Lycoming IO-320 excessive fuel use

[GeneC]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

 

[RichNY]

Some ideas. When you lean and feel the roughness, just enrichen enough to stop the roughness, no further.
Your tach might be reading low and you might be at 2700 rpm.

 

[Henning]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

If it takes that to keep the engine smooth, you likely have an induction leak. Do you have a 4 cylinder engine analyzer or one point probe for EGT? Do you have a fuel flow, or are you just measuring what you needed to fill it?

 

[pigpenracing]

Sounds like to much rpm for good mileage.... Back it down some.

 

[Henning]

Sounds like to much rpm for good mileage.... Back it down some.

Depends on the prop. My 310 did fine economy wise at 2675. I'd get the same 9.5 nmpg from 2400-2675/redline going for the same speed.

 

[GeneC]

Single point egt. We consistently ran 50 ROP. I don't think that it would take that much to keep it smooth. Since I am just learning this plane, maybe just smoothing it out will be the right ticket. I'll have to try that next flight. Is not 50 ROP best economy?
Also, leaning in the climb to altitude, would 100 ROP, be the right choice above 3500, or is full rich to 5000 correct? During the trip, once above 2000 due to density altitude, leaning increased RPM substantially. I kept at least 100 ROP on the climb out to altitude.
Fuel burn was calculated from time, take off to touch down and used gallons. The Cheetah comes down at very well at 130 kts at low power settings. Figured it made up for the climb.
How would one find an induction leak?

 

[Graueradler]

How would one find an induction leak?

Pressurize the induction with the discharge side of a shop vac (or other vac. cleaner). Rig a filter so that you don't blow dirt into the engine. Use soapy water to look for the leak. On Lycs. the place where the intake tubes are rolled into the oil pan in a possibility

 

[docmirror]

Look for fuel leaks anywhere. From the tank, all the way to the carb. Don't forget the primer system. You've got to look at your plugs too. See if they show rich burning. That fuel use is too high.

 

[kyleb]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

Do this:

Fill both tanks. Take off and climb to your cruise altitude on one tank. Upon reaching your cruise altitude, switch tanks. Fly on that tank for an hour at a steady state power setting with the mixture properly leaned. At the end of the hour, switch tanks, land and refill the "cruise" tank. That will give you an indication of your real cruise fuel consumption.

Also, check your tach. Optical tach's run ~$50 and you might be able to borrow one for free...

 

[philiplane]

I would start with the ignition system before worrying about induction leaks, based on the ratio of those faults.

Do you have Champion plugs? Slick mags? Both are a recipe for poor performance.

 

[Ted]

I'll have to try that next flight. Is not 50 ROP best economy?

No. Lean to the edge of roughness is best economy.

Also, leaning in the climb to altitude, would 100 ROP, be the right choice above 3500, or is full rich to 5000 correct? During the trip, once above 2000 due to density altitude, leaning increased RPM substantially. I kept at least 100 ROP on the climb out to altitude.

Probably the best thing for you to do is note where the EGT is on takeoff and lean to maintain that.

 

[alfadog]

I also would have the tach and ff gauges checked if no visible fuel leaks before going further.

 

[flyingriki]

Look for fuel leaks anywhere. From the tank, all the way to the carb. Don't forget the primer system. You've got to look at your plugs too. See if they show rich burning. That fuel use is too high.

Has anyone mentioned looking at the pipes to see how lean he's running? But hey, I don't have 37,000 opinions so what the hell do I know?
I do know when I am leaned properly my pipes look great, and they usually do....

The OP needs to look at an engine graph that explains the relationship between power, RPM, mixture and temps to appreciate how it works. Where the power is, where the economy is and where the temps lie, both lean and rich of peak. Mike Busch would be a good start. He knows his stuff, its proven and he doesn't live to blabber on forums, he's too busy with reality.

Too bad helmet guy can't explain it that simply. But hey, after 37,000+ opinions you just start to get stale bragging about past fancy planes and explaining why life revolves around a forum...

Gotta go fly and lots of other things that would scare geico! (Is that an ad for something?)

 

[Dan Thomas]

Don't forget the primer system.

