Adjusting the idle and low speed system on the WG or the WB
by Had Robinson
updated September 16, 2023
If your carburetor is the WB, follow this link to adjust the idle and low speed system.
Note: For clutched motors: If the clutch engages at idle (< 2,200 RPM), it is impossible to adjust the idle properly. Fix this first.
Note: If your engine will not idle correctly or is erratic no matter how you adjust it, the carburetor probably needs a rebuild. Very rarely, it may have a leaky or gummed up main nozzle check valve. See the main nozzle check valve section of "Rebuilding and tune up" of the Walbro WG carburetor on how to do the easy test to see if it is leaking or stuck.
Note: The idle and low speed system must be re-adjusted whenever there is a significant change in the operating altitude (> 1,000') or ambient temperature.
Here are the numbers and locations for the various internal parts of the WG (referenced below).
1. Set idle needle If it has been removed or tampered with, set the idle needle (part #39, the one without the cone on it) to 1.25 turns out from all the way screwed in. If you live above 4,000' MSL, set the needle to 1.0 turn. Be careful not to force the needle against the seat or you will damage the carburetor. Warm up the engine to 70C. If the carburetor is far out of adjustment, screw in the idle speed screw (part #27 with the cone on it) so that the engine will idle without stalling. It does not matter at this point if the engine idles too fast or too slow.
2. Get max speed Screw the idle needle (part #39) in or out by 1/8 turn increments until the engine runs the maximum speed. It may take a few seconds for the adjustment to have an effect. Screw in the needle and note the point when the engine begins to slow (lean drop-off point). Unscrew the needle and note the point when the engine begins to slow (the rich drop-off point). Your final setting point is exactly half way between the two drop-off points. This needle adjusts the ratio of air and fuel entering the engine. You can experiment with this adjustment, screwing the needle too far in (engine slows from a lean mixture) or screwing the needle to far out (engine slows from a rich mixture) to see how the idle needle works. This must be set correctly before adjusting the idle SPEED in the next step. It is better if this adjustment is on the rich side.
3. Set the idle speed screw (part #27 with the cone on it) just fast enough so that the clutch does not engage, causing the propeller to rotate. If your engine does not have a clutch, set the idle speed to specs or about 2,200 RPM. If your carburetor was way out of adjustment, you may have to unscrew it a turn or two to bring the engine speed down. The engine must not idle so slow as to engage the starter pawls, if you engine has this type of starter). This speed is around 2,200 RPM. As with the idle needle adjustment, wait a few seconds between adjustments in order for the engine speed to stabilize. Stop the engine and restart. The idle should be steady.
4. Final idle needle adjustment It is best to do this adjustment after taking a flight. If the engine stumbles on acceleration, then open the idle needle 1/8 turn. The WG does not have an accelerator system like an automobile. SUDDEN OPENING OF THE THROTTLE CAN STALL AN ENGINE. The only way to fix this is to install the FSM. Always monitor your CHT while flying. A sudden and partial clogging of any of the filters can cause an extreme lean condition and overheat your engine.
5. High speed system Once the idle and low speed systems have been adjusted, the high speed system may be adjusted, if necessary. Unless there are problems with the engine running at 3/4 throttle and above, changes there should not be necessary.
If carburetor adjustments do not work as expected, there may be other issues. Check the fuel delivery system and make sure that it is working properly. The metering lever may need to be checked and adjusted. If the ignition system has any defects, starting will be difficult and the top end performance will suffer. If the carburetor is worn out or needs a rebuild, it will not idle or run smoothly. If the reed valve block is not torqued down properly it will leak and the fuel pump will not work properly, especially at 3/4 throttle and above. If the carburetor mounting nuts are tightened excessively, the gasket between the carburetor and the reed valve will be deformed which can affect fuel pump operation.