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Equalizing you Gas Engine Carburetor

2-Stroke Engine with Walbro Carb
2-Stroke Gas Engine (DL50).
Most of the 2 stroke Gas Engines used in the R/C Aircraft hobby have a Walbro (or similiar) carburetor. These Walbro carburetors work very well for this application, but because they were never really purposely built for R/C Aircraft use, there are certain scenarios where the user can run into problems. One of the most common problems has to do with the Walbro carburetor being exposed to uncontrolled atmospheric pressure changes.

Walbro Carburetors and Atmospheric Pressure

Diaphram Plate
Walbro Carb - Diaphram Plate.
Walbro Carburetors use a diaphram metering system to control the amount of fuel that is pumped through the carburetor. One side of the carburetor has a raised metal plate with a small hole in it. Under this plate is the float diaphram. The hole (or vent) in the plate exposes the diaphram to the atmoshperic air pressure. For the carburetor to function properly, it must have a stable source of atmospheric air pressure. If the atmospheric air pressure changes, problems can result. For example: With a cowled engine, certain flight manuevers may pressurize the inside of the cowl (where the carburetor is located) and cause more atmospheric air pressure to push on the carburetor diaphram. This forces more fuel into the carburetor, causing the engine to run rich. A fix for this problem is to supply the carburetor diaphram with a reliable stable source of atmospheric air pressure. The idea is to supply a constant atmospheric air pressure source that is the same (or equal) throughout all of the airplane's flight manuevers. The best place to find this source of atmospheric air pressure is inside your plane's fuselage. Generally, the air pressure inside your fuselage does not change too much during flight.

Items you will need

Items you will need

Here's a list of Items you will need to install a carburetor equalization kit.

  • Pressure Fitting for your carburetor. These are available at at this link: Click Here
  • Appropriate Drill and Tap for the pressure fitting you decide to use.
  • Fuel Line. This is available at at this link: Click Here.
  • A 35mm Film Canister or Pill Bottle
  • Soldering Iron and Solder (Silver Solder works best).
  • Dremel or Rotary Tool.

Alternatively, you can get the 35mm Film Canister (pre-drilled with a pressure fitting), a carb pressure fitting, and Fuel line, in a kit from at this link: Click Here

Remove the Diaphram Plate

Diaphram Plate Removed
Diaphram Plate Removed.
After you have removed the carburetor from the engine, the first thing to do is remove the diaphram plate. Once the plate is removed, you can see the diaphram. Notice how it easily it can be moved. A small amount of air pressure can completely change the amount of fuel that the carburetor pumps. This is why the atmospheric air pressure affects the way the carburetor performs.

Solder the existing Carburetor Vent Hole

Diaphram Plate Vent Hole
Soldered Vent Hole
With the diaphram plate removed, notice the small hole in it. This is the vent hole. Solder this hole shut (so that no air can move through it). Try to keep the solder to a minimum and sand or grind any excess solder away. Make sure that the plate can sit flat without any solder obstructing it.

Drill and Tap the Pressure Fitting

Pressure Fitting Installed
Pressure Fitting Flush
With the existing vent hole covered. Drill and tap a new hole for the pressure fitting. Drill the hole in the center of the plate. Make sure you use the proper drill size for the tap you are going to use. For example: If you use a 6-32 size pressure fitting, use a #35 drill bit. Once the new hole is tapped, insert the pressure fitting (make sure you use a gasket or some gasket sealant and thread locking compound. Grind or sand down the extra threads on the pressure fitting flush with the inside of the diaphram plate. Allow the thread locking compound and/or gasket sealant to cure. (Don't allow any thread locking compound or gasket sealant to come into contact with the carb diaphram.

Reinstall the Diaphram Plate

Pressure Fitting Installed
Diaphram Plate Reinstalled
The finished diaphram plate should look similiar to the picture on the left. Once you are finished installing the pressure fitting, make sure the diaphram plate is clean (remove any metal dust, etc). Also, make sure that the pressure fitting allows air to freely pass through it. Once that is done, reinstall the completed plate on the carburetor.

Prepare the 35mm Film Canister

Film Canister Drilled
35mm Film Canister drilled.
The 35mm film canister (or pill bottle) will provide a stable source of atmospheric air pressure inside your fuselage. It for some reason, you have any air currents entering your fuselage, the film canister will disperse these and provide the proper static air pressure. Drill one hole that is suitable for your selected pressure fitting in the film canister lid. Drill another small hole (~1/8") in the bottom of the film canister (this will make sure that a vaccuum does not exist inside the film canister).

Install the Pressure Fitting on the Film Canister

Pressure Fitting fitted
Film Canister Top Done
With the holes drilled in the film canister, insert the pressure fitting into the film canister lid (use a gasket or gasket material) and apply thread locking compound to the exposed threads. Tighten the pressure fitting with the appropriate nut.

Reinstall the Carburetor and Attach Fuel Line

Kit Installation Complete
Kit Installation Complete.
Reinstall your carburetor on your engine. Attach the fuel line to the new pressure fitting on the carburetor and onto the pressure fitting on to the film canister. This setup will allow the carburetor to use the atmospheric pressure in the film canister to properly meter the diaphram.

Install the Film Canister in the Fuselage

Photo of the Item
Film Canister mounted in fuse.
Install the film canister inside your airplane's fuselage. Make sure it is secure (use wire ties or velcro). Route the fuel line through the fuselage and into the pressure fitting on the carburetor. After installing this setup, you may need to adjust your carburetor needles.

That's it! - Go fly!