TUNING GASOLINE ENGINES WITH WALBRO CARBURETORS
This article is based on Al Watson’s March Club presentation on tuning gasoline engines with
Walbro carburetors. The presentation was specifically for the DLE 30 engine; most of the
information is applicable to any model gasoline engine.
Recommendations
1.
Use a NGK CM-6 spark plug. Set the gap to 0.024 inch.
2.
Disconnect the throttle spring so that the throttle servo does not have to work against
the force of the spring. Leave the spring on the carburetor.
3.
During break-in use an 18 x 8 prop that will not heavily load the engine. Fly the engine
instead of running it on the ground, but avoid vertical maneuvers.
4.
Use gasoline that is fresh or no more than a few months old.
5.
Use a 32:1 gasoline-oil ratio for break-in and thereafter.
6.
Use Stihl synthetic HP Ultra 2-Cycle oil.
7.
Use a quality tachometer to measure RPM; your ear is not accurate enough. The
Fromeco TNC tachometer works well and displays both peak RPM and current RPM.
Tuning Procedure
1.
As a starting point, set both the high speed and low speed needle valves 1-1/2 to 2 turns
open. If the setting of the needle valves are unknown, turn them fully clockwise (CW);
don’t apply force that would jamb them into their seats. Next, turn each needle valve 11/2 to 2 turns counter clockwise (CCW). Most engines will run at these settings. Note
that turning a needle valve CW leans the air-fuel mixture; turning a needle CCW
richens the air-fuel mixture.
2.
If the engine carburetor is bone dry, inject a small amount of fuel into the throat of the
carb using a syringe. Open the choke and throttle before doing this.
3.
Turn the ignition “on”, open the throttle about quarter way and “close” the choke.
4.
Smartly flip the prop and as soon as the engine fires open the choke. Start the engine
and let it warm up by running it at about three quarter throttle. Don’t adjust an
engine when it is cold because the settings are affected by temperature.
5.
Set the throttle to maximum and adjust the high speed needle valve for maximum RPM
as indicated by the tachometer. Begin this process by turning the high speed needle
CCW thus richening the mixture. As the RPM begins to drop turn the needle CW
leaning the mixture. The RPM will increase and once over the peak will begin to drop.
Turn the needle CCW to the peak RPM. Then set the throttle to idle so that the engine
cools or just shut the engine off and let it cool.
6.
Restart the engine if shut off. Set the throttle for 2,5000 to 3,000 RPM as indicated by
the tachometer. Maintain this throttle setting. Adjust the low speed needle for
maximum RPM using the same technique as the high speed needle.
7.
Move the throttle to maximum and wait for the engine to clear out. Adjust the high
speed needle valve for maximum RPM. This should only require very small needle
movements. Shut the engine down and let it cool.
8.
Start the engine and let it idle for a few seconds. Quickly move the throttle to
maximum. If the engine RPM does not quickly accelerate, the low speed needle valve
should be leaned (CW) slightly, using very small movements.
9.
The engine should now be ready to fly, but probably is not tuned to perfection.
Conduct a flight and note the results. If it sags on a long upline, richen the high speed
needle valve slightly. If it burbles during level flight at half throttle, lean the low speed
needle valve slightly. Some engines will burble regardless of the settings. Continue to
note now the engine performs in flight and make very small changes to the needle valves.
General
As the engine breaks-in the needles will need to be adjusted to maintain maximum RPM. When the
break-in prop is replaced with the regular prop the needles will need to be adjusted again.
If the “gasoline to oil mixture” is changed, then some retuning will also be required.
Reading the color of the spark plug is a good way to determine if your engine is “rich” or “lean”.
The center electrode of a new plug is encased in a white ceramic insulation. If the insulation is
“black” then the mixture is rich and it is “grey” then the mixture is lean. For an ideal mixture the
insulation and the electrodes should be a light brown color.
A good idle for the DLE-30 is around 1500-1600 RPM. Don’t expect to get a reliable idle this low
until the engine is fully broken-in.
Resources
1.
NGK CM-6 spark plug: auto parts store such as O’Reilly
2.
Stihl HP Ultra 2-Cycle oil: Stihl dealer such as Goodsell Power Equipment
3.
Fromeco TNC tachometer: Fromeco Scale Products (fromeco.org) or Don’s Hobby
Shop (donshobbyship.com, 1-800-972-6237)