Suggestions for diagnosing lowered static rpm?

[humptybump]

For those keeping track ...


I flew this evening to warm up the engine. Airport elevation is 42'. The OAT was 88F and humidity was 65%. The altimeter was 29.79 and the DA was 2400'.

Static RPM was 2260 rather than the old 2300.


Two A&P's arrived this evening to "have s look".

Compression tests were done hot (130-140F): 80, 79, 80, 80 - yes, we triple checked. We usually do the compression check with the engine cold and in December and get numbers in the 74-78 range over the past several years.

Timing was right at 25 degrees.

Engine monitor Tach checked was with a strobe light at three RPMs: 1050:1050, 2000:2001, 2100:2098

The intakes are tight. The couplers and gaskets are new as of last condition inspection.

The borescope showed nearly pristine cylinder walls and piston heads. The spark plugs were clean.

The mags have 450hrs since last overhaul so they may be on the list at the condition inspection.

[aerhed]

Egap. Plus extra words to post.

[lr172]

There is not a lot that changes same condition power output

airflow/volume (In & out - int / exh)
air / fuel mixture
ignition timing
Compression (ratio & leakage)
Friction (doesn't usually change at a meaningful level)

You seem to have covered most of the bases here. If you are convinced that performance is down, you should check for a worn intake cam lobe (worn intake lobes cause more power reduction than exh, but wouldn't hurt to check both). If the valve is not fully opening, the air volume in that cylinder will be down and so will power output You might also see lower CHTs on that cylinder. You can pull the valve covers and put a dial indicator on the rockers (valve side of the rocker) and pull the prop through a full cycle to observe the total lift. They should all be within a few thousands of one another. THese point to different causes.

You should also check your exhaust for any blockage, as that can rob power in varying degrees. Birds and insects can build nest that may not fully burned or blown out.

Also try to compare your EGT Peak behvior (e.g. spreads) from '12 to '16. Meaningfull differences here can be clues to problems that are affecting individual cylnder power output. Remember, you have 4 little engines working in harmony. Your looking for clues on which cylinder is down on power or is it all of them down the same amount.
Larry