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Old 11-03-2005, 11:38 AM
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Question Irregular Idle

Whether idling or in gear under load, the RPMs on my A-4 seem to vary more both up and down than they should. It's kind of erratic. Sometimes it seems like random power surges, more often like random power losses. What do you think might cause that?
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Old 11-03-2005, 11:41 AM
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Don Moyer Don Moyer is offline
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In the case of your irregular idle, we prepared the following guide which may help. The guide doesn’t address the issue of irregular RPM above idle. My best guess is that you may have some sloppiness in the throttle cable and levers somewhere between the throttle handle in the cockpit and the throttle lever on the carburetor.

“The following problems within the idle system (listed somewhat in decreasing order of likelihood) can cause uneven or unreliable idling:

IMPROPER IDLE SET SCREW ADJUSTMENT: Your idle RPM may be set – not by the idle set screw – but by the throttle cable itself. In other words, the idle set screw is backed off so far that the lower limit of the throttle cable is reached before the idle set screw. The problem with this scenario is that throttle cables do not have an accurate end point, and they don’t always stay where you put them. If you pull the throttle cable to its minimum position and walk away, it’s likely to spring back up or down a bit, moving the throttle control arm with it.

To check the idle stop, place the throttle lever in the cockpit at idle, then go down to the carburetor (with a flashlight and mirror if necessary) and note whether or not the end of the idle set screw is actually touching the idle stop coming out from the housing of the carburetor. If it is not, reposition your throttle cable so that the idle set screw does touch the stop.

This adjustment will probably result in too slow an idle, and the next time you start the engine, it will probably shut down as you slowly move the throttle to idle. So the next (and final step) is to readjust the idle set screw to the idle RPM that you want. In this new setting, the idle RPM will actually be set by the idle stop and not by the random positioning of the end of the cable.


1) Dirt in the idle jet.

2) A small speck of dirt in or near the upper of two tiny outlet ports of the idle system. This is the port (about the size of a fly speck) that you see just off the edge of the throttle valve when looking down from the mounting flange of the carburetor.

3) A faulty seal around the idle passage way between the upper and lower halves of the carburetor housing. This passageway is in the very center of the carburetor housing and is about the size of a drinking straw. If the gasket between the upper and lower housing is not completely sealed around this passageway, air can be drawn in and break the suction necessary to draw fuel up to the upper idle port.

4) A leaky float valve which provides such a rich mixture that it floods out the engine at idle RPM.

5) There is an air bleed hole in a small brass plug that is pressed into the lower face of the upper half of the carburetor that sometimes becomes blocked. This plug is located between the idle fuel jet and the venturi tube in the main throat of the carburetor and can be easily seen when the upper half of the carburetor is separated from the lower half. Sometimes a small speck of dirt lies on top of this small plug and interferes with the air being drawn in through the bleed hole to mix with the idle fuel.

MANIFOLD LEAK: You could have a vacuum leak in the manifold gasket or carburetor flange gasket. Some folks have been successful in diagnosing a vacuum leak in these areas by spraying some starting fluid around the manifold gasket and carburetor flange gasket while the engine is at idle. If the engine RPM changes at all while spraying the starting fluid, the gasket is almost certainly leaking.”


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