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Thread: Variable Fuel Flow and Power Loss on Startup

  1. #1
    Join Date
    Apr 2019
    Location
    Macomb Township, MI
    Posts
    7

    Default Variable Fuel Flow and Power Loss on Startup

    Aircraft: PA-22 135
    Engine: O-290-D2 (135 hp), 1600 hours
    Avionics: JPI FS-450 Fuel Flow
    Fuel: 87 (ethanol free), 100LL

    Symptoms / Problem Descriptions:
    • Flight 1
      • Fuel flow on FS-450 suddenly began fluctuating by about 3gph during cruise flight (normal at that RPM would be 6gph, FS-450 read 5-8gph, changing every second)
      • Flew for 1 hour back to home base in light turbulence, ran up on the ground in front of hangar
      • Pushed back into hangar, some fuel poured out from bottom of cowl, likely from carb air box

    • Flight 2
      • Fuel flow flux continued, albeit by more like 1gph instead of 3gph, less turbulence
      • Flew for 1.5 hours, ran up in front of hangar, no fuel came out when pushing back this time

    • Flight 3
      • Engine started, went up to my throttle setting of 1000 RPM, then lost power down to idle for a second, then came back up
      • Wasn't too much prime, only 2 shots (normal) and RPMs didn't spike, just dropped
      • Fuel flow continued to fluctuate, but again only by about 1gph

    • Flight 4
      • Same exact problems as on Flight 3, power loss on startup, fluctuating fuel flow


    Summary of Problems:
    • FS-450 fuel flow meter fluctuating wildly
    • Power loss right after startup, then comes back within a second
    • Fuel poured out from under cowl (carb air box most likely) after 1st of 4 flights (flight where fuel flow flux was greatest)
    • Engine does not feel rough, maybe a bit rich, but not rough


    I have discussed the issue with a couple of mechanics who were unable to recreate the problem on the ground. Here is what was discussed:
    • Stuck carb float? - maybe... seems like it would be intermittently stuck, though, for the flow to fluctuate, and I have NOT been turning off the fuel selector, no fuel running out (except for that one time after a flight)
    • Bad fuel? - I've used 87 ethanol free and 100LL on these flights, and that doesn't explain the fuel flow flux, right?


    I'm stumped, but there is definitely a problem. Maybe the 2 issues (fuel flow and power loss) are unrelated? Seems like they should be related.

    I should mention that my battery might be aging, not sure how old it is, but I notice slow turnover on startup sometimes. Never had an issue actually starting, though.

    What do you Short Wing Piper experts think?? I'm ready to answer any questions you might have to help me solve the problem!



    Jeff Muehl

  2. #2
    Join Date
    Nov 2006
    Location
    Mason, Ohio
    Posts
    370

    Default

    Jeff,

    I would check the screws that hold the two halves of the carb together and make sure they are tight and safety wired. Check the carb attachment to the oil sump while you are at it. I would also check and make sure all the intake tubes and tube connectors are tight and can not leak. Beyond that, I agree that it could be the float sticking. When was the last time the carb was rebuilt? Also, do you check your sumps before each flight? That is a must do to prevent water from getting to the carb. If it happens again check to see if the carb heat or mixture have any affect on the fuel flow. This is the time of year when icing can occur, although it's not common on that engine.
    Relative to your battery, the new glass mat batteries typically last 3 to 4 years, although some have gone much longer. Not sure why. A trickle charger will help in the short run.

    Let us know what you find.

    Tom Anderson

  3. #3
    Join Date
    Apr 2019
    Location
    Macomb Township, MI
    Posts
    7

    Default

    Hey Tom,
    So I have flown several more times since the issue popped up, but have had no further problems:

    • Fuel flow has been steady, no fluctuations
    • Engine continues to run fine, no roughness
    • Checked carb, nothing seemed out of place, everything seemed tight, no leaks
    • Carb head operated normally, nothing out of the ordinary
    • Mixture control is normal, again nothing weird here
    • I always sump before flight, haven't had water in there. I run auto fuel (STC), however always smell it (since it's mostly clear), no issues


    I am guessing this was an issue internal in the carb: stuck float, needle seating issue, etc. I'm no expert here, so I don't really have any other ideas. I had mechanics look at it, they found nothing and also couldn't recreate the issue either! I'm going to just keep an eye / ear on it and see if I find anything else.

    Regarding the battery, maybe I'll consider a trickle charger. I fly often (a couple times per week generally), not sure if that changes anything regarding a trickle charger. I was wondering if anyone knows of a lightweight battery option? Always looking to shed weight... haha!

    Thanks for your help, Tom, I really appreciate it!

  4. #4
    Join Date
    Nov 2006
    Location
    Mason, Ohio
    Posts
    370

    Default

    Jeff,

    I installed the light weight battery using the Svenn's Aviation STC https://svennsaviation.com/product/s...-installation/. This saves about 15 lbs. in battery weight versus a Gill or Concord. In the short run the total is a little more than a new Concord initially, but a new Flight Safe battery is much cheaper and they last as long if not longer than either Gill or Concord. Make sure you use copper cables if you do this installation. Also, I only did the battery conversion. I did not put in the circuit breaker switch.

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