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Thread: Fuel Burn and Mixture Control

  1. #1
    Chris Iriarte's Avatar
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    Default Fuel Burn and Mixture Control

    Hey all,

    Just completed a KLRU to KBQR to KLRU cross-country ... 35.2 hours total. Just sat down to calculate the fuel burn for each leg and it's all over the place, from 7.9 to 9.8 gal/hr. I did have some prolonged climbs at lower elevations that could have increased the numbers a bit, but I'm pretty aggressive with leaning so I'm wondering if my stock mixture control is creeping.

    Anyone have any similar experiences? I know there's at least one thread on installing a vernier control so I'll look for that.

    Chris

  2. #2
    Brian's Avatar
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    Default Re: Fuel Burn and Mixture Control

    Chris when I fly from the West Coast to OSH which involves altitudes from near SL to 12,000, both hard full throttle burns and some efficiency throttled back burns, I see the wide GPM range numbers like you recorded. My first and very hot leg across the Majove is sometimes 10+ GPM as I'm keeping the mixture rich to mitigate high Cyl Head & Oil Temp numbers. So you have normal fuel burns in my opinion.
    Last edited by Brian; 08-16-2015 at 11:29 AM.
    Brian
    Monrovia, CA

  3. #3
    Chris Iriarte's Avatar
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    Default Re: Fuel Burn and Mixture Control

    Thanks Brian,

    I used to to have a Cherokee 140 with a 160-hp O-320 (don't remember the suffix) and it was a pretty consistent 8 gph engine so these fluctuating numbers had me a little concerned. But I fly the Tripacer at lower altitudes, too, so that could be a contributing factor.

    Chris

  4. #4
    59pacer's Avatar
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    Default Re: Fuel Burn and Mixture Control

    Just back from 60 hours in 6 weeks around Australia in our 160 hp Pacer. I measure fuel burn as what I put in divided by flight time, as the taxi times are always fairly similar.
    Every takeoff was, dare I say it, overweight, so I try to avoid climbing more than a few thousand feet for the first hour or so, then climbing to a bit less than 5000' (we can do that here!) for the rest of the flight. I always run at near to 75%, and lean to rough running, then richen to just smooth running, plus a tiny bit more (old school, I know). OATs varied from 32*F to 85*F.
    Fuel burn varied from 8.6 to 9.5 USG/Hr on the 30 'legs' involved. The higher average burn was always associated with prolonged climbs at heavy weights-- no surprise there.

  5. #5
    Theo's Avatar
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    Default Re: Fuel Burn and Mixture Control

    I have a 150 Tri-Pacer and When I did the trip from 34TS to KSRR I burned about 8.8 GPH going there and 8.1 coming back. I just figured I got better mileage on the way home because it was down hill the whole way. When cruising at 8500 ft MSL I pretty much had to keep the throttle almost WFO to maintain cruising speed. It was warm to hot out and I had to keep it pretty rich to keep the CYL head temps down.

  6. #6

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    Default Re: Fuel Burn and Mixture Control

    Just got back from a round trip to Michigan from Texas. T31 to KOZW to T31. Did not keep exact track of fuel burn, but it appears to be right at 8 GPH for the whole trip. My PA22/20 has an O320-B2B.

  7. #7
    proav's Avatar
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    Default Re: Fuel Burn and Mixture Control

    Just bought my 1956 PA22/20-150. On the first leg home (2.3 hours) cruising at 4,500msl, 2450rpm, at +20c, TAS was about 122mph. Heavy with fuel (44 gal) and luggage, fuel burn for the first leg was 10gph. CHT on the single probe was about 350 in cruise. EGT in climb with WOT was about 1400. Level at 4,500’ throttle pulled back to 2450rpm, leaned out, the EGT peaked at 1600 which about coincided with when the engine would start running rough. Richened up to smooth RPM (50 ROP) would produce about 1550 on the EGT.
    2nd leg of the flight, after about an hour at 4,500’, I climbed up to 6,500’. At WOT, 2500rpm, 22.5 MP the CHT was about 325 and the EGT stabilized after leaning to 1475. My fuel burn for that leg averaged 10.2 gph. My TAS after burning off fuel was about 125mph.
    Do these numbers sound about right for what you guys are seeing?

  8. #8
    d.grimm's Avatar
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    Default Re: Fuel Burn and Mixture Control

    I had high fuel burns on my PA22/20 with a 150.
    Mixture was very sensitive. Had the carb overhauled
    and flow bench checked. Brought fuel burn down
    2 gph and made mixture very linear.
    A field installation of the one piece carb Venturi
    Was the culprit. Flow bench checking is a requirement
    in my book.
    One of the best things I did on that plane.
    Dave

  9. #9
    Gilbert Pierce's Avatar
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    Default Re: Fuel Burn and Mixture Control

    When you are at full throttle the enrichment valve opens to provide extra fuel for cooling. I always lean in a climb. If you do that when you level off and pull the power back you have to push the mixture in a little. As you come of the enrichment Valve when leaned you will be really lean if you donít richen up a little.

  10. #10
    proav's Avatar
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    Default Re: Fuel Burn and Mixture Control

    Thanks Dave.

    Thanks Gilbert, I'll be sure to look out for that next time. It seems that the fuel burn is a little on the high side to me and the EGT seems higher than I have seen in other carb engines that I have flown. There is very little travel on the mixture knob when I do lean it out, maybe 1/2" to 3/4" until peak EGT. I am wondering if I have an air leak somewhere or maybe if the carb may need to be overhauled. Engine was overhauled in 2007 (240 hours SMOH). I'm not sure if the carb was overhauled at that time or not.
    Bert Rodgers
    N20PP PA22/20-150

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