Adjusting the idle and low speed system on the WG-8 or the WB-37

by Had Robinson

If your carburetor is the WB-37, follow this link to adjust the idle and low speed system.

For clutched motors: If the clutch engages at idle (< 2,200 RPM), it is impossible to adjust the idle properly. Fix this first.

If your engine will not idle no matter how you adjust it, you may have a leaky or gummed up main nozzle check valve (a rare event).  See the main nozzle check valve section of "Rebuilding and tune up" of the Walbro WG-8 carburetor on how to do the easy test to see if it is leaking or stuck.  Usually, poor idle is a result of the need to rebuild the carburetor.

Note that 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 (±20ºF).  Here are part numbers for the WG-8 (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, if necessary, for a few minutes.  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.  Screw in the needle and note the point when the engine begins to slow (lean drop-off point).  Unscrew the needle and not 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 amount of air + fuel entering the engine.  Wait (10) seconds between each change of the idle needle because it takes time for the adjustment to take effect.  You can play 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.

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 (when they strike the starter pawl, they make a ratcheting sound).  This speed is around 2,200 RPM.  As with the idle needle adjustment, wait (10) seconds between adjustments 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-8 does not have an accelerator system like an automobile.  SUDDEN OPENING OF THE THROTTLE CAN STALL AN ENGINE.  There is no way to fix this but to be gentle on the throttle.  Always monitor your CHT while flying.  A sudden clogging of any of the filters can cause an extreme lean condition and burn up 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.

Do not idle the engine excessively, as is often done before launch.  This will foul the plug, slow the idle, and cause carbon build up inside the cylinder.


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, 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 while in the air (the time of the greatest load on the engine).  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. 

Turkey Vulture