If your tractor or small engine equipped with a Marvel-Schebler carburetor is surging or “hunting” at light throttle, it means the engine is constantly speeding up and slowing down without throttle input. This annoying issue is most noticeable when the engine is warm and running under light load. Fortunately, the causes are well known and usually easy to track down.

Surging occures when the engine can’t maintain a steady RPM and oscillates up and down. You’ll hear it rise and fall rythmically, as if the engine is trying to correct itself over and over. This usually happens when the air-fuel mixture is unstable at part-throttle.

VACUUM LEAKS

Air leaking into the intake manifold can lean out the mixture, especially at part throttle. Check:

• Throttle shaft bushings (worn shaft = extra air)
• Carburetor to manifold gasket
• Cracked vacuum lines or ports

Spray carb cleaner around these areas while idling. If you hear a change in RPM, you’ve found a leak.

Throttle shaft bushings can be replaced to tighten up the throttle shaft, unless the shaft itself is worn or uneven in which case you’ll need to find a replacement.

CLOGGED IDLE OR TRANSITION PASSAGES

Surging is often caused by partially blocked idle or transfer ports. The idle circuit delivers fuel below the throttle plate, and the transition slot fills the gap between idle and the main jet.

If these passages are dirty or varnished:

• Remove idle mixture screw and clean with a soft wire or nylon brush, then blow out the passages with cleaner or compressed air.
• Run soft wire or nylon brush through the transfer slot (if your carb is equipped with one)

IMPROPER IDLE MIXTURE ADJUSTMENT

A lean mixture will cause unstable combustion at light throttle. Set the idle mixture screw by:

• Gently turn the screw all the way in just until it seats, the back out 1 1/2 turns.
• Fine tune the screw while the engine is idling warm - adjust for highest smooth RPM, turning the screw out 1/4 turn at a time.
• DO NOT over tighten the screw or you will damage the metering tip.

Too lean = surge or stall

Too right = lumpy idle or black smoke