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High fuel pressure, carburetor, landing configuration

Tom C

Member
RV-6A, O-360, fixed pitch, MA-4-5 carb

I was playing around at altitude in landing configuration: full flaps, idle, fuel boost pump on, 65 knots when the high fuel pressure (8 psi) went off. Fuel tanks were full.

The pressure went back to normal with the boost pump off. I’m thinking it might just be the combination of a nose down attitude, full fuel tanks, and the boost pump. It rises slowly, so I’m pretty sure I didn’t notice it in normal landings as the boost pump was on with the nose pointed down less than 30 seconds.

The electric fuel pump is rated for 4.5-6 psi of boost.

The top of the fuel tanks may have been higher than the tap at the carb for the pressure to add 1 psi.

But perhaps I should subtract 1 psi for the height of the pressure transducer mounted on the fire wall. In a descent, the transducer might be level with the top of the tank.

It’s not hard to believe the engine driven pump wouldn’t add 2-3 psi.

I couldn’t find a spec for the carb for maximum fuel pressure. But the overhaul manual for the carb has you do a needle valve leak test at 6 psi with the bowl full. The bowl level is not suppose to increase for 5 minutes.

The engine (Superior) spec’s 8 psi for the fuel inlet. But we are measuring the output.

Anyone have any thoughts if this might be a problem?
 
Tom: There have been a number of threads on this site over the years about the fuel pressures on the carbureted engines -- both high and low readings. I can't say that anyone has ever figured it out but there are a number of opinions including what you mentioned re: the location of the pressure transducer and the flight configuration of the aircraft.

For a data point, I'll relate my own experience.

I recently replaced the electric fuel pump and fuel pressure gauge on my O-360-A1A powered RV-8. The fuel pressure gauge had always read low and the electric pump was 15 years old and sounding a bit tired. On the ground with just the electric fuel pump I would get a reading no higher than 3 psi. Sometimes on takeoff at full power I would take a quick look at the fuel pressure gauge and it would read near zero even with the electric fuel pump on (which of course it would be for takeoff). First time I saw that, my heart almost stopped until I realized the engine was running fine. Likewise, I've seen a range of readings during straight and level flight from below the green line to above the green line. Typically, if I turned the electric fuel pump on in flight, the fuel pressure would rise between one and two psi. (As a side note, make sure your fuel pressure gauge is adequately grounded as a poor ground can effect your readings.)

The new fuel pressure gauge I installed last month is the same as the original Van's gauge but without the Van's logo. It cost about $50 dollars from Chief Aircraft in Oregon. I also replaced the Facet electric fuel pump with a new unit of the same model. And I replaced the pressure transducer since the existing unit coincidentally crapped out while working on the other fuel related items. Now, when I turn the electric fuel pump on before engine start, my pressure gauge registers between 6.5 and 7.5 psi (above the green arc). Red line is 8 psi. This has caught my attention, but during flight it usually stays in the green range though tends to be near the top of the green. I haven't flown it enough since installation to have chanced a glance during takeoff. I'll have to do that soon.

My suspicion is that some, if not all, of the issue is the quality of the instruments; these aren't top of the line but they're adequate. I suspect, if I changed these units again I might get some other combination of readings.

Carbureted engines require very low fuel pressures unlike their injected brethren. The physical placement of the various items in the fuel system relative to the aircraft attitude and engine may also be a factor but I don't know that anyone has clearly established that. I can only say that fleet-wide this apparently is not an issue.

Sorry for the long-winded commentary. I think you're okay with your setup. Monitor it of course, and see what others have found. Perhaps do a search on this site for old threads on the issue. It seems our old carbureted technology has its own quirks, but it works.

Have fun flying.

Chris
 
Chris, thanks for the thread link. Interesting. A good reminder that unlike rope, warm gas would much rather be pushed rather than pulled.

Didn’t see that one in my thread searches on VAF. My situation seemed different enough and I’d run down a bit of data that I thought I’d throw it out there.

The surprising thing, to me anyways, when I look at the numbers for fuel pressures is that the overflow dump on the carburetor will drain fuel at times, just not in cruise. Why you want to have a good location for the fuel dump to happen, i.e. not on the exhaust.
 
My anecdotal experience: I have an O-360, fixed pitch, MA-4-5 carb. I would get intermittent high fuel pressure readings on the ground sometimes. I would get also get erratic readings in the air occasionally. I thought I had a poor ground somewhere. I replaced my fuel pump and it fixed all my problems.
 
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