Fuel Pump Assembly and Pressure Test

Time: 1.0 hrs

The fuel pumps went together quickly. I purchased some Oetiker ear type clamps from Belmetric. I purchased 14.5mm and 19.8mm size clamps which seemed to fit well. The 14.5 were a tad tight over the fittings, but I was able to get them to work. After I had the pumps assembled I used a differential pressure tester that I purchased from Sky Geeks as well as some extra fittings and a hose that has quick change connectors on the ends (the hose that comes with it one has quick change on one end). This pressure tester can also be used to test the pressure drop on the 914 cylinders using a 12mm adapter to connect to the spark plug hole which may be handy in the future when I do my annuals.

I tested the fuel pumps for leaks up to 30 psi. I found that the Rotax docs for the rubber rebuild on the IS (fuel injected) type engines tests the pumps up to 87psi, but the carburated engines don’t run at as high a pressure, the normal fuel pressure is around 2-5psi over the airbox pressure (which is max around 18psi). I didn’t want to damage anything so I figured 30 psi would be reasonable. The only leak I had was on one of the adapters I used to get the tester connected to the -6AN fitting on the pump. I’m happy that the 5/16″ hose barb to -6AN converts I found don’t leak. Now I have AN fittings on the pumps and don’t need to use rubber to run out of the pumps to connect to the stainless steel hoses.

Just a quick check I ran the tester up to 10psi and set the slow fill back to off. I noticed the pressure dropped off on the second gauge after a few minutes so I knew there was a leak. It turns out the leak was between the M14 adapter and the M14 to -6AN fitting. I was able to tighten that up again and ran the test up to 20, then 30 psi. I left it for about 5 minutes with no noticeable pressure drop and a check with some soapy water on all the fittings just to make sure.

Fuel pumps all assembled and fit into the Rotax fuel pump box. I’m going to check with TAF to see if they have a been made bottom plumbing manifold. I noticed from Craig’s build log that his has a better, tighter bend to it that would fit much better in the Rotax box than the one I have. Also I may work on a better way to attach the pumps to the mounting plate. I’m currently using the clamps that came with the pumps, but I don’t like that I had to mount them towards the ends of the pumps to avoid the screw from hitting the check valves. It’s such a tight fit to get all this stuff into the box. I would prefer more centered so the pumps are more secure or using 2 clamps per pump.

A photo of the differential pressure tester (model E-2A) I got. Aircraft Tool Supply has them as well, but they were out of stock and it wouldn’t have been available for 3-4 weeks. This is the 0.040 orifice one with no master orifice. The hose is the ATS extension hose which has quick disconnects on both ends so I was able to purchase a 12mm (for the cylinder compression checks) and a 14mm (to make the conjoin to the fuel pumps) with quick disconnects that can be swapped out easily. I chose a 14mm adapter just because I was able to find an M14 female to -6AN male fitting.

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