[QUOTE=Silver Flight;11300]Mine was around 900 rpm for a couple of years, after which time the transmission failed and the mechanic told me it was because my rpm was too high. Mine won't be over 750 again.[/QUOTE]

Transmission failures have many different reasons, and not all of them are related to engine rpms. If you can live with the shake rattle and roll of 750, good for you! At that rpm my M25 simply stops. The Hurth transmission manual has something to say about the range of acceptable rpms. IIRC, it's up to 1200.

Quote from Hurth manual:

"The HBW permits direct reversing at full engine speed, for example in emergency cases."

So I am thinking that having the idle at 900 rpms is not going to cause transmission failure. Have to agree with Stu, at 750 rpm the idle is so rough it's causing more trouble than its preventing, especially with a standard spring type flex plate. But once it hits 900-1000 rpms it quiets right down. That is our experience with both the M3-20 and the M35A (3 and 4 cylinder respectively). Mechanics say the damndest things at times. This thread sort of morphed didn't it?

I agree with Bud and Stu that an idle near 1000 rpms could not likely be the cause of transmission failure unless there was already something else very wrong.

I calibrated our tach and keep the idle about 900 rpm and that works just fine to reduce the rattle and vibration.

I idle well at about 800-850. Rough idling can cause excessive vibrations that do more harm than good, example being breaking brackets that hold the HE to the engine. More than anything you should check the engine alignment and the prop for correct pitch and balance. Also worth checking is the shaft alignment as Catalina uses Bronze and not SS for shafts.
John Meyer
La Paz & Cruising Sea of Cortez