912ULS hiccuping

[Dave S]

Frank,

When you mentioned the broken module bracket - that is what inspired me ask about the aircraft model - some earlier versions (not the S7) had a modified module bracket with both module grounds going through the bracket - S7 you should have both grounds going to separate screws on the intake manifold so that should be no factor.

It can be pretty perplexing to separate what is caused by fuel deals and what is caused by ignition things. I'll throw out a couple ideas; however, I think your diagnostic procedures so far seem to indicate a fuel deal.

1) DF 4's comments about proving the fuel flow are spot on. Based on your description, it sounds like the fuel pressure sensor is picking up at the split where the fuel goes to each side in separate hoses. Pulling the hose off at the carburetor and getting an actual fuel flow at that point for each side should determine if there is an issue between the fuel line split and the carburetor inlet.

2) As to the indicated fuel pressure under the conditions of the miss......if the gauge is right, the pressure is adequate. It is normal for the pressure to drop a small amount with increased fuel flow, but I believe the minimum fuel pressure Rotax specifices is a lot less than what you are registering. When doing fuel pressure ground checks, I use a direct reading automotive fuel presure indicator spliced into the fuel line at whatever point I want to check - not possible airborne.

3) On the fuel flow rate - again, if the gauge is right (my faith in gauges is rather low unless they have been currently calibrated against a direct measuring technique) Low fuel flow can be due to either interference with supply or low demand (less than full throttle or a prop that is way over pitched limiting RPM). If the flow rate actually does decrease to 3.4 gph...no way can the engine develop full power.

4) Again, DF4's comments about checking the carburetor, even though it has been rebuilt is a good call, likewise, a new fuel pump is not necessarily a good fuel pump.

5) If you get adequate static flow rate at the carburetor inlet - I'd be real suspicious about the carb or mechanical fuel pump.

Keep us all in the loop - the rest of us might find knowing what is going on useful.

Good luck,

DS

[HighWing]

My thoughts would be to check fuel flow first. This is because of an experience several years ago when I had gone through the system at annual and replaced the fuel hoses. I was very careful to avoid cutting the internal lining of the fuel hoses when working over the barbs and thought I was careful in other areas as well. The test flight after the annual proved me wrong on the careful score. The engine ran well on idle and the run-up was normal, however, after about thirty seconds as I climbed at full throttle, I had a severely rough running engine. I flicked the switch for the aux fuel pump, reduced the throttle to about 4000 and things smoothed out a bit and was able to return to the runway.

What I found is that on tightening the clamps at the fire sleeve ends, I had tightened one sleeve beyond the barb and tight enough that it constricted the fuel line reducing fuel flow. I had sufficient fuel flow for idle and to keep the bowls full at low power, but once at full throttle, the bowls would empty enough that the engine ran rough.

A further note: When evaluating my installation, I pulled all the hoses off one by one and checked them. I discovered that my constriction was very difficult to find as superficially, everything looked fine. It was only by trying to look down the tube against a light that I found one section where the light never became visible.

[N5929]

Folks
If it walks like a duck, . . .
I checked the fuel flow and found it restricted. Flow with the Facet pump was ~13 gph. I removed the wafer in the fuel filter and found flow to be ~32 gph, about what it should be. I replaced the filter with a different one and found flow to be ~ 31 gph.

Test flight showed good fuel flow steady at ~6.0 full power on climb out. Fuel pressure ~5.8 psi. (These are relative #s which may include instrument/sensor/calibration errors)

Problem solved!

Now, why the problem?! I have used this fuel filter for about five years. It's a K&N canister type (chrome with one end that unscrews giving access to the wafer filter secured in place with a spring). I have cleaned it the same way for each condition inspection, using brake cleaner squirted in from both sides. What happened this time to make it unserviceable I don't know (another mystery of the universe).

Since I had used this filter for some time and had cleaned it twice, once for the condition inspection and a second time when symptoms appeared, I was hesitant to believe this was the problem. Thanks to the multiple common sense suggestions to check the fuel flow I was prompted to get on track to deduce the problem. My learned lesson: Common sense (more powerful than splitting atoms) beats second guessing.

Thanks folks!

[Dave S]

Hey Frank,

So good to hear you got 'er done. And not even an expensive fix! (I love it when a fix is inexpensive.)

Back to good flying and clear skys now!

Even though it isn't clear what changed the the flow rate on the filter disk, sounds like this is a item to discard and replace periodically if that type of filter is being used.

Good catch,

Fly Well

Dave S

[DesertFox4]

Sounds good Frank.

[Esser]

Not sure if this s the case, but some fibrous paper filters catch water which causing the to swell and lower the flow rate.