Engine Answer Pages

Set that needle valve!

A common problem seen time and time again is the mis-adjustment of the needle valves on both 4 and 2-cycle engines.

The traditional carburetor uses two separate needle valves to adjust the fuel mixture, one for the idle circuit and the other for high speed adjustment. A critical factor when adjusting these needles is that in most cases the adjustment of one needle has a cross-couple effect and will alter the setting of the other needle.

Many times I see modelers lean or richen the low end needle and then wonder why this has also leaned out the transition from idle to midrange and altered full speed mixture setting. This will happen in virtually all carburator designs. Richening the high speed needle does not have as dramatic an effect as altering the low end but there is usually a slight cross-couple between the two needles that will have to be adjusted after setting either needle.

It is sometimes difficult to tell whether your needle settings are rich or lean -- especially at idle. There is an easy way to determine what side of rich or lean you are on. Simply pinch the fuel line close to the carb and listen to what happens. If the engine dies with no increase in RPM, it is lean. If the engine increases in RPM and then slowly dies of fuel starvation then it is on the proper side (richer) of the mixture setting and you can adjust accordingly.

It is always important to remember to always be on the RICH side of the mixture setting, especially on the high speed needle. Running lean on the bottom end will not allow the engine to transition cleanly to idle after it has been run at higher RPM's. The tell tale "ring-ding-ding" sound a two cycle makes coming down to an idle is a sign your idle mixture is too lean.

Adjust the idle screw until you get a clean and consistent transition up to mid throttle where the high speed needle takes over. When you are happy with this setting the engine should settle down into an idle as soon as the throttle is dropped. There should be no stumbling or hesitation if the carb is adjusted correctly.

If you are too rich it will burble and load up with the excess fuel and if too lean it will stumble and slow from the lack of fuel. This is a process that takes a little time but you will be rewarded with an engine that responds instantly to throttle and will rarely if ever let you down by causing dead stick landings.

Grab the screwdriver and set that needle!!