A few weeks ago while searching for a possible cause for my slightly elevated CHTs, I decided to re-check the engine timing. This was at about Tach: 91.6, Hobbs: 102.2. The timing had drifted from 25 BTC to 27 BTC on one mag (didn’t record which) and 25 to 29 BTC on the other mag. Resetting this to 25 & 25 helped cool the CHTs on subsequent flights, but the engine became more difficult to start when cold (maybe unrelated). Since removing & gapping the lower plugs to 0.016 (earlier entry) seemed to help with the subsequent cold start (may be my imagination), I also removed and gapped the upper plugs (earlier entry) to 0.016. Subsequent starts did not seem to improve in difficulty. I’ve tried various combinations of priming durations and no longer suspect it is a rich/lean mixture thing, rather a weak spark on left (starting) impulse coupled magneto.
Yesterday, a very experienced aviation maintenance technician stopped by (Nabil) for over an hour to help me trouble shoot. He wanted to confirm several things before agreeing with me that the mag spark was weak. First we removed top 4 plugs and re-checked the timing. He agreed it was correctly timed at 25 BTC on both mags. He was also suspicious of my left&right independent DPDT mag switches (since I used the 2nd poles to route my starter relay wire to prevent starter engagement with any combo other than left mag on, right mag off). He wanted to verify that the mag timing box lights didn’t change when pressing the starter switch (they shouldn’t and didn’t). Since the prop cranks with gusto when starting, and coughs several times before catching, he finally agreed with my diagnosis that it could be a weak spark from the left mag. Still seems strange since after start, all running is beautiful and mag drop checks are OK at 1800 RPM with engine slightly leaned from rich (140L drop/150R drop). I understand 175 max drop and 50 differential is the Lycoming limit.
He offered several ideas: Buy new left impulse coupled mag…he estimated $900. Send left mag out for inspection and overhaul…he estimated $300-600. I said what about buy new P-Mag EI for left mag replacement (more expensive and reputed to be much less dinosaur technology with much hotter spark and easier starting). His opinion was that was “too advanced technology” that didn’t belong in this style plane/engine…I let that one go by. He also was a fan of eventually replacing the right mag with an impulse coupled mag also since that would allow me to wire for starting with both mag switches on. I was very grateful for his help and learned a bunch. He wasn’t charging, but I figure I will buy him a six pack.