In my last posting I noted how engine vibration was unacceptable above 2540 rpm. This was after I had the engine and prop dynamically balanced by a local balancing shop.
There had to be another cause for this vibration. The vibration first appeared in the Fall of 2004, when I was flying to the Rough River fly-in. The vibration had been getting steadily more noticeable, especially on longer flights at altitude.
As a result of my experience on returning from Hondo, I limited my flying to local flights with a rev limit of 2460 rpm until I resolved the issue.
I began by getting my A&P, John Hooker of J&A Aviation in Sherman, to inspect the engine as part of the Annual Condition Inspection process. We performed a compression test (all numbers in the 70-73 range), removed and inspected the spark plugs (all wearing normally with no signs of issues inside the cylinders), and John inspected the inside of the cylinders. We also checked the exhaust system and engine mounts for any evidence of deterioration or vibration.
John's conclusion was that there was nothing wrong mechanically with the engine. The cause of the vibration was therefore elsewhere, most likely in the ignition or fuel systems.
We then broadened the scope of the investigation. My engine has 1 electronic ignition system (the Electroair system originally sold by Jeff Rose) and a Slick 4370 magneto. When turning over the engine by hand, we heard 2 solid clicks and 1 weak click from the magneto per engine revolution. This was not correct. If the magneto is an impulse magneto, all of the clicks should be the same amplitude. If the magneto is not an impulse magneto, then there should be no sound when the engine is rotated by hand.
Whichever variety of magneto I have on the engine, there was something not right with its behaviour. The magneto had been on the engine since the plane was built (just over 430 Hobbs hours) and had not been rebuilt or overhauled for 10 years. John recommended a magneto overhaul.
I therefore pulled the Magneto and sent it to Select Aircraft Services in Lancaster, TX for overhaul. One thing I discovered from talking to Select is that Slick sets spare prices very high to discourage field overhauls. They really want you to simply purchase an exchange unit from them, for upwards of $375. Also, there is a brand of Slick magneto (the 4350) that was designed to be disposable. Literally, after 500 hours, you open the trash bin, drop the magneto into it, and fit a new one. Select's opinion of the quality of that magneto was barely repeatable in mixed company...
After 4 days my overhauled magneto with a yellow tag was ready for collection. The cost was a surprisingly reasonable $171. The work sheet revealed that internal gears in the magneto were loose. That probably explained the clicking sound when rotating the engine by hand.
I fitted the magneto back to the engine, replaced a defective spark plug lead to #3 cylinder (this was a saga in itself which I may discuss in future) and timed the magneto.
I then fired up the engine and took off for a test flight. For the first 10 minutes, I was convinced that I no longer had an engine in the back of the plane; it appeared to have been replaced by a watch. On the way back from a local airport, I was able to run the plane at full power at 7500 feet and 70 degrees OAT for 5+ minutes. The rpm settled at 2740. There was very little vibration, and only slightly more noise than at lower rpms.
The conclusion: the magneto had been deteriorating for some time - at least 1 year and probably longer. However, the deterioration of a magneto is a slow, subtle process, that you may not notice until the engine is really suffering. The issue will show up at elevated rpm and altitude, where the lack of an automated timing advance in the magneto, coupled with its poorer performance, will affect engine smoothness a lot more than at low altitudes or engine speeds. It is important to note that the engine was not actually misfiring; it simply ran roughly, and generated a significant level of vibration.