s an internal leak.
Soapy Water Test:
Fill the carb body with light air pressure (~5 psi) and spray with soapy water. Bubbles = leak.
Fuel Dye Test:
Fill with dyed fuel (like 2-stroke mix) and watch under bright light for seepage through the casting.
Magnifying Glass / UV Light:
Sometimes cracks show up as faint lines or stains along cast ribs when viewed under UV or strong light.
🧰 4. What To Do if a Crack Exists
Unfortunately, there’s no reliable permanent repair for an internal micro-crack in a carb body. Epoxy or sealant might work briefly, but fuel, heat, and vibration eventually break it down.
So, once confirmed:
Replacement is the only dependable fix.
Cheers
Max.
