|Carburetor equalization||Carburetor adjustment screw guides|
A few months ago I came across the topic of carburetor equalization. In a nutshell, the Walbro and Walbro-like carburetors that are typical to gas airplane engines were originally designed for things like chainsaws and weed whackers. The flow of fuel is regulated by a diaphragm in the carburetor that's activated by air pressure entering through the metering cover vent - a small hole in the metal plate that covers the diaphragm. This system is fine on the ground where the carb doesn't get moved around a lot but flight - easpecially aerobatic flight - can cause swirling air inside the cowl. The moving air can change the pressure on the diaphragm, causing the engine to run too rich or too lean.
Enter carburetor equalization, which relocates the source of the air pressure to somewhere more stable. In this case, inside the fuselage. To the shop!
Assemble the parts
You can source the parts separately; I bought Thunderbolt RC's carburetor equalization kit:
Fill the existing vent
Remove the metering cover from the engine. In this case it was not necessary to remove the engine from the airplane.
Secure the metering cover and solder the vent closed:
Remove any excess solder from the inside:
Remove any excess solder from the outside:
Drill and tap the new vent
Mark the centre of the metering cover:
Drill a pilot hole in the metering cover:
Drill a 7/64" hole in the metering vent and tap it with a 6-32 tap:
Install the metering cover pressure fitting
Slide the gasket onto the pressure fitting threads and add some thread lock, then thread the pressure fitting into the newly tapped vent and tighten:
Grind off the inside of the pressure fitting:
Complete the installation
Reinstall the metering cover on the carburetor
Attach the pressure line to the metering cover pressure fitting:
Secure the film canister inside the fuselage and route the pressure line:
Carburetor adjustment screw guides
Because this plane is new to me and because I have added carburetor equalization I figured it would be nice to have an accurate way to access the carburetor adjustment screws without having to remove the cowl. I had heard of using tubes or straws so I decided to give it a try. To the shop!
Install and route the guides
Remove the cowl.
Find a tube of suitable inner diameter, cut it to length and slide it over the carburetor adjustment screws:
Mark the cowl where the guides need to exit and drill a pair of holes:
Install the grommets
Find a suitable size of grommet for the guide tubes and determine the diameter of the panel hole:
Drill two holes for the grommets:
Reinstall the cowl to determine where the guide tubes come to rest:
Open the grommets at the back to allow the guides to enter, glue them in place and allow the glue to cure:
Complete the installation
Fashion a hook from a piece of push rod or other handy material:
Pop the guide tubes into the grommets and trim off any excess:
Find the right size screwdriver and you're good to go:
When I brought the completed SBach to my February club meeting for bring'n'brag, somebody asked me if the guides could fall out. That got me thinking. I started by adding some heat shrink tubing near the tips, then binding them together:
Same deal at the base:
I added a longer piece of heat shrink tubing to cover the joint and the springs on the needles:
Here are the tubes viewed from the inside:
And the outside. Note that the tips are now black to help them blend in a bit better: