912ULS Engine stumbled in flight
I've got about 35 flight hours to date and my 912ULS engine has started and run flawlessly for most of that time until a hiccup the other day. I had been flying for an hour or so and was cruising at about 4500 rpm at 2000' altitude and 85F ambient when all of a sudden the engine started to miss like it was being starved of gas and the vibration level at the engine went up noticeably during the missing. When I tried to increase the throttle it bogged down and when I reduced throttle it ran better, including at idle during the approach to a nearby airport. On the approach I checked the R & L mags and they seemed to have the normal response. The gas valve was ON and engine temps and pressures were all within their respective green bands. The whole event only lasted about 5 minutes till I got on the ground. While on the ground I ran the throttle up and down and everything seemed back to normal. I checked the fuel tanks and header tank for evidence of water in the fuel sample, with none seen. I took off and climbed without any missing to an altitude that I could glide home from and made it home with no further issue.
With the cowlings removed I checked gas levels in both carb bowls and one seemed a little lower than the other. A closer look at the float valve armature in the low side bowl revealed that one of its two arms was slightly higher than the other. I realigned the armatures to match each other and tweaked the float level tab of the armature to meet the 10.5mm dimension specified in the Rotax heavy maintenance manual. I also checked all the spark plugs for color and condition and they appeared the normal tan color, indicating acceptable mixture settings. I also checked differential compression on each cylinder and they were all very good. I'm hopeful that this tweak alone has solved my issue and during subsequent ground runs I have not seen any engine running issues. The following are additional items that I'm wondering about and is it possible that they may have contributed to my engine missing issue:
Fuel pump: My fuel pump is the original German made Pierburg pump that came with the kit (new in 2001) and I've heard from trusted sources that this pump is "bullet proof". Even though it only has 35 operating hours on it, its age alone makes its internal rubber diaphragm suspect and I plan to add a fitting in the pump output line to monitor the fuel pressure to look for any anomalies. Question #1: Is anyone out there got a high time Pierburg fuel pump that is still operational?
Fuel line routing: I've installed some see through gas filters temporarily so I can keep an eye on gas flow and potential fuel contaminates during my phase one test program and I noticed that the filter upstream of the fuel pump inlet was more than half full of air (all the time). That filter happens to sit in a high spot location of the fuel line routing between the firewall and the fuel pump inlet. I disconnected that line going into the fuel pump to recheck the fuel flow rate due to gravity and it still flows better than the spec rate. However, I'm wondering if the filter location at the highest spot in that fuel inlet line routing can be trapping air that should or could otherwise be purged from the line by rerouting the line to slope it steadily upward from the firewall to the pump inlet without a high spot. I'm also wondering if that trapped air bubble could have somehow migrated into the fuel pump inlet during operation and contributed to my recent engine missing issue as described above. I suspect that the fuel pump doesn't have enough suction to pull in any air from such a bubble but looking for opinions. Question #2: Is the air bubble in the fuel filter a problem and should I reroute the line to avoid the high spot and the bubble all together?
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