912ULS Engine stumbled in flight

[Birdseyeview]

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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[Slyfox]

what kind of fuel are you using?

[Birdseyeview]

I was originally burning 100LL until my 25 hour first oil change then I switched over to 91 octane Autogas with no ethanol. Been burning Autogas ever since.

[Av8r_Sed]

Don’t use paper element filters. Any water will soak into the paper and block flow. SS mesh only. I donkt believe those are likely needed in that location anyway.

[Slyfox]

well that could be your problem. The word is out that they been using winter gas longer and the rotax doesn't like it in warm climates. I say add a gallon of 100LL to each tank and see if the problem goes away.

[Dave S]

Larry,

A few thoughts - may or may not be relevant.

Running out of power on the high end can be the result of a reduction in fuel supply - the engine gets enough fuel to run with a low fuel demand but not full power.

Diaphragm type fuel pumps have additional failure modes besides the diaphragm leaking. 1) There are two one way poppet valves inside the pump - if either one sticks open the system essentially becomes gravity feed. Poppet valve problems and be intermittent and sometimes clear but sometimes come back. 2) Wear on the plunger can be an issue with pumps that have been in service for a long time but not likely since this one is low hours. 3) The plunger spring, if it breaks or is weak has a direct bearing on the resulting fuel pressure - although that particular part is not likely to fail and it certainly would not "fix itself".

Having a permanently installed fuel pressure gauge is an excellent idea.

Agreed - a 21 year old fuel pump is well beyond service life. I don't even know if Rotax publishes a shelf life for pumps in storage that long. An additional benefit of the "new style" pumps is that have an external fitting to attach a line to direct potential diaphragm fuel leaks away from the hot parts of the engine.

I'd be real leery of using any plastic housing fuel filter in the engine compartment. I believe the original build manuals show a metal case automotive fuel filter, but located closer to the inlet of the fuel pump.

I am familiar with the winter gas problem in the spring. Back before Rotax used an orifice/bleed between the carbs and fuel pump this problem was typically manifest on hot restart. Basically the low vapor pressure of the winter fuel would allow the fuel in the line between the pump and carbs to expand from under cowl heat and flood the carbs.

[Slyfox]

something interesting here. My thought is do you have an electric fuel pump, if not get a facet on there 40105. just put in inline with the mechanical. Another thought. I been through hell with my problem similar to yours. today it flew fantastic this morning and than I landed and taxi to hanger. guess what, rough running and shut down and fuel was coming out of the right side. not a bunch but when I turned on the facet pump, holy cow just poured out the vent, not drips but a bunch. smacked on the carb and still coming out a bunch. go back a few weeks and this happened and removed the bowl and found my new blue float stuck on the pin, on the bottom. I took and made sure the openings were ok and didn't find any more problem at least until this morning. did it again. did some checking and I find out the pins are having trouble in the bowl. to be exact the left carb, well in my model 4 that carb is now on the right. needs that for things to fit with a 912. you put the left carb on the right. well anyway I called lockwood and yes a redesigned left bowl almost black in color. I asked for the right carb bowl and was told that one is an original design. crazy, so I got the black bowl coming. oh and my problem a couple months ago was identical to your problem.

[Dave S]

Hey Steve,

Can you help me out - I am not sure what a Blue Float is?

Thanks,

Dave

[Slyfox]

Hey Steve,

Can you help me out - I am not sure what a Blue Float is?

Thanks,

Dave
they are a new float by sheiber I think the name is. you want to see expensive, these are

[Slyfox]

here you go. https://msacarbs.com/product/rotax-bing-blue-epoxy-float-ms80-430/

[Slyfox]

I just looked at your pics and your setup looks just like mine. meaning the left carb and intake was put on the right side and vice versa on the other carb. so your right carb will have the issue with the float bowl. hope this helps

[Birdseyeview]

Thanks for all the input. I replaced the see through plastic paper element fuel filters, took the high spot out of the fuel-in line to the fuel pump and realigned a float valve armature that was slightly misaligned. I also discovered that the same carb had rotated inboard about 8 degrees. I re-leveled the carb and a subsequent test flight proved the engine was running fine (no missing). So far so good after an hour run time.

[Birdseyeview]

On a subsequent recheck of the carbs to see if they had rotated inboard again I discovered that the left carb had backed out of its rubber socket about 1/4". I had previously installed a short metal tube spacer on the screw that tightens the carb hold down clamp to prevent over tightening. Apparently it didn't tighten the carb clamp down enough to prevent it from walking out of its rubber socket. I highly suspect this was the culprit to my rough running all along. I removed the spacers and tightened both clamps down. Also, when I replaced the temporary plastic see through fuel filter I blew through it so I could compare its resistance to that of the original metal filter that Kitfox supplied. The metal filter was dramatically more free flowing (less resistance) that the plastic see through filter. Another reason not to use the see through plastic fuel filters. On a subsequent flight the engine ran smooth as silk. Lesson learned for me is to add a step to my condition inspection to see if carbs have backed out or rotated in their rubber sockets. I hope others benefit from my findings.

[napierm]

here you go. https://msacarbs.com/product/rotax-bing-blue-epoxy-float-ms80-430/

Do you have any thing that says whether or not these are resistant to alcohol?

[Dave S]

Hi Larry,

One thing I didn't pick up on right away from your photos had to do with a change Rotax made to help with keeping the carbs in place on the sockets. Please see my attached photo - top front of the carb diaphragm chamber. At one point Rotax added a clamp to the manifold crossover tube on each side to connect a spring to the top front screw of the carb diaphragm chamber. I don't see this on our original photo so suspect the engine was made before this change occurred. I believe the purpose of this additional securement was to remove some of the downward/backward pressure on the carb socket. I can't tell from the photo if your application has any securements other than clamp going around the carb socket.