: The dry tappet clearance might be part of the problem.
: The clearances can increase as the cylinder lengthens with heat, and a
: few valves might not be opening all the way. But it should be a
: consistent lack, not 1 out of 25 takeoffs.

I may not have been completely clear in my zeal to describe my frustration... There are two
power issues we're trying to resolve. There's a chronic loss of power (never made more than the very
bottom of the static RPM). The occasional loss is so subtle, infrequent, and irreproduceable, that's
it's very difficult to track down. We thought maybe gunk in one of the lifters that occasionally
prevented pumping up which is why we decided to look. I'm now wondering if the 1-in-25 thing is the
lifter spring snapping out of its little shoulder due to the excessive clearance. Then it might
occasionally snap back in.

: I would suspect that carb icing might be your problem.
: It's way more common than many folks think. What are the atmospheric
: conditions when this happens? Look for small temperature/dewpoint
: spreads. It doesn't have to be cold outside; ice can appear at temps
: up to 100?F. Lycomings are ice-resistant because their carbs are
: mounted on the oil sump, but if the oil isn't really hot yet, or if
: the temps are cooler and the dewpoint isn't far away, ice will form.
: Sometimes even if the oil's hot, and even in the full-throttle climb.
: Does the engine stumble once in a while in the climb? Probably ice. We
: see ice all the time here, even in a dry climate and on Lycomings and
: on warm days. Got to know what to watch for.
: Check the carb heat immediately before takeoff, at around
: 12 or 1400 RPM. Should get a drop that stays dropped; any rise after
: that with carb heat on, or excessive roughness before it rises,
: indicates ice. There have been accidents where engines didn't generate
: enough power to sustain a climb, and the cause was pegged as ice that
: formed during the taxi, after the runup.

I've gotten carb ice a few times... even in fairly warm temps. I'm skeptical that this is the
cause because it doesn't seem to be temperature related. It's done it on nice cold (35) dry, winter
days, as well as hazy hot (95) summer days. Also, I've got a manifold pressure gauge on this plane even
though it's fixed pitch. Manifold pressure indicates correctly when it's acting up... if it were carb
ice it'd indicate less MP.

I'm glad to hear someone agreeing that chronic loss might be due to valve clearance.

-Cory


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* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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