...................have you changed or inspected the plug?
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No, haven't changed the plug. In fact, I hadn't even considered it till you mentioned it, but should have. Fact is, glow plug trouble has been so rare for me that it just slipped my mind. Sure would be a cheap fix anyway. I currently have 16 O.S. engines in 7 different planes, and I can't remembr the last time I changed a glow plug! The first failure occured about 3 minutes into the flight and 3/4 throttle, and the second one ocurred within about 10 seconds after lift off at full throttle.
As far as pre-flight testing, I start each engine individually, perform the vertical test, shut it down and do the same for the other 3. I don't even put a tach on them, but listen. Then I start up all four, vertical test again to see if they sound OK, and fly. On subsequent flights, I usually just start all 4 and do a vertical test. I don't go so much for trying to get them all running together as much as having them all running at a setting each one is happy with. The plane has plenty of power as it is, and there is little point in trying to get the last ounce of power out of the engine by leaning it out.
Randy
P.S. I'll be offline for the next 6 days due to vacation. (isn't hot enough here at 90˚+ today, we're going to Vegas!!)
Randy