Saturday, September 24, 2016

Completing Carburetor Vacuum Synchronization Fixture

Continuing on with the fixture for mounting the vacuum gauges, figured it would be necessary to install a piece of metal that would extend aft over the upper forward fuselage skin to prevent the fixture from flipping aft when the engine is running. This was easily accomplished by bending a joggle in a piece of metal so it cleared the hinge and extended aft over the upper forward fuselage skin.

Once happy with the overall fit of the fixture, I decided to also place hinges along the side edges of the fixture. This will allow for side mounting the fixture so the gauges can be seen while standing alongside the engine when another person is available to sit in the cockpit at the controls while the carburetors are being adjusted.
The almost completed carburetor synchronization fixture after the piece of metal with the joggle was bent and the fit tested. One of the side hinges was also in place for the test fit. Note the piece of rubber trim over the edge of the tab will prevent scratching the upper forward fuselage skin.

Installing the side hinges created a minor issue in that there are two rivet holes (one on each side hinge) that needed to be machine countersunk. This is because there is one rivet for each side hinge that ends up under the cowling hinge … so those two rivets need to be flat to the base of the fixture so there is no interference. After all the drilling was completed for the side hinges and the two aforementioned rivet holes countersunk, the pieces were primed and painted. The completed fixture can be seen in the photo below.

Return from the future: The gauges shown in the photo below are incorrect for use on the Rotax engine ... the vacuum gauges shown are calibrated for inches of water. The correct gauge for this application is 0-30 inches of mercury ... AKA ... in Hg. The amount of vacuum presented to the gauges when the engine was running destroyed the internal workings of the gauge. Below is a link to the post covering the gauge destruction.


The completed carburetor synchronization fixture. Note the gauge on the left has a blue dot on the bottom of the glass. I need to make a label noting this gauge reads .25 inches of water lower than the right gauge. The gauges shown in the photo are wrong, they should be 0-30 in Hg ... not inches of water.
The hinges on either side allow for side mounting the fixture. Forgot to install the rubber trim piece on the end of the tab before taking this photo, but it can be seen in the first photo of this post.

Synchronization Fixture Version 2.0 … After all the fussing around with the side hinges and painting the fixture, I thought of a much better and simplistic way to make the fixture. I would suggest others interested in building their own carburetor synchronization fixture to do it a little differently than my 1.0 version. If I had it to do over, instead of bending one large piece of metal, I would make the fixture from two pieces of aluminum. I would keep the design of the bottom plate with the long hinge and the tab with the joggle exactly the same as it is now… however, there would be NO side hinges. The gauges and valves would be mounted onto a second aluminum plate that is bent 90 degrees below the valves like it is now … however, a single centered  bolt and some washers would be used to attach the gauge’s mounting plate onto the base plate. This would allow the base to remain in one secure position on the aircraft, yet the plate the vacuum gauges are mounted onto could easily be rotated so the gauges either face aft, to the left or to the right side of the aircraft. Simplistic, yet functional. It would have been much easier to construct and much more utilitarian. I feel fellow builders would be much happier with the 2.0 version as opposed to my clunkier 1.0 version.

Now that I have a suitable carburetor balancing fixture, I’m hoping to complete the carburetor synchronization over the weekend and get this bird in the air in the next few days.