That's a rare one, but I've seen it. If that primer isn't closed and locked properly, the engine can suck fuel through it. If the needle and seat inside the primer are leaking, that will also let the engine get extra fuel that can't be controlled with mixture. It will suck more fuel at lower throttle settings when the manifold pressure is much lower.

That tach will be suspect if it's the same age as the airplane. I've seen tachs that old that are reading 200 RPM low, making the pilot think he's using less power than he is. The spinning magnet in the tach gets weak with age and doesn't drag the cup and needle as far around as it should.

Dan

 

[docmirror]

Has anyone mentioned looking at the pipes to see how lean he's running? But hey, I don't have 37,000 opinions so what the hell do I know?
I do know when I am leaned properly my pipes look great, and they usually do....

The OP needs to look at an engine graph that explains the relationship between power, RPM, mixture and temps to appreciate how it works. Where the power is, where the economy is and where the temps lie, both lean and rich of peak. Mike Busch would be a good start. He knows his stuff, its proven and he doesn't live to blabber on forums, he's too busy with reality.

Too bad helmet guy can't explain it that simply. But hey, after 37,000+ opinions you just start to get stale bragging about past fancy planes and explaining why life revolves around a forum...

Gotta go fly and lots of other things that would scare geico! (Is that an ad for something?)

I don't know, why don't you read the thread and see what's been posted. I also don't know what the hell you know so I can't help you there either.

I don't think my post count is even a decent fraction of 37,000 but really I don't keep track. Surely not all of them are opinions. But - of the ones that are opinion, I stand behind them, unless you have something specific that you'd like to share about my post?

 

[coloradobluesky]

You can check the accuracy of your tach yourself. At night, taxi to a florescent street lamp and have it at your back. With a 2 blade proop, it will "stop" at 1800 rpm. For 3 blade it is 1200 and 2400. Its an accurate measurement because the power company makes 60 cycles with very good accuracy.

 

[coma24]

50 ROP would be best power. Best economy would be LOP.

Without a 4 point engine monitor, you have no idea where the other cylinders are along the curve, so it's a total guess.

 

[bnt83]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

What kind of fuel flow indicator? Shooting from the hip, you digital fuel flow gauge needs to be set.

 

[bnt83]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

You say your AA5A is fuel injected? Don't you have an O-320-E2G?

I just installed a brand new EI fuel flow and got dialed in,

I have a 150 horse O-320-E2D (in a Cessna) burning about 6.5 gallons at 2300 RPM @ 5000 MSL in 75 degrees F, going about 100-105 knots. Wide open full throttle with 70 knot climb is between 10 and 11 gallons per hour

 

[poadeleted20]

The PoH for my 1979 Grumman AA5A says I should be @ 8.3 GPH at 5500 and leaned to smooth operation after leaning to rough them back a little. On bringing the plane home from Texas to Ft Lauderdale experienced @10.5 GPH at 5500' 2600RPM, leaned 50 ROP. This is @ 71% power if I recall correctly. The plane runs fine. All comps in the 70's minus one at 68. Cannot figure where it is going. Oil is good, temps good. Indicating @ 115kts at altitude. Lower altitude and it gets worse.
Any help or ideas?

First, are you sure your tach is accurate? If it's the original mechanical tach from 35 years ago, it's probably reading significantly low (100-150 low is typical), and you may be pulling more power than you think. Note that at 5500 feet and 71% power, I'd be expecting about 8.5 gph and about 120 KTAS. You're indicating 115 knots, which at that altitude is about 125 KTAS. Typically, it takes well over 75% power to push a Cheetah that fast, and that's going to take well over 8.5 gph.

Further, the cruise fuel flows from the POH are based on leaning per manufacturer's recommendations of "lean to rough then enrich to smooth", and after a couple of decades of experimentation with full engine analyzers, that's going to be right around peak EGT in a Lycoming O-320, and 50 ROP is going to be at least 0.5 gph greater flow. Put that together with the other data, and 10.5 gph at nearly 80% power leaned only to 50 ROP is about what I'd expect.

So, I'd start by checking the accuracy of your tach, either with a strobe or one of the smart phone "head speed" apps. Then, try leaning as they recommend rather than richer. When you've done that, see what fuel flow and speed you get -- bet they'll be right close to "book".

