912ULS hiccuping

[N5929]

During takeoff climb out I experience a hiccup in the engine. If I keep full power in it get worse by more missing.

Pre-takeoff check is perfect--both "mags" same drop, ~70 RPM; fuel pressure 6.3 psi

On takeoff with full power fuel pressure (initially 6.3) and fuel flow (initially ~5.3 gph -- fuel flow not precisely calibrated but close) gradually declines. Hiccup consistently first occurs when fuel flow declines to 3.4, usually on crosswind but sometimes early downwind.

Facet pump will return fuel flow & pressure momentarily, eliminating hiccups. Reducing power also restores flow & pressure.

Upon landing and doing repeat run-up all indicators are again normal. Repeat take off again results in hiccups but more of them.

Tried a flight yesterday. Climb out was good, fuel flow & fuel pressure good until about 2700 AGL/3500 MSL then had hiccup. Reduced power and no hiccup. Did slow flight for 15 min and then experienced hiccup again.

Fine fuel filter is clean. Cleaned second time--no change. Fuel cap vents clear. No kinks in fuel lines. Rubber fuel lines replaced 18 mos ago.

Fuel plumbing (in order): Wing tanks with coarse filter, header tank, fuel valve, fine fuel filter, Facet pump, engine fuel pump (newer style pump), "T" for fuel return line (with fuel pressure sensor then carb jet restrictor then feeding into L wing tank), fuel flow monitor (Red Cube), "T" for split to carbs.

700+ hrs on engine. Recent problem occurring. Problem began after condition inspection. Very smooth except for hiccups.
Carbs rebuilt by Lockwood 160 hrs ago. New type floats.
New plugs at condition inspection. New plugs again after experiencing problem. No change.
Replaced broken module bracket (pn 851-480) and installed fuel pressure sensor during condition inspection.

Related or not??? About two months ago engine would not fire at all with several attempts. Normally quick starting. Next day I tried again and it started without hesitation, without any changes/corrective action.

My plan is to check electronic modules by trying to start engine individually with each module.

Thanks in advance for any suggestions/observations?

[DesertFox4]

Just for reference - my 912 uls at full power flows 6.9 to 7.2 gallons per hour.
At cruise with 5,400 rpms I will show a fuel flow of 5.6 to 5.8 gallons per hour.

To me your flow numbers seem too low (with boost pump off) for power settings mentioned.
No way a 912 uls at full power will demand only 5.3 gallons per hour. Engaging the boost pump and the engine returning to normal operation seems to indicate a fuel starvation at your mid to high rpm settings.

Sounds like you made changes to your proven fuel system recently. The problem may have been introduced with the changes. If you are flowing sufficient fuel to the carbs then suspect the carbs even with a recent rebuild.
You need to do a fuel flow check right away before flying again and see if your engine is really getting enough gallons per hour to the carbs at full power plus a safety margin. My system static flows 11 gallons plus per hour before the mechanical fuel pump. ( I also have the new fuel pump style.) If you changed the fuel system between the mechanical pump and carbs you may need to check there also. If you flow test at the carbs you will need 5 gallons or better per hour at each carb.

I do not have a fuel pressure gauge on my 912 uls so can't give you comparable pressure numbers.

Best of luck with the fix.

[N5929]

Thanks much for your input, Desert Fox! I appreciate your reference #s and suggestion.

Yes, I agree, I must have done something to bring this on. I will do as you suggest--do a fuel flow test.

Frank

[Dave S]

Frank,

What model Kitfox is the engine on?

DS

[N5929]

Dave
It's a Series 7
Frank

[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