28th August 2012

Analysis, no diagnosis

Posted in Flight Test

The fuel spider/purge valve assembly arrived at Airflow Performance this morning, and by early afternoon I had a phone call with the results.  They flow tested it and found it met the specifications.  To be thorough, they disassembled it for inspection, checked over all the internal parts, finding nothing wrong, cleaned & reassembled it.  A second flow test also showed everything to be within new limits, so it’s being sent back to me, hopefully to arrive before the weekend.

Talking with the Airflow folks for a good half-hour (plus the 20 minutes last week), they feel that the colder EGT is of no consequence, and as long as the engine is running smoothly, can be safely disregarded.  We seem to have tied it to fuel flow though, whether due to the spider port or the injector line.

Setting aside that problem for a moment; it is their belief that the hot, low-RPM stumble is related to fuel vaporizing in the line between servo and spider, due to heat absorption and the fact that the fuel in that line at idle is very low volume (<1gph) and pressure (essentially 0 psi, since the flow is so low).  We verified that I have the correct (.022 for the IO-320) restrictors installed.  He said that they get regular calls in the summer months with similar symptoms, and the call volume dries up in the colder months of the year.  Some suggestions were offered for remounting the fuel flow transducer and shortening the servo->spider fuel line as much as possible to minimize heat absorption.

So, the current test plan…

  • reinstall the purge/spider assembly
  • ground run to verify proper operation
  • flight test, observe EGT’s and hot stumble
  • bypass FF transducer with union & firesleeve (or a different hose), see if hot stumble remains and/or improves
  • shorten FWF fuel lines as much as possible/remount transducer in cooler loc’n if testing shows benefit

We’ll see if the forecast holds for Labor Day, which currently is expected to be the crummiest weather of the weekend — guaranteed, since that’s my day off this week…

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  1. 1 On August 29th, 2012, Bob Collins said:

    Joshua, how close does the line from the servo come to the exhaust stack on the way to the spider? I’ve got a couple of those steel heat shields that Van’s sells and you’re welcome to try them out and see if they change anything.