IO-360-L2A - 2350 Max RPM in the Air with Vibration
I've been troubleshooting this engine for months now, and I'm out of ideas. Each time this aircraft gets written up I've done the basics, compressions, plugs, etc.
First time I found No. 4 exhaust valve sticking, zero compression. Replaced cylinder.
Second time, found the mags were at 1100+ hours with no 500 hr inspection. Put two different fully inspected mags on.
Third time found No. 2 exhaust valve sticking, zero compression. Replaced cylinder.
During this time, I had to keep leaning the servo back in order to get the appropriate idle/mixture settings. It kept creeping up. Finally I started noticing blue stain on the ground under the airbox drain line. I determined the servo to be going bad and not shutting fuel fully off, replaced servo. I also ran through a thorough fuel system check per the Precision Airmotive troubleshooting guide. No defects noted.
Finally, everything is checking fine, compressions are holding in the high 70s, idle mixture is stable, mag drops are fine after a clearing run, ground run is 2350 static rpm, but in the air pilots can't get more that 2350 RPM and are getting a vibrating tach, a noticeable vibration in the engine through the 1800 - 2100 rpm range you can feel and hear, and the fuel flow needle is bouncing.
So this week, grasping at straws, I check my valve clearances after shut down. I get anywhere between 0.010" - 0.050" and most are a little spongy. I replaced all 8 tappet plungers, checked dry plunger clearance, 3 were in the high 0.070" range and 3 in the 0.080" range, 2 around 0.40". Replaced the 6 above 0.070". I'm a little annoyed feeling that I've gotten a little gipped by my overhaul guy, but happy that I've probably figured it out.
Test flight revealed no change in symptoms.
Beyond pulling the engine, or putting new mags on, I'm out of ideas. I do have a optical tach I'm going to use to verify RPM, but the vibration makes me believe that it's probably not going to reveal anything.