Maintenance Matters

December 2017 Issue

Carburetor Idle Issues: Check the Mixture First

If you live in a climate with seasonal temperature swings, adjusting the carburetor idle mixture may be required more often than you think. There are some clues.

Have you noticed that carbureted engines behave differently in varying temperatures? We have, and it’s a pretty good argument for the simplicity of fuel injection.

But if you’re stuck in the carb age, tweaking the idle mixture (perhaps often) for the intended operating climate could be a necessary evil for properly maintaining it. Unless you’re an A&P, adjusting carburetors may not be an approved owner-performed task. But you can help your tech get it right to your liking by knowing the basics. Here’s a primer.

By The Book, By The Tach

Face it, from a fundamental standpoint, the carburetor on your Cessna is hardly much different than the one you maybe had on your small-block Chevy engine. Dealing with its maintenance is the familiar nuisance of dealing with stuck float bowls, chasing fuel leaks and doing preventive rebuilds.

At the least, if you want your engine to run well on and off idle, commit to frequent adjustments. Lycoming even suggests resetting idle mixture on a seasonal basis, if you live in an area with wide swings in density altitude. But we say only if there’s an issue. With finicky carbs, we’ve long subscribed to the “if it isn’t broken, don’t fix it” theory. And, we wouldn’t attempt any adjusting without a current service manual, plus service data for any modifications made to the fueling system.

If you’re still using an analog tachometer, don’t go any further without knowing when it was last calibrated. If there’s doubt, send it to an instrument shop for a bench cal.

As for symptoms that suggest it’s time for an idle mixture adjustment, you might observe the engine running rich in summer and lean in winter temperatures.

Some engines are more affected than others due to carb location, intake plumbing, baffling and so on. In warm weather the tipoff is, first, a noticeable RPM gain on pulling the mixture control to idle cutoff—maybe as much as 100 RPM or more. The reverse may be the case when it’s cold. You might also find that spark plugs are fouling easily, even with a lean mixture set for taxiing.

Pull It Back

Don’t adjust anything unless you’re confident in the health of the magneto timing, the condition of the spark plugs and obviously the carb itself, although attempting the adjustment is the first step. Head to the runup area (or fly it first) to warm the engine. With carb heat off and a clean air filter in place, face the airplane 90 degrees to the wind and adjust the throttle to give minimum RPM. This is typically around 650 RPM. Secure the throttle friction lock to keep it from creeping. Move the mixture control toward idle cutoff and monitor the tachometer.

As you come to the last inch or two of mixture travel, you should notice a 25- to 50-RPM rise in engine speed before the engine falters from lean misfire. Keep it running and record the actual RPM rise, whatever it is. If your lean-out gave more than a 50-RPM rise, your carburetor (or fuel injection—the same test applies) is set too rich and needs to be compensated in the lean direction.

Conversely, if you saw little or no RPM rise, your idle mixture is set too lean. The idle mixture adjustment on a Marvel-Schebler carburetor is in the form of a large knurled screw (or small slotted knob) on the throttle casting, high on the carb. On MA-3 and MA-4 series carbs, for example, find the bowl drain plug, then run your finger (or eyes) straight up the side of the carb until you come to a slotted knob with arrows on it pointing to “R” (rich) and “L” (lean). That’s the idle mixture adjustment. Give this screw a turn in the desired direction, then repeat the lean-out procedure described above.

Of course, alterations in idle mixture have an effect on idle speed as well. If your engine was set too rich (i.e., 150-RPM rise on shutdown) and you corrected this by turning the idle mixture screw as needed to give the desired 50-RPM rise, your engine will now probably idle about 100 RPM faster than before. Accordingly, you’ll want to adjust the idle speed to put it back in the 650-750 RPM range. Any changes require supervision by an A&P.

On a Marvel-Schebler carb, as on an automotive carb, the idle speed adjustment is made with a set-screw on the low-RPM stop at the throttle arm on the carburetor. Make adjustments as needed to bring the idle RPM back to 650-750. Repeat the idle mixture lean-out check. You want no more than a 50-RPM rise on lean-out.

Obviously, several iterations of the basic procedure may well be necessary to get the carb set up correctly for both idle mixture and idle speed, since one affects the other. Try not to idle the engine so long on the ground, however, that CHT reaches in-flight indications.

And, be aware that with prolonged ground operation at full-rich mixture and a low RPM, spark plug fouling is encouraged, even with a perfectly adjusted idle mixture. That’s because in the 700-RPM range, the plugs operate too cold to burn clean. It really helps to have even the most basic graphic engine monitor for maintenance and normal ops.

Once you have it set, record the RPM rise on every shutdown. It can help you spot float saturation and internal leakage. And while you’re at it, record the date and outside temperature. After a while, the calendar will tell you it’s time to do this all again.