Adjusting Carburetors

If your engine won’t start or runs poorly, check all the other possibilities (see Troubleshooting Small Engines) before you adjust the carburetor. But if the problem is nowhere else, it may be in the carburetor—particularly if you have recently had it overhauled.

There are two basic adjustments you can make on the carburetor: the idle speed and the flow of fuel. The idle speed is regulated by a screw attached to the throttle linkage near the carburetor. To adjust the idle speed, simply turn the screw one way or another. This puts pressure on, or releases pressure from, the throttle. Tighten the screw to increase the idle speed, and loosen the screw to lower the idle speed. Make this adjustment when the engine is running and thoroughly warmed up.

Full-flow adjustment valves are controlled by one or more screws called needle valves. These screws have tapered tips to control the amount of fuel that flows into the carburetor. Tightening the screw moves the needle tip deeper into the valve and reduces the flow of fuel into the carburetor. The needle-valve screw is brass—therefore soft—so don’t overtighten it.

Before you make an adjustment, note the precise location of the screw slot. If you find that making an adjustment has not solved the problem, return the screw slot to its original location.

To make the adjustment, let the engine run; turn the screw a quarter-turn to the right and listen for any change in the engine’s sound. Now give the screw a quarter-turn to the left past the original position and compare the sound of the engine. If your problem is in the needle-valve adjustment, you should be able to find one particular position that will make the engine run faster and smoother. Turn the needle valve until the engine sounds best, and leave the valve there. If there is no position that will make the engine sound right, then the problem is not in the needle valve adjustment, and you should return the screw to the original position and check elsewhere for the problem.

If you have completely lost the original position of the screw, carefully tighten it all the way down and then back off one and a quarter turns. Start the engine and make small adjustments until the engine sounds best.

Some carburetors may have two needle valves side by side. The one closest to the source of air into the carburetor is for adjusting low speeds and idling. The other is for adjusting operating speeds.