Suspected induction leak

Discussion in 'Maintenance Bay' started by Baron62, Aug 17, 2018.

  1. Baron62

    Baron62 Pre-takeoff checklist

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    My Piper Archer (O-360 Carb) has been having a hesitation problem at around 1200 to 1500 RPM when advancing the throttle so I suspect an induction leak. I also notice no slight RPM rise when changing the mixture to idle cut-off and the Idle Mixture adjustment does not produce an RPM rise. The engine runs fine otherwise with good static RPMs. I did an in-flight induction leak test and found that the number 1 and 2 cylinder EGTs had very little temperature drop when comparing WOT and 2000 RPM power settings. The number 3 and 4 cylinder EGTs both had a drop of about 150F temperature drop when comparing WOT and 2000 RPM power settings. I believe this indicates there is a leak on the number 1 and 2 cylinder intake paths.

    What is the simplest way to to ground test and find the leak(s) or rule out leaks? I am reading that air pressure from a clean vacuum cleaner on the carb heat port is a good way to pressurize the intake and then spray on soapy water to look for bubbles/leaks.

    Is there a common leak area associated with number 1 and 2 cylinders or possibly both intake tubes are leaking?
     
  2. bnt83

    bnt83 Final Approach

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    Plug the one end and run shop air or a vacuum cleaner (outlet) in the other. (Ends = intake & exaust) Spray areas with soapy water.
     
  3. Dan Thomas

    Dan Thomas Final Approach

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    An induction system leak big enough to cause trouble at those RPMs will cause rough idling or even a stopped engine at idle. More likely the idle mixture adjustment is set far too lean. You should be seeing a 25-50 RPM rise when you pull the mixture control out slowly when the throttle is set for 800 RPM or so.

    Lycomings sometimes blow their intake flange gaskets out at the cylinder flange. Turning the mags off and on again accidentally during the runup can do that. The big bang as raw air/fuel mix explodes in the muffler can send a shock wave back through the induction system if it happens during valve overlap in one cylinder. Overtightened flange bolts can result in poor gasket contact as well. Other places to check are the hose clamps and the carb gasket. Not much else to look at.

    That said, 90% of engine troubles involve ignition. All sorts of symptoms can be blamed on weak spark, including hesitation. Chacking the mags during runup doesn't necessarily catch all the propblems. When were the mags last off the engine for inspection? How many hours and years ago?
     
  4. Stewartb

    Stewartb Final Approach

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    The tubes aren’t connected. Two cylinders on opposite sides makes even less sense. If you had an induction leak I’d expect that cylinder to running lean and hot. Have you checked to make sure your carb isn’t coming loose? That’s not uncommon.
     
  5. Baron62

    Baron62 Pre-takeoff checklist

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    Thanks for the reply. Increasing the Idle mixture 1.5 turns out did not produce any RPM rise when setting the mixture to idle/cut-off. Mags are 250 hours since inspection and the run up drop is less than 100RPM. I also did an in-flight mag test and both L/R seemed to run fine. I think I need to pressurize the induction system and look for leaks. I did accidently turn off the mags a while ago so I will closely inspect the intake flange gaskets.
     
  6. Baron62

    Baron62 Pre-takeoff checklist

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    I tried to wiggle the carb the last time the cowling was off and did not notice looseness but maybe there is a gap in the gasket. The Mx said the carb was loose when I first bought the airplane 1.5 years ago and he tighened it up.
     
  7. Baron62

    Baron62 Pre-takeoff checklist

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    Shop vacuum pressure was applied to the intake air filter. Number four cylinder had leak as indicated by bubbles. All riser clamps and nuts were retightened and vacuum pressure was retested with no leaks found. Next is ground and flight test.
     
  8. Baron62

    Baron62 Pre-takeoff checklist

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    Good news! Looks like tighting up the intake tube junctions solved the problem. I tested the airplane and found that the hesitation is now 90% gone with very slight hesitation now only at 1100 to 1200 RPM which I think is normal for a Lycosaur. The engine feels smoother too. The carb Idle Mixture is now adjusted for a small RPM rise when going to idle/cut-off (it would not adjust correctly with the leak present). I'm glad that I didn't have to send the carb in for rebuild.
     
  9. Dan Thomas

    Dan Thomas Final Approach

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    That small hesitation might be caused by the accelerator pump pushrod being in the wrong hole in the throttle shaft lever. Move it outward one hole.
     

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