(John Clonts) wrote
My mechanic posited that the broken valve spring somehow facilitated
the clogging of the injector. I'm not sure I can see that...

That's because it makes no sense whatsoever. Your mechanic is
grasping at straws because he doesn't know what's going on either.

First off, while it was certainly necessary to replace your broken
spring, the truth is that the spring can be broken and still work for
a long time. The ONLY function of the spring is to keep the valve
closed and seated when the lifter isn't pushing on it, to keep gases
from leaking through and eroding the valve/seat. If your compression
is really 74/80, then the spring was doing the job. Otherwise, you
would see leakage past the intake valve.

The fuel injector is actually a nozzle - it sprays fuel onto the
intake valve. The spring never gets near it. Even if some flakes
came off the spring when it cracked, there's no way for them to get
into the fuel. What you're positing here is that a metal flake, small
enough to get into the injector, but big enough to clog it, somehow
came off the spring and bounced around and got in. Pretty far
fetched, IMO.

In all likelihood, the problems (clogged injector and broken spring)
are unrelated. I know how that sounds, but bear with me. The art of
troubleshooting is really all about Occam's Razor. We look for the
least hypothesis - what ONE point failure will explain all the
symptoms. A good troubleshooter who knows the system well will find
the problem quickly, because the list is usually pretty short. When
you stump a good troubleshooter, it's usually because there is no one
point failure. It's two or more. The planes we fly are so old and so
poorly designed, and the parts are so poorly made, that unfortunately
multiple failures are not too rare. Every time a problem stumped me,
that's what it turned out to be.

Also, it's kinda hard to resolve the "intermittent" nature of the
problem from flight to flight beginning a month or so ago, i.e. "When
did the valve spring break?", and "When did the injector clog-- and
was it suddenly, or gradually?".

I will be pondering these things during my next few flights!

Intermittent fuel injector clogging is the result of one of two things
- bad fuel or rust. Did you save the material that clogged it? You
should put it under the microscope and look at it. Metal is shiny,
rust is reddish, both are magnetic. Shiny metal would suggest that
the one in a million happened, and a piece of spring did get in there.
If this really happened, lunch is on me next time you make it to
Weiser.

Non-magnetic crud implicates fuel. Flush and clean the tanks, and
hope for the best.

Rust implicates the fuel servo. Check everything past the fuel filter
carefully. I had this problem before (and it lead to an inflight
failure) and tracked the problem to a corroding fuel filter plug. Be
careful - next time you may lose more than one cylinder.

Michael