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Monday, June 16, 2014

DLE/Walbro Carb Metering Diaphragm Mod

As I read up on gas engines and their problems, I ran across a solution to a common problem my 2-stroke 30cc DLE engine exhibits (we think from a poor break in by the original owner). It already 4-cycles (runs rich) a bit more than it should, running even more rich once at altitude. One of the causes can be high and variable air pressure inside a relatively closed circuit cowl with a running engine at the lower pressures at altitude affecting the fuel metering diaphragm and valve. That solution is a metering vent modification I found online at Flying Giants that effectively moves the vent to a more stable air chamber at the appropriate pressure further aft in the fuse.

The metering diaphragm helps control fuel flow as described in this Walbro Tech Tip.

Use a small nitro muffler pressure fitting. Picked mine up for a couple bucks at Orange Grove Hobby in Gulfport, MS.

Drill out the original metering vent hole to fit the 6-32 fitting. If you can tap it, do it, but I don't have one. It still screwed in nicely.

Obviously the fitting protrudes into the metering chamber and needs to be ground down.

To keep it in place and seal it, I used a torch to heat the metal and solder it in place. I did this before grinding down the inside end (see below) as it made sure the fitting stayed put.

Better living through Dremel!  Nice and smooth, finished off with 220 sandpaper, and a small drill bit to reopen the hole to its full size. Smooth as a baby's bottom.

There are variations on the distal end of the vent tube. I used an extra plastic fuel clunk and a deep plastic top of a bottle. The idea is to have the opening of the vent in a still or laminar stable pressure flow. I drilled the top of the plastic bottle to accept the thick part of the clunk so it rests deep inside the plastic top.

I used Tygon tubing to run from the pressure fitting into the fuselage, and secured the distal "cabinet" inside the fuse next to the gas tank. There is good airflow through here, drawn out by the low pressure louver vent I put in the aft fuse. 

The plastic top opening is open to the air in the fuselage, facing aft.

Looking forward to seeing how this affects the engines performance.

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