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Question:  Why would an O-290-D2 installed in a Lancair 235 stumble at
cruise?  Full power operation is fine, but at 2400-2600 it stutters
(shudders, stumbles etc., and sometimes backfires). At 2700 on up it runs
like a scalded dog.

The engine has 5 hours on it - Just back from a full overhaul, including new
studs, bearings and so on.  The carb was also gone through, and the engine
was test cell run for break-in.

It runs fine on the ground, but the mag check is about 150 rpm drop - In the
test cell it dropped about 100.

Now before I get all of the "Something's wrong with the carb - ignition" and
so on... consider this...

The engine had 45 hours on it when I tore it down (VERY long story) and
exhibited the SAME characteristics BEFORE it was rebuilt.

In an attempt to cure the problem, the mags were replaced (brand new Slick
mags), the carb was gone through and everything else was checked.  The one
thing I am left with is the induction from the cowling.

Does anyone else have any ideas as to the cause of this?