Carburetor Basics
by BOB GILBERT
as appeared in Model Airplane News


SET-UP AND OPERATION

I'm an old-timer in the hobby, and my motto is "Keep 'em flying." Toward that end, I'll try to help you keep those engines purring!

As simple as possible, this guide will enable you to properly set up 90 percent of the carburetors available today. I'll address 2-stroke sport-type engines up to .65 displacement that don't have tuned pipes, pumps, or three-needle valve carbs.


THROTTLE? CHECK! FUEL? CHECK! ...

To start, set the throttle linkage. If your throttle servo doesn't have an adjustable arm, get one. It will help you immensely when you make these adjustments. (This won't be necessary with newer radios that have servo-travel adjustment built in.) With the linkage disconnected, hold the throttle arm so that you can rotate the barrel toward the closed position. Rotate the idle-speed stop screw so that the barrel closes fully, then rotate it half a turn more. Don't back the screw out any farther, because on some engines this screw also holds the throttle barrel in place. If there's a locknut on the screw, tighten it.

Now connect the throttle linkage to the servo, and adjust it so that the with the stick and trim fully down (emergency cutoff position) the barrel is completely closed, and with the stick and trim fully up (full throttle) the barrel will be 100 percent open. For idle, set the trim at the position that will open the throttle barrel 1/32 of an inch (about the size of a round toothpick at its thickest point) when the stick is fully down and the trim is fully up (idle position).

Next, check the fuel system. The spraybar/needle-valve assembly should be positioned (relative to the tank) no less than a third, but no more than halfway, down from the top of the tank. The fuel tank will be pressurized with muffler pressure. Inspect all fuel lines, metal and otherwise, for kinks and splits. Look inside the tank unless it's brand new. Fuel filters can be a source of air leakage, so remove them for the inital setting.

Always install a new glow plug. The object here is to be as sure as possible that other systems won't interfere with the carburetor when we're setting it up.

By this point, you should have run the engine a little and found that the carb settings are good enough to start turning the engine. Refer now to the pictures of the carburetors, and identify the one that looks like yours. You only need to identify the idle-mixture screw; the high-speed valve is always the longest and most prominent of the adjustment screws.


CARB ADJUSTMENS

If you can, have a helper hold the transmitter. Tell your assistant to open the throttle fully when you point you finger upward, and to close it fully when you point your finger downward.

1. Start the engine and, using the throttle trim, set the idle speed. Typically 2,500 to 3,500 rpm is correct. Then test to see whether, with the throttle stick fully down, moving the trim to the full-down position will shut off the engine as an emergency cutoff. Adjust it as required.

2. Open the throttle fully. If the engine speeds up momentarily and then dies, open the highspeed needle valve one turn (richer) and restart. Once the engine is running smoothly, slowly turn the high-speed needle valve clockwise (leaner) until the engine runs maximum rpm. If you turn the needle valve too far, the engine will die. Open it half a turn (richer) and restart. Note: always restart at 1/4 throttle or less. It makes starts easier and safer. By now, you should have a feel for the maximum rpm, so run the engine up to the maximum rpm point and turn the high-speed needle counterclockwise (richer) just until you detect a slight slowing of the engine. This will put the setting a little on the rich side.

3. Now set the idle-mixture screw. For safety, stop the engine when you do this. Refer to the pictures, and find the carb style that matches that on your engine. From the picture, you can determine where to make the idle-mixture adjustment and the direction in which to turn the needle for a rich or a lean mixture. For example, on a two-needle carb such as a Webra, you'd turn the needle counterclock-wise to richen the mixture, becasue the fuel is being metered. With an air blend, such as that found on many Enya engines, you'd turn the screw clockwise to richen the mixture because the air is being metered.

Slowly pull the throttle stick on your transmitter down, and have the throttle trim full up (idle position). If the engine quites right away, richen the idle-mixture a little and restart. Repeat this if necessary. If the engine slows down after a little while, runs roughly, and then quits, make the idle mixture leaner.

When the engine idles at a reasonably slow speed (3,000 rpm or so), you can fine tune the idle mixture: idle the engine for 30 seconds, then quickly open the throttle fully. If it bogs down, and/or eventually quits, the mixture is too rich; if the engine quits abruptly, it's too lean. Make the suitable adjustments. One it has been set, the idle mixture rarely, if ever, has to be adjusted.

4. Now you can make the final adjustment on the high-speed needle valve. Hold the plane level, open the throttle fully and adjust it as you did before for just-below-maximum rpm. Now point the plane's nose straight up. If the engine stalls, open the needle valve slightly (richer) and try again. Sometimes it's best to make the maximum rpm setting with the nose already up. The engine must run at maximum rpm with the nose up if you want to prevent your engine from stalling just after takeoff.

5. This is very important! Don't change the adjustments you've just made. Don't close the needle valves at the end of a flying session. Leave the settings where they are. The next time you go to the field, just fill up the tank, open the high-speed needle valve (richer) half a turn and start up the engine. When the engine is warm, open the throttle fully and point the plane's nose up. Repeat the final adjustment described in the previous paragraph.


TROUBLESHOOTING

The only types of needle valves that I've ever had trouble with are those that don't have a spring riding on the straight knurl of the needle body. These are often seen on old K&Bs and old Super Tigres. It's difficult to set their friction locknuts to allow the needles to turn and yet not vibrate loose when the engine runs. I once locked an engine tight on the rich side and flew it all summer without changing it because of just that problem!

If you start to notice problems, look elsewhere before you start twisting the needle valves (e.g., check glow plugs, tubing, dirt in fuel system).

That's all there is to it! Make your settings; don't change them, and keep 'em flying!


TECH TERMS

CCW - counterclockwise

CW - clockwise

High-speed needle valve - sets the flow of fuel to the engine at high speeds (also called the "high-speed mixture valve"). It's always located on the side of the engine opposite the muffler. It's the most prominent needle valve.

Idle-speed stop screw - limits the motion of the throttle barrel toward the closed position. It's also called the "idle stop screw," the "throttle stop screw," and the "rotor setscrew."

Idle-mixture screw - sets the flow of fuel to the engine at low speeds. It's also called the "air-bleed screw," the "mixture control screw," (on some O.S. engines) and the "low-speed mixture setting screw." It's usually opposite the high-speed needle valve.