Challenger Seasonal Carburetor Needle Adjustments

In Canada and the northern United States, the carburetor needles on Challenger Ultralights must be adjusted seasonally to accommodate the large temperature changes. This needle adjustment procedure is relatively easy and only takes a few minutes when the procedure is done properly. It is also a good time to check the carburetor balance and the condition of the air filter.

If the carburetor needles are not adjusted with the seasonal temperature changes, the engine will receive a very lean fuel mixture as the denser cold air is mixed with the liquid fuel. This leaner mixture will cause the exhaust gas temperatures (EGTs) to soar above the maximum allowable range and the engine will seize.

The EGTs are really the most sure indicator of when it is time to adjust the carburetor needles. The normal EGT operating range is 930 to 1150 degrees F. The maximum allowable EGT is 1200′F. It is advisable to stay at least 50 degrees F below this maximum EGT. Above this temperature the engine will be damaged by scored pistons and may even seize.

A rule of thumb for some Challenger pilots is to fly the air craft at the summer setting when ambient outside temperatures are above 55 degrees F ( 12 degrees C). When The temperatures drop below 55′F but are still above freezing, set the needle to the fall setting. When the temperatures are below freezing, the needles should be at the winter setting.

The needle settings are easy to remember. As the temperatures go down, the circlip on the needle goes down to a lower groove on the needle with the winter setting being the lowest groove. As the temperatures go up, the circlip moves up. The summer setting has the O-ring in the top groove and the circlip in the groove right below it.

Here is the procedure for setting the carburetor needles.  See the table below for corresponding pictures of the procedure.

1. Pull the throttle lever to full idle. This will give you the maximum amount of carburetor cable exposed and it will make the following steps much easier.
2. Remove the two screws that hold down the top cap of the carburetor with a slotted screwdriver. These screws are captive so they will not fall out and be lost.
3. Pull off the top cap of the carburetor and pull the the carburetor piston assembly out of its cylinder.
4. Pull the spring up out of the carburetor piston and fully compress it and hold it as such with one hand. Turn the carburetor piston up-side-down and push the carburetor wire up into the piston. The ferrule on the end of the wire will poke out of the end of the piston. Push it over to the large hole closer to the edge of the piston. The ferrule will slide back through the large hole and come right out of the piston.
5. Carefully remove the white plastic spring guide and the carburetor needle assembly. Set the loose pieces aside in a safe area where they will not get dirty or lost.
6. Taking the carburetor needle, remove the snap in circlip and move the rubber O-ring to the groove above where you want to have the circlip. For the winter setting, the circlip will be on the bottom groove and the O-ring in the groove above it. For the summer setting, the O-ring will be in the very top groove and the circlip in the groove second from the top. It is easier to move the O-ring when the circlip is not on the needle.
7. Drop the needle assembly back into the central hole of the carburetor piston. The circlip should swing around so that it is over the other holes. It will not fit down into the piston otherwise because of the shape of the metal on the inside of the piston. Set the piston aside for a moment or have a friend hold it.
8. Pick up the top for the carburetor and pull the cable wire all the way through it. Slide the spring over it. Slide the white plastic spring guide over the end of the spring and compress the spring with the wire passing through the center of the guide. Hold the spring fully compressed in one hand. Pull the wire out again to make sure it is all the way out.
9. Pick up the carburetor piston and pass the ferrule on the end of the wire through the large hole beside where the needle sticks out on the bottom of the piston. When the ferrule is all the way through, hook it over into the smaller hole just inboard of it. The ferrule will catch there.
10. Release the spring and the white plastic guide. Slide the grooves in the white plastic guide over the 3 matching guide rails in the interior of the piston.
11. Take the carburetor piston assembly and slide it down into the carburetor cylinder. Note that the is a groove along one side of the carburetor piston and a matching locator pin sticking in on the right side of the cylinder barrel (near where the choke mechanism is located). The groove in the piston must slide over this guide pin in order for the piston to move all the way down the inside of the carburetor cylinder.
12. Replace the carburetor tops and screw them down snug. Note that there is another locator pin on one side only of the carburetor top cap and it must line up with the matching receptacle hole on the top face of the carburetor.
13. Repeat this process for the second carburetor.

This procedure is very frustrating the first few times you try it. It requires some manual dexterity to hold the compressed spring in one hand and stick the ferrule through the little hole in the bottom of the piston. It comes with practice.

Here are some photos of the carburetor needle adjustment procedure. Click on the photo and it will appear in a new window. Click on it again for a full size image. Use the Back button to return to this page.

Carburetor Top Cap	Removing the Piston from the Carburetor Cylinder
Carburetor Piston Assembly with Top Cap	The Carburetor Piston Parts
Bottom View of the Carburetor Piston	Piston Side View Showing Locating Groove
The Winter Carburetor Needle Position	Holding the Spring and Guide Compressed to the Cap
Cap Locator Pin Receptacle	Carburetor Cap Locator Pin Circled
Sliding the Piston back into the Carburetor - the piston will need to be rotated to line the groove up with the pin inside the barrel	Challenger Carburetors all back together