Low fuel flow at low altitude in a 1963 C Model

[Greg Ellis]

I have a 1963 C model with a Lycoming 0-360 A1D engine with about 900 hours SMOH.  I have struggled with an issue ever since the overhaul and would like some guidance since no one can seem to figure out what the problem is.  On takeoff I get 15-16 gph fuel flow which is perfect from what I have been told for a carbureted engine.  The issue is when I level off at a low altitude, at or below 2500 feet MSL.  The fuel flow drops down to less than 11 and usually in the lower 10's.  This is with the mixture control at full rich and the RPM/MP at 25/25.  This is very lean for this altitude which the book value says should be running in the mid 12's GPH.  So while down low the engine will occasionally run rough due to the low fuel flow.  If I climb above 3000 feet MSL the fuel flow increases up into the 12-13 range and the engine runs fine.  Above 5000 I lean the engine so of course the fuel flow drops off but the engine does very well above 5000 feet at 10 gph.

The carburetor has been checked and everything seems fine.  So I am baffled as to what is causing a low fuel flow at a low altitude when it should be running above 12 gph and then an increase in fuel flow when climbing above 3000 feet.  Thanks for any help.

[Drumstick]

I thought fuel flow at takeoff should be around 18ish GPH? (sea level - standardish kinda day)

Edited July 10, 2017 by Drumstick

[Bob_Belville]

3 minutes ago, Drumstick said:

I thought fuel flow at takeoff should be around 18ish GPH? (sea level - standardish kinda day)

18 is about right for my E model, I suppose a 180 HP C would be 10% less... 16+

[goterminal120]

At WOT you get into a enrichment jet for cooling at high power. When you pull back to 25 inches at 2500 feet your out of the circuit. When you go up and add power to maintain 25 inches you get back into the enrichment ciruit, so with out leaning fuel flow will increase. Go higher and start leaning fuel flow comes back to what you would expect.

[Bob_Belville]

24 minutes ago, Greg Ellis said:

I have a 1963 C model with a Lycoming 0-360 A1D engine with about 900 hours SMOH.  I have struggled with an issue ever since the overhaul and would like some guidance since no one can seem to figure out what the problem is.  On takeoff I get 15-16 gph fuel flow which is perfect from what I have been told for a carbureted engine.  The issue is when I level off at a low altitude, at or below 2500 feet MSL.  The fuel flow drops down to less than 11 and usually in the lower 10's.  This is with the mixture control at full rich and the RPM/MP at 25/25.  This is very lean for this altitude which the book value says should be running in the mid 12's GPH.  So while down low the engine will occasionally run rough due to the low fuel flow.  If I climb above 3000 feet MSL the fuel flow increases up into the 12-13 range and the engine runs fine.  Above 5000 I lean the engine so of course the fuel flow drops off but the engine does very well above 5000 feet at 10 gph.

The carburetor has been checked and everything seems fine.  So I am baffled as to what is causing a low fuel flow at a low altitude when it should be running above 12 gph and then an increase in fuel flow when climbing above 3000 feet.  Thanks for any help.

That's weird. You're climbing through 2500' at 25 square and seeing 14(?) GPH but if you level off, still at 25" and 2500 RPM, FF drops and you haven't done anything except trim the nose down? 

[Greg Ellis]

22 minutes ago, Bob_Belville said:

That's weird. You're climbing through 2500' at 25 square and seeing 14(?) GPH but if you level off, still at 25" and 2500 RPM, FF drops and you haven't done anything except trim the nose down? 

Sorry, no.  I am climbing at WOT.  Level off at 2500 feet msl, pull the power back to 25 squared and the fuel flow drops into the low 10's.  Book value says it should be in the 12's.  The engine runs rough like it is too lean.  If I increase the MP and RPM I will get back up above 12 and the engine smoothes out.  My concern is the book value should be at 12 but at 10 it runs rough.  

[Greg Ellis]

29 minutes ago, goterminal120 said:

At WOT you get into a enrichment jet for cooling at high power. When you pull back to 25 inches at 2500 feet your out of the circuit. When you go up and add power to maintain 25 inches you get back into the enrichment ciruit, so with out leaning fuel flow will increase. Go higher and start leaning fuel flow comes back to what you would expect.

