Engine modifications

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Carburetor equalization Carburetor adjustment screw guides

Carburetor equalization

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:

New parts - carb equalization kit

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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:

Diaphragm plate clamped for fill soldering

Remove any excess solder from the inside:

Stock hole filled - inside

Remove any excess solder from the outside:

Stock hole filled - outside

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Drill and tap the new vent

Mark the centre of the metering cover:

Pressure fitting hole marked

Drill a pilot hole in the metering cover:

Drilling the pilot hole for the pressure fitting

Drill a 7/64" hole in the metering vent and tap it with a 6-32 tap:

Drilled and tapped to 6-32

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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:

Pressure fitting installed

Grind off the inside of the pressure fitting:

Inside of pressure fitting ground flush

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Complete the installation

Reinstall the metering cover on the carburetor

Metering cover reinstalled

Attach the pressure line to the metering cover pressure fitting:

Metering cover plumbed

Secure the film canister inside the fuselage and route the pressure line:

The whole system

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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:

Carb needle adjustment - nylon tubes to guide the screwdirver

Mark the cowl where the guides need to exit and drill a pair of holes:

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Carb needle adjustment - Nylon rods exit the cowl

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Install the grommets

Find a suitable size of grommet for the guide tubes and determine the diameter of the panel hole:

Grommet panel diameter

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Drill two holes for the grommets:

Carb needle adjustment - two 19-64 in holes in the cowl

Reinstall the cowl to determine where the guide tubes come to rest:

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Carb needle adjustment - position of the access tubes

Mine end up right above the access holes

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Open the grommets at the back to allow the guides to enter, glue them in place and allow the glue to cure:

Carb needle adjustment - grommets open at the top

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Complete the installation

Fashion a hook from a piece of push rod or other handy material:

Carb needle adjustment - hook to push or pull nylon rods

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Pop the guide tubes into the grommets and trim off any excess:

Carb needle adjustment - access tubes

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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:

Carburetor adjustment screw guides - bound together

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Same deal at the base:

Carburetor adjustment screw guides - more heat shrink at the base, joined

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I added a longer piece of heat shrink tubing to cover the joint and the springs on the needles:

Carburetor adjustment screw guides - joined at the base

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Here are the tubes viewed from the inside:

Carburetor adjustment screw guides - view from the inside

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And the outside. Note that the tips are now black to help them blend in a bit better:

Carburetor adjustment screw guides - back through the cowl

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