2nd September 2012

Step forward, step back

Hours: 3.0 | Posted in Flight Test

Headed into the Sunday-Monday “weekend” with high hopes…  one afternoon later, we’re no farther ahead than we were a week ago.

Reinstalled the fuel spider and did a ground run to check operations, then replaced the cowling and went for a test hop.  Something must have been flushed out during all the testing and inspecting, because the EGTs on all four cylinders tracked pretty closely together during the flight.  The hot, low-RPM stumble, however, is just as bad as ever.

Airflow Performance is of the opinion that the stumble is being caused by fuel vaporizing in the lines between the fuel servo and the injectors, most likely in the line from the servo to the purge valve.  They recommended shortening that line as much as possible, and removing the FF transducer for testing purposes.  So, I dug out the original hose that was sent with the engine, and set to work.  It turns out that hose is 2 or  inches too short to run from the servo to the purge valve…it measures 30″ end-to-end, and 32 or 33 would work nicely.

So, having failed on the “short as possible” avenue, I reinstalled the lines I had (which total 40″, split partway through to accommodate the FF transducer), but with the FF removed and a steel union, wrapped with ceramic felt & silicone tape, in its place.  A second test hop revealed that the stumble is still there in this configuration, so the removal of the FF did not help.  The stumble may have been reduced in severity — or it may have just been my wishful perception.  In either case, it is still present.  I suppose the final test would be to procure a shorter hose of the appropriate length to install and test.  The hottest-to-touch part of the hose is the vertical run where it heads up between the cylinders; there’s nothing else to be done there, though.

And, to top it all off, I found that I now have a fuel leak at the spider.  It appears that the nipple line fitting out of the spider for the #3 cyl somehow got a scratch across the seating face.  So I’ll have to order a new one of those, too.

Essentially, that means we’re back on the ground for another week and a half — at least — while I wait for more parts to arrive, family obligations requiring out-of-town travel for a weekend to pass, and evening work commitments to pause.

I’d expected to be at least half done with the test flight hours by now, given that I had a couple weekends with nothing else to do; as it is, I haven’t been out of sight of the airport, and the only flying I’ve done has been to test changes and modifications which have proven fruitless.  There were some disheartening, throw-it-all-away moments during the build; the feeling I have after all this testing (that has only created more problems, solving none) is that, times three.

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