 

[poadeleted20]

Single point egt. We consistently ran 50 ROP. I don't think that it would take that much to keep it smooth. Since I am just learning this plane, maybe just smoothing it out will be the right ticket. I'll have to try that next flight. Is not 50 ROP best economy?

Yes, it is not. It's close to best power, which is around 75-100 ROP.

Also, leaning in the climb to altitude, would 100 ROP, be the right choice above 3500, or is full rich to 5000 correct? During the trip, once above 2000 due to density altitude, leaning increased RPM substantially. I kept at least 100 ROP on the climb out to altitude.

If you don't have an all-cylinder CHT system, stick with the book recommendations on climb (full rich to 5000 DA, then lean to peak RPM). You cannot rely on a single-cylinder EGT to keep you out of trouble when leaning at full throttle.

Fuel burn was calculated from time, take off to touch down and used gallons. The Cheetah comes down at very well at 130 kts at low power settings. Figured it made up for the climb.

It doesn't. You'll be running about 140% of cruise fuel flow in full throttle climb but only about 85-90% of cruise fuel flow during a cruise descent of 500 ft/min.

 

[poadeleted20]

Also, check your tach. Optical tach's run ~$50 and you might be able to borrow one for free...

If you have a smart phone, you can get an app for this for about $3-5. Here's the one I use on my Android phone, and it's dead accurate compared with electronic tachs.

 

[poadeleted20]

You say your AA5A is fuel injected? Don't you have an O-320-E2G?

Unless he bought one of the four Hyper STC'd Cheetahs with a 200HP IO-360, or one of the three Sabertooth STC'd Cheetahs with a 260 HP IO-540, he's not fuel injected, and if he had one of those, he wouldn't be running the numbers he's saying no matter what he did with mixture or how inaccurate his tach was. And there are no Cheetahs with an IO-320, so I'm guessing the "I" in the thread title is a typo.

 

[bnt83]

Unless he bought one of the four Hyper STC'd Cheetahs with a 200HP IO-360, or one of the three Sabertooth STC'd Cheetahs with a 260 HP IO-540, he's not fuel injected, and if he had one of those, he wouldn't be running the numbers he's saying no matter what he did with mixture or how inaccurate his tach was. And there are no Cheetahs with an IO-320, so I'm guessing the "I" in the thread title is a typo.

Tach calibration is a good easy cheap check.

A carbureted O320 with fuel flow via JPI/EI/OTHER may need some k factor tweaking or he has sketchy flow transducer. Easy to verify over a few flight by keeping track of fuel consumption

 

[MyassisDragon]

FWIW I had an interesting issue with an AA5A that I occasionally take on trips.
At first I was only able to get around 10.5 - 11 gph ( went by fuel stops on long trips so its an average) but one day in the cold winter I had issues with the carb heat not closing all the way and eventually freezing. Found out it was a bad cable and after it was replaced and working good I could get the plane down to 9 to 9.5 GPH on trips

simple....but made a difference....

 

[bnt83]

FWIW I had an interesting issue with an AA5A that I occasionally take on trips.
At first I was only able to get around 10.5 - 11 gph ( went by fuel stops on long trips so its an average) but one day in the cold winter I had issues with the carb heat not closing all the way and eventually freezing. Found out it was a bad cable and after it was replaced and working good I could get the plane down to 9 to 9.5 GPH on trips

simple....but made a difference....

Right. Total fuel used for the flight / hours flown is always higher than gallons per hour flow @ cruise.

I don't have vernier mixture, it clicks while moving it, not the old Cessna tab thingy type.

 

[poadeleted20]

A carbureted O320 with fuel flow via JPI/EI/OTHER may need some k factor tweaking or he has sketchy flow transducer. Easy to verify over a few flight by keeping track of fuel consumption

It appears the OP does not have a fuel flow system, and is just dividing gallons used by flight time. Not a really accurate method of determining cruise fuel flow. While I'm regularly running about 9.2 gph at 70% cruise in my Tiger, I get about 10 gph total burn by the gallons used/hours flown method, and I suspect it's the disproportionately high fuel flow in climb which causes that.

