Why do they install a spring open choke on RC engines?
That is a very good question. It is very difficult and unsafe to hold the choke closed and prop your plane. Switching to a friction choke is simple. A few dollars and a few minutes you can have this changed over. If you feel comfortable pulling your carburetor partly apart then continue, if not ask around at the field.
Instructions are for Walbro Carburetors, you will also need to know what model you have.
Pick up a few parts online or at your local repair shop. Ask for the following parts for your carburetor. Parts should only run you a couple bucks. You can save a few dollars by modifying the shaft you have. See section at the end on modifying your current shaft.
Shaft Assembly – Choke 40-1121-1 – Shaft assembly – choke part number may be different depending on what model of carb you have.
Ball-Choke Friction 89-13-8 – Ball – steel .125 DIA.
Spring – Choke Friction 98-162-7 – Spring – choke friction
This modification was done on a WT-613. It is recommended that you pull the carburetor and work on it on top of an old towel.
NOTE. If you plan on modifying your current shaft, open the choke. Looking at the photo below mark the shaft thru the hole where the arrow is pointing with a permanent marker.
First make note of the orientation of the choke plate and remove the screw holding it to the choke shaft. Set the plate and screw aside for now. Pull the choke shaft out of the carburetor. You should also remove the spring that holds the choke open.
Looking at the photo you will see a small hole going down that will intersect the choke shaft
Inside this hole you will want to drop the spring and follow it by the ball. Now we are ready to insert the new choke shaft. The shaft can be a little difficult inserting due to having to compress the spring and ball.
Rotate the shaft until the flat spot on the shaft is facing you. Reinstall the choke plate and screw. You now have a friction operated choke. Work it a few times; you should feel it stop at full open and full close. Some shafts may also have a half open stop.
Modifying your current shaft.
You should have marked the shaft in the above instructions. With a .125 drill, you will want to drill a small indention on the shaft on the opposite side of where you placed the mark. Refer to the above photo.