I would open up the cowlings and look over the engine very thoroughly for
any obvious problems (cylinder cracks, loose spark plugs, loose spark plug
wires etc.) then pull all of the plugs, make sure they are all in good
shape, clean and gap them and swap the bottom and top plugs if they all
appear serviceable. Be sure to torque to the correct specification when
reinstalling them (it's about 32 ft lbs for my Archer.) Also be sure to
clean the 'cigarettes' at the spark plug end of the spark plug wires with
something like MEK to avoid arcing around the spark plug. Then I would do a
quick run-up check and, if the problem persisted, ground the plane until a
mechanic had found and fixed the problem.

If the engine sounded fine I can't imagine that the engine RPM was actually
going up and down several hundred RPM. A bouncing RPM indication is usually
caused by a tach cable in need of replacement.

"att news group" wrote in message
...
I purchased a 1971 Cherokee 140 shortly after getting back into flying the
first of this year. It is a marvelous plane, relatively inexpensive to
fly
and to maintain.

The other night while flying with my instructor he asked me if I
typically
got this much vibration. I told him that it felt normal, however when I
looked at the GPS on the steering column it was vibrating so much I
couldn't
read it. All indicators were normal, the engine sounded fine, the one
exception was the RPM indicator was moving back and forth a couple of
hundred RPM from the 2500 I had it set at.

I haven't had a chance to fly again and my mechanic is on vacation until
tomorrow.

Any thoughts or ideas?

Charles Graef