15th September 2012

Two hops, one hose, zero change.

Hours: 3.0 | Posted in Flight Test

Off to the hangar this afternoon, armed with parts: a new nozzle line fitting to replace the leaky scratched one, and a 90° -4AN fitting to enable the installation of a shorter servo-to-spider fuel line.  A few hours later, everything was installed and secured.  A test run showed no leaks, so I put the cowling on and went to fly, a quick local flight to get things up to temp and test for changes.  Land, inspect, refuel, and off to fly some more…headed up to Menomonie and back, with a detour to follow a river valley and then up Lake Pepin.

On the up side: the nozzle line fuel leak is, as expected, gone.  Also, the main seal that was weeping a bit of oil seems to have sealed up; there’s no more oil being slung off the ring gear.  And, the EGT drop on #2 seems to have been taken care of by the test & rebuild of the fuel spider.  All 4 cylinders stay together now when pulled to idle.

On the down side, the thing still stumbles as much as it ever did; both subjectively and as shown in the data logs.  I pulled out 40 inches of -4 hose between the servo and spider, and replaced it with 29 inches, which is really about as short as it can be.  The line is routed as far as possible away from exhaust pipes.  I find the hottest place in the line to be the area where it rises up to the purge valve, between the #1 and #3 cylinders.  I was really hoping that the line change would produce some result, even if only a marginal improvement…that’d tell me that we’re on the right track.  The vapor explanation makes sense, given the symptoms and the data logs, but the FI folks seem to think we’re at the end of the road with this, that the hose was going to improve it, and there’s not much more to be done.

A thread popped up on VAF last week that dealt with this same problem, and it’s not unheard of.  Some people seem to accept it and move on.  However, feeling the engine stumble during descent to land and final approach is not confidence-inspiring, and I don’t know that it would cause happy feelings in passengers, either.  Not good, as far as I’m concerned.  Surely there is something that can be done…plenty of folks are running FI without these problems, and it’s not that hot outside right now.

So.  No progress on the biggest issue.  Perfect…I’ll start calling again on Monday and see what “they” want me to do next.  It’ll be two weekends out before I can get back to the problem, since next weekend is stuffed full at work.  (This does make it hard to work with…by the time I’ve been able to test something out on a weekend and report back, the technician has moved on to other problems and I have to reacquaint them with the entire story.)

I also did a ground run with the MP lines disconnected from the Pmags, to get ahead of the question train on ignition problems.  No change.  And, while it was stumbling during the ground run, I alternated ignitions on/off to see if one was different than the other, or causing the problem.  It ran equally lousy on either ignition.  Anything 1200 RPM or above still runs fine.

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