I do understand this and thanks for the response.  This is exactly what is happening.  However, at 2500 feet and 25 squared I should see 12 gph fuel flow or there abouts and I get 10 and the engine runs rough.  I have a good friend who has an A model with the same engine.  When he levels off at 2500 feet and pulls back to 25 squared he sees around 12 gph and no roughness.

[Greg Ellis]

33 minutes ago, Bob_Belville said:

18 is about right for my E model, I suppose a 180 HP C would be 10% less... 16+

I was told by a long time Mooney owner and mechanic that 15-16 gph on take off is what to be expected in a carbureted O-360.

[Bob_Belville]

33 minutes ago, Greg Ellis said:

I do understand this and thanks for the response.  This is exactly what is happening.  However, at 2500 feet and 25 squared I should see 12 gph fuel flow or there abouts and I get 10 and the engine runs rough.  I have a good friend who has an A model with the same engine.  When he levels off at 2500 feet and pulls back to 25 squared he sees around 12 gph and no roughness.

The '66C Owners Manual shows 12.3 GPH for 2500' & 25/2500

FWIW, at 2500', 27", 2600 RPM (95%power) the OM shows 14.4 GPH.

But I'm puzzled as to why the FF goes up when you climb above 3000' 

I'm not so sure your "occasional rough" is too rich. Next time try leaning a little. See if it gets better or worse.

[Drumstick]

38 minutes ago, Greg Ellis said:

I was told by a long time Mooney owner and mechanic that 15-16 gph on take off is what to be expected in a carbureted O-360.

I just reviewed my FF data from the last years flights.  I peak at 18-19GPH on takeoff roll at sea level.  FF tapers off during climbout and looks to avg around 16 GPH in a climb to 10K.  My FF meter is spot on.  I have never flown 25^2 at 2500 feet so I can't pull any of my FF data to compare.   I have no idea how to fix your FF issue but just offering my own data to help - not sure if mine is correct either but it seems to be working without roughness.

[Hank]

Goterminal is correct about why fuel flow increases at higher altitude--it's because you are pushing the throttle forward and getting back into the enrichment circuit.

I never fly at 25 / 2500, that's too high of a power setting for me. I like it when the MP and RPM numbers add to 46 or 47, not to 50 (2500 RPM on the gage is only "25"). 

This is the sea level page of the Performance Tables from my 1970 C, looks like your fuel flow at takeoff is about 2 gal low. And it is also 2 gal low at 3000 msl.

My charts are in 2500' increments, so I don't have anything for 3000 msl, but at 2500 msl I show 25 / 25 to be 84.7% power and 15.1 gph. When I'm flying that low, generally for nearby lunch run or flightseeeing, I back off to 23 / 23 and lean the engine; can't give fuel flow as I'm not instrumented for it, but my breakfast runs out and back used to log 0.9 hours and pretty much 6 gallons at the pump.

Edited July 10, 2017 by Hank

[carusoam]

A couple of things, Greg...

Do you have:

1) An engine monitor, aka JPI?

2) Fuel flow instrument?

 

Are you familiar with:

3) The second fuel jet in the carb?

4) How and when it works?

 

Have you checked the accuracy of your instruments?

5)  rpm (phone app)

6) Mp (atmospheric pressure while not running)

7) FF and EGTs and CHTs?

 

I think I'm sensing something missing in the observations, more than something not working properly...

Generally speaking the T/O power available at SL has twice the FF of a nicely leaned engine in cruise (broad observation)

FF at T/O power is a setting / book value.  Without the fancy instruments listed above, it is hard to check things in flight.

 

The reason I bring this up...

You are going on what you were told.  When you can go on actual data that you can look up and verify.

Knowledge is power.

Some of what you have been told was good information.  

Best rgeards,

-a-

 

 

[Greg Ellis]

1 minute ago, carusoam said:

A couple of things, Greg...

Do you have:

1) An engine monitor, aka JPI?

2) Fuel flow instrument?

 

Are you familiar with:

3) The second fuel jet in the carb?

4) How and when it works?

 

Have you checked the accuracy of your instruments?

5)  rpm (phone app)

6) Mp (atmospheric pressure while not running)

7) FF and EGTs and CHTs?

 

I think I'm sensing something missing in the observations, more than something not working properly...

