Bela P. Havasreti wrote in message ...
On Fri, 12 Nov 2004 07:43:34 -0500, Ron Rosenfeld
wrote:

On Fri, 12 Nov 2004 03:16:35 GMT, (John_F) wrote:

A spark plug that is broken internally will cause this at high power
settings. Find a spark plug tester that uses air pressure and fires
the plugs under pressure to test the plugs.

I've had bad plugs that have passed that test. I think a problem is that
the pressure in those testers is only a fraction of the pressure that
exists in the cylinder. But it is a place to start.

The problem was rough running. The affected cylinder and plug was
determined by a multi-probe EGT and running on single-mag. The plug passed
the 'test' in the box. But the engine ran properly after that plug was
replaced.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Ditto. Those air pressurized testers/cleaners can't begin to
adequately emulate the actual pressure/heat a sparkplug
is subject to while the engine is making 70% (or whatever)
power at cruise.

I had a plug that would go off line intermittently, but with a 6-cyl
Continental, all I noticed was a slight drop in RPM @ cruise.

Bela P. Havasreti

Typical compression pressures are what the plug has to deal with,
though at elevated temps, as you say. An 8:1 compression ratio will
produce 120 psi or so at TDC, and that's where we test our plugs and
often find them failing at that point. Even if it's only one plug in a
cylinder that's missing, roughness will show in many engines. If the
engine data specifies mags timed at two different angles, the
later-timed plug may not fire because the cylinder pressure is already
higher, with combustion underway.

Dan