I own a 1961 172B. Recently I took a cross country trip and my fuel burn was 12GPH on the first leg. My cruise was 5000ft @ 2300rpm full rich. On the return leg my burn was 10.5 gallons, 7500ft @ 2300rpm leaned.
My manual states that I should burn around 7 GPH. Does anybody have any suggestions what might be causing this excessive burn rate?
You probably need to clean your spark plugs. After flying rich, they might be crudded up. Lean aggressively, from start up to shut down. The exception would be takeoff and climb out. My 172 with a Continental 145 engine burns 6.6gal per hour at 2400 rpm.
12 GPH does seem quite high, however,I would not depend on fuel consumption numbers in the POH. I think they are always lower than the actual fuel consumption.
A couple of possibilites, did you accurately measure your tanks, before takeoff and after landing? Could the gaskets on your fuel caps be worn to the point that fuel is being siphoned out by the slip stream? Is your tach accurate? Tachs typically read low at the high end as they age
Do you have the one piece venturi and the new main jet in your carb? I've heard of increased fuel consumption as a result of that "fix"
Any chance you have a fuel leak in the tanks, the sump valves or the gascolator sump valve?
I have a 150 hp 172 and only see 12 GPH on climb, cruise is anywhere from the high sixes to the low nines depending on my power setting and leaning.
My 172-A with the O-300-D burns around 8.5 GPH running wide open at 5,500 msl.
Your's must have a leak somewhere.Primer,carb,or gas caps would be the places to start.
What about the fuel tank vent behind the left side strut? Could it be improperly positioned and pressurizing the tanks thereby forcing fuel overboard?
That high of a fuel burn rate would foul spark plugs right and left I would think.
Mark Hough Wrote:
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> I own a 1961 172B. Recently I took a cross
> country trip and my fuel burn was 12GPH on the
> first leg. My cruise was 5000ft @ 2300rpm full
> rich. On the return leg my burn was 10.5 gallons,
> 7500ft @ 2300rpm leaned.
>
> My manual states that I should burn around 7 GPH.
> Does anybody have any suggestions what might be
> causing this excessive burn rate?
>
> Any input would be greatly appreciated.
Just curious if you have found the cause of your high fuel burn rate, and what was the cause?
Check your tach - I couldn't figure out why my fuel burn and speed were both below book - turns out my tach was reading 100 rpm higher than actual (rpm's 100 lower thank I thought they were) Speed :) and fuel burn :( now match book values. You can buy an optical tach tester for $40 to $50 which counts the revolution of your propeller blades.
I fly a '66 172 w/ O300D. I burn around 6.8 gal/hr @ 2350rpm leaned @ around 3500ft. There is an easy way to check the accuracy of your tach. - After sunset, taxi close to or under a sodium lamp post such as used @ a fuel farm. From idle, increase power very slowly to around 1800 rpm watching for the strobe effect to "stop" the prop. At this point the tach should indicate 1800rpm exactly. 1800rpm is a resonant frequency of our 60Hz power. Once the prop is synchronized the engine will be 1800rpm. Now look at the tach @ see how for off 1800 it is (if any). This test will only validate the mid range of the tach but will give an idea of the tach in general.
Interestingly enough, mine was accurate up till about 1800 rpm, but reads 100 rpm higher than actual around 2500 rpm. If you don't want to buy a tach tester from someone like ATS or Spruce, see if you can borrow one from your local Radio Controlled model club. The one I got from ATS was identical to ones sold by the local hobby shops.
