Thread: Pawnee problems
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Old April 29th 10, 07:12 PM posted to rec.aviation.soaring
BDS
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Default Pawnee problems


"bildan" wrote

Going back to 100LL probably contributed to the problem. Lead
deposits build up on the valve stems and inside guides causing them to
stick. Unleaded fuels are one reason cars last so long these days.

First check your engine for intake leaks (and for exhaust leaks while
you're at it) The rubber hose sections on the intake runners are
notorious for developing leaks. One way is to pressurize the intake
system with the blow side of a very clean shop vacuum (new is good)
and spray the outside of the engine with a soap solution looking for
bubbles.

If the intake system is tight, your suspicions on sticking valves may
be correct. You'll need to read up on the infamous "Lycoming rope
trick" which reads in part: "Insert about 8 feet of 3/8 inch nylon
rope through the spark plug hole; then turn the crankshaft until the
piston moves the rope snugly against the exhaust valve."

See:
http://www.lycoming.com/support/publ...fs/SI1425a.pdf

This procedure allows the valves to be removed from their guides and
cleaned without removing the cylinders from the engine. You should
also clean the valve guides while you're at it. Be prepared to find
bad things that lead to cylinder replacement.

In the event this doesn't solve the problem, I'd look at the
possibility a magneto has gone intermittent. Mags have a service life
of about 500 hours but almost nobody replaces them until they fail -
and they can fail in some weird ways. In my airplane days, I replaced
mags every 500 hours with a new ones - it was my butt in the left seat.

******************

We did check the intake tubes and couplers and tighten up the clamps but we
haven't done anything like the test with the vacuum cleaner - that sounds
like a good idea.

I recall reading something about the Lycoming rope trick last year when we
had the stuck valve on the one cylinder. I am thinking that if we had one
stick, it is reasonable to assume that we likely have problems with others.

Bruce