O-360 Higher Than Normal Fuel Burn

[jeffk]

In our RV-7 we have a O-360-A1A (carbureted and dual mags) with a constant speed Hartzell. Fuel flow in the climb has been 12-15 gph and in cruise at 4500 (e.g. 20.6 man, 2430 rpm, egt ~1470, ff 14.2). I have a new commute for work and it's only a 50 NM flight (hence the lower altitudes). There are some rocks to cross and altitudes I've tried range from 4500 to 7500. I'm able to get the fuel flow down around 9-10 at 7500, but I've also seen excursions to 12 and 13 gph.

Historically, I've been able to flight plan for 9-10 gph if I was in a hurry and ~7-8 if I'm pulled back. My leaning technique has been to set the power (20-23 man, 2000-2100 or 2300-2400) and pull the mixture back until the EGTs are in the 1450-1470 range (never exceeding 1500). CHTs are 350-375.

My engine monitor is a vision micro 1000 (fuel flow, 4 EGT, 4 CHT). In an effort to determine if the fuel flow sender or monitor was acting up, I filled the tanks to the brim, flew to work, noted the 4.3 gallon burn, topped the left tank up and noted 4.3 gallons on the pump. This suggests the fuel flow sender and engine monitor are not the problem.

There are no signs of blue fuel stains, the cockpit is not awash in fuel (no drips noted, no scent of fuel with boost on or off) and the interior of the cowling does not give an indication of a fuel leak.

I'm looking for some suggestions on next steps to track down the source of my higher consumption. The engine was overhauled more than twelve years ago, has ~800 hours on it, compressions are all in the mid-70s, condition inspection was five months ago and I've put ~50 hours on the aircraft since then.

Thanks for your help and suggestions.

(I do plan to put on at least one electronic ignition in the future once this issue is sorted.)

Jeff

[Ironflight]

My first thought would be to establish a nice cruise at about 65% power (you can't hurt the engine there) and then pull the mixture knob back until it runs rough, then sweeten it up just a touch until it is almost smooth - just the way we learned to do it in teh days before everyone had engine monitors. See what the fuel flow shows then. Don't worry about absolute values of EGT's - there are not actually any limits for the engine (not that can be reached with the fuel you're using).

See if you get leaner than you were before.

[jeffk]

Hi Paul, thank you for your reply. I flew home at 7500 last evening and (over an airport just in case) I set the power to 19 inches of manifold and 2100 rpm. I worked the mixture back until the engine started to get grumpy (like I used to do in the Citabria). I saw fuel flows in the 7-10 gph range. More testing is required as I was able to turn the mixture knob a full turn plus. I'm going to go on a higher altitude flight 10-12K today and see what sort of numbers I can get. This may be an issue of my over reliance on the EGTs on the cockpit 'video game' and running too rich.

More soon.

[jrs14855]

Take a look inside your exhaust stacks. If they look black and sooty you are running way too rich. At some point you will want to look at the spark plugs, see if there is evidence of one or more cylinders running rich.
Really hard to get good data on such a short flight.

[DEWATSON]

How hot can EGT temps get before you have leaned too much?

[jrs14855]

Lycomings position years ago was that you can't hurt a normally aspirated Lycoming below 75% power. As Paul stated there are no EGT limitations(except on turbocharged engines, and that is usually tubine inlet temperature, NOT EGT.
Read the Mike Busch articles from Sport Aviation, they will help you understand this.

[Bubblehead]

Originally Posted by DEWATSON

How hot can EGT temps get before you have leaned too much?

Absolute EGT is meaningless. 1/4" closer or farther from the cylinder will dramatically change the EGT reading. What matters is how many degrees lean or rich of peak you are. At 65% power the risk of detonation is small or none depending on who you read, so you can lean or over-lean (undefined term) without risk to the engine.

I don't understand the statement about fuel flow varying so much. "7 to 10 gph range" Is that from full rich to lean or is it varying that much in flight after leaning? If that is the change from full rich to lean then no problem.

[jeffk]

Originally Posted by Bubblehead

"7 to 10 gph range" Is that from full rich to lean or is it varying that much in flight after leaning? If that is the change from full rich to lean then no problem.

The 7 to 10 gph was the fluctuation I was experiencing in cruise after leaning.


It turns out I was wrong about the fuel flow being accurate. I've gone on a number of long flights and made use of the 'fill the tanks, fly and then fill the tanks again' method to determine fuel flow.

On one flight, I burned 17.2 gallons flying from Monterey to Burbank. We were at 11500 and I was around 21" and 2400 rpm (a.k.a. not economy cruise). I'm burning between 8 and 9 gph if I keep the prop spinning. I can get down to ~7 gph if I dial it back.

I'm going to check my wiring as well as the fuel flow sender to see what's causing the high flow numbers.

The good news is the everything appears to be fine with the engine with the exception of my instrument not reading correctly.

Thank you to everyone for their input. Appreciate your comments and suggestions.

Jeff