 

[bnt83]

It appears the OP does not have a fuel flow system, and is just dividing gallons used by flight time. Not a really accurate method of determining cruise fuel flow. While I'm regularly running about 9.2 gph at 70% cruise in my Tiger, I get about 10 gph total burn by the gallons used/hours flown method, and I suspect it's the disproportionately high fuel flow in climb which causes that.

Just having a visual reference that shows flow in gallons per hours is encouragement to slow it down IMHO

 

[GeorgeC]

I've been meaning to try my hobby-grade IR tach for just this purpose.

...
So, I'd start by checking the accuracy of your tach, either with a strobe or one of the smart phone "head speed" apps. Then, try leaning as they recommend rather than richer. When you've done that, see what fuel flow and speed you get -- bet they'll be right close to "book".

 

[GeneC]

It is not injected. Sorry about the typo. There is no fuel flow meter either. Basic, tach and EGT gauge. I am going to check the tach, it is possible it is off. With the high compression STC the RPM's are limited to 2650, I never seen this. 2600 was about the max. It is possible the tach is reading low. I am going to try the IPhone RPM app. It also does not have wheel pants, so maybe the RPM's were high to give the numbers seen. TAS worked out to 122 if I recall, which would be good for no wheel pants. I am also going to try the lean till rough and enrich till smooth. If I pull it back and lean as indicated, I will assume it will get better. I am definitely going to check the tach anyway. Thanks for all the advise. I should be going in the right direction. First plane, lots to learn. Love the Grummans.

 

[poadeleted20]

It is not injected. Sorry about the typo. There is no fuel flow meter either. Basic, tach and EGT gauge. I am going to check the tach, it is possible it is off. With the high compression STC the RPM's are limited to 2650,

Which prop? Original 59-pitch McCauley or the STC'd 61-pitch Sensenich you can use with the HC STC'd engine?

I never seen this. 2600 was about the max. It is possible the tach is reading low.

You should have no trouble at all getting that engine to overspeed a bunch with HC STC and the stock prop -- it will turn near 3000 RPM at full throttle in level flight at low altitude (not something you actually want to do). Makes me think even more your tach really is reading significanly low.

I am going to try the IPhone RPM app. It also does not have wheel pants, so maybe the RPM's were high to give the numbers seen. TAS worked out to 122 if I recall, which would be good for no wheel pants.

It would be very good without wheel pants -- the numbers I gave were for with pants, and typically you lose about 4 knots without them. And it would have to be -8C at 5500 for 115 KIAS to be 122 KTAS, and that's bloody cold this time of year (ISA-13C).

 

[coloradobluesky]

You can check your tach accuracy by doing a runup with an airport florescent light in back of you. A two blade prop will appear to stop at 1800 rpm, 3 blade at 2400rpm.

 

[GeneC]

Original 59 from what I have found int the logs. Tried the RPM audio app on the iPhone. Numbers came out pretty close. Within 25 up to 1800 RPM. Going to take her flying tomorrow so we will see at higher RPM then. Just to note, I do not get much of a carb heat drop during run up. Maybe 15 RPM. Which got me thinking and poking around. Found the carb heat flapper rod that runs through the airbox wiggles around. My guess is there is at least some carb heat carrying through even when turned off. I found it requires aggressive leaning on the ground not to foul the plugs. I also am guessing this would cause extra fuel burn. Any other ideas? Thanks for all the input.

 

[docmirror]

That post reminded me of something. There is a gap in the front cowl where the air inlet is and it mates to the airbox. The gap can allow hot air to get in and reduce the power available. I had to repair this very issue on my Grumman and according to Fletchair, it's a common area of failure. I think their solution was either a special thick gasket, or two gaskets sandwiched together. Could also be why your carb heat isn't very different.

 

[GeneC]

Ron from my E6B from the day we flew..
Indicated 5500', 18C, 29.99
Press Alt 5430, Density Alt 6995
Indicated air speed 111kts, yields 123 kts TAS.
Without wheel pants that is very close to POH numbers. At 2600 RPM indicated.
With the RPM indicating low, this may be causing the increased fuel burn.

 

[GeneC]

Thanks I'll have a look.