Engine doesn't make minimum static rpm

Aircoupe Alon A2 with C90 engine with two props, one 48" pitch and the
other 52". It doesn't make the static runup RPM minimums with either
one, for instance the 48" prop should be 2225 to 2325 rpm, I averaged
out to 2162 rpm. Likewise the 52" prop falls short 100 rpm or more.

Three years ago when I checked the tachometer, it read a little
slow--maybe 10-20 rpm at speed, that is it would indicate 2300 and the
electronically measured rpm would be 2310 or so. Assuming that's
still true, what would cause the engine to not make minimum rpm?
Where should I start troubleshooting?

I would start with the Tach.

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE


Where should I start troubleshooting?

tony roberts wrote:

I would start with the Tach.

Good suggestion. There is a program available for a Palm Pilot that
indicates RPM using its built in microphone.

In rec.aviation.owning Bob Fry wrote:
: Three years ago when I checked the tachometer, it read a little
: slow--maybe 10-20 rpm at speed, that is it would indicate 2300 and the
: electronically measured rpm would be 2310 or so. Assuming that's
: still true, what would cause the engine to not make minimum rpm?
: Where should I start troubleshooting?

In rough order of liklihood
- Tach
- Mixture (Here at 2100 MSL, often too high DA to get full RPM in the summer)
- Timing
- Air filter
- Muffler blockage
- Compression
- Ignition (plugs)
- Ignition (others... unlikely: mag internal timing (egap), wires)

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Borrow a optical tach from a A&P.
Dynamically balance the prop(s)

Cory hit upon a good idea ie muffler blockage. You could have a blown baffle
or collapsed tube in your muffler.

Jim

I just went thru this with my Beech Sundowner. It was not making
minimum static rpm and would not go over 2500 at 7500 ft leaned.
Everything checked out, then pulled the prop and had the prop shop
check it and it failed. Not enough metal to overhaul it and it
surprised me cause it looked good.

Put a new prop on it and now getting 125 rpm more at static.


Bob Fry wrote:
Aircoupe Alon A2 with C90 engine with two props, one 48" pitch and the
other 52". It doesn't make the static runup RPM minimums with either
one, for instance the 48" prop should be 2225 to 2325 rpm, I averaged
out to 2162 rpm. Likewise the 52" prop falls short 100 rpm or more.

Three years ago when I checked the tachometer, it read a little
slow--maybe 10-20 rpm at speed, that is it would indicate 2300 and the
electronically measured rpm would be 2310 or so. Assuming that's
still true, what would cause the engine to not make minimum rpm?
Where should I start troubleshooting?