Generally speaking the T/O power available at SL has twice the FF of a nicely leaned engine in cruise (broad observation)

FF at T/O power is a setting / book value.  Without the fancy instruments listed above, it is hard to check things in flight.

 

The reason I bring this up...

You are going on what you were told.  When you can go on actual data that you can look up and verify.

Knowledge is power.

Some of what you have been told was good information.  

Best rgeards,

-a-

 

 

No engine monitor as such.  I do have the old CHT single probe and a 4 lead EGT but they are analog, not digital and difficult to use other than a glance to see if you are close.  I do have a JPI Fuel scan and that is what I am getting my readings from on the fuel flow.  I have checked the RPM using an RPM tester that you point at the prop and it checks out very close to the gauge.  I will check the MP when not running.

I appreciate each and every response on this post and all the help and time you guys are willing to give me.

[DXB]

A few potentially relevant thoughts:

1. 15-16gph for takeoff seems too low to me - it should be above 17gph at least assuming you're not at a highDA.  If you're really running that lean at WOT, you should have serious CHT issues on takeoff and down low at high power - is that the case?  Also <15gph and WOT at takeoff at low DA will make my engine monitor alarm immediately from astronomical EGTs.  Don't ask me how I know this.  

2. Make sure your mixture cable pushes the lever on the carb all the way forward when set to full rich - a mis-adjusted cable could be a very simple explanation for your observations

3. How is your fuel pressure under conditions where fuel flow seems too low?  If  pressure is low, this might suggest your issue is in the fuel system somewhere before your carb.  

4. Turns out there  are 3 flavors of the Marvel Schleber MA-4-5 carb that are approved for the O-360-A1D in the C model: 10-4164-1, 10-3878, 10-3878-M.  They differ in how rich they run - it may be worth knowing which one you have and if you got a different one at last overhaul when your problems started.

5. Of course fuel flow meter calibration could be an issue, but seems less likely if you also have objective evidence of it running too lean.  As I note above, your low fuel flows at high power settings should give you serious engine temp issues.

 

[carusoam]

I spent a decade flying a C with similar instrumentation... less the four EGTs, just one. I didn't have a FF gauge.

One thing that you can check on while cruising is adequate FF...

Having adequate FF means you can enrichen the mixture to about 300°F ROP.  Good for cooling during the climb.

Being able to lean to peak, and beyond for cruise.

 

Be real while doing this...

25/25 is a high power setting.  

Staying ROP by 200°F+ is a good idea while climbing...

Climb procedure using the blue box for an IO550...  The blue box is 200-300°F ROP.  Maintain while climbing...

If you are cruising at low altitudes with a 25/25 setting I would expect CHTs and other temps to be pretty warm...

 

PP thoughts only, not a mechanical genius...

Best regards,

-a-

 

[Greg Ellis]

6 minutes ago, DXB said:

A few potentially relevant thoughts:

1. 15-16gph for takeoff seems too low to me - it should be above 17gph at least assuming you're not at a low  DA.  If you're really running that lean at WOT, you should have serious CHT issues on takeoff and down low at high power - is that the case?  Also <15gph and WOT at takeoff at low DA will make my engine monitor alarm immediately from astronomical EGTs.  Don't ask me how I know this.  

2. Make sure your mixture cable pushes the lever on the carb all the way forward when set to full rich - a mis-adjusted cable could be a very simple explanation for your observations

3. How is your fuel pressure under conditions where fuel flow seems too low?  If  pressure is low, this might suggest your issue is in the fuel system somewhere before your carb.  

4. Turns out there  are 3 flavors of the Marvel Schleber MA-4-5 carb that are approved for the O-360-A1D in the C model: 10-4164-1, 10-3878, 10-3878-M.  They differ in how rich they run - it may be worth knowing which one you have and if you got a different one at last overhaul when your problems started.

5. Of course fuel flow meter calibration could be an issue, but seems less likely if you also have objective evidence of it running too lean.  As I note above, your low fuel flows at high power settings should give you serious engine temp issues.

 

I did check way back when this first started and I do have the lean carb.  I will check the mixture cables when I go to the airport this weekend.  Fuel pressure is fine when this occurs.  Fine meaning well in the green.  With my limited engine monitoring gauges, when running in the mid 10's GPH on fuel flow down low, the CHT gauge reads below 400.  It is an old analog gauge so I assume it is around 360-380.