I wouldn't even attempt to candle them before day 5, and preferably day 7. The embryo is very fragile the first few days, and easily killed by jarring it around the slightest bit. Just my 2c...Hmm... Just Candled all 14 eggs (Day 3) And don't see any veins in any of them. Some have definently grown though... Maybe my candler is just old and I can't see them? Or at least I hope..
It's not so much how much the temps fluctuate, but the length of time it stays at one extreme or the other. The core temp of the egg changes much slower than the air temp in the incubator.
My incubator showed as much as 3 degrees between high and low temps during a 24 hour period, and I had a great hatch. I think it'll do better after it's been used for awhile. This is my second batch in mine, and it hasn't changed over .6F in two days. If it goes high and stays high, or goes low and stays low... then you have a problem. Try not to worry too much about it. Time will tell if it did good or bad.
