Recently I've been having problems starting my engine. I have a 6 with a new lycoming O-360 A1A. Carb, standard mags and a wood prop. It always started fine. I've got 300 hours on the plane now and I seem to be having trouble starting when cold. I have a vans primer, solinoid and primer lines to three cylinders. I used to prime for a few seconds and she would always start fine. Now it pops, backfires tries to start. Sometimes I think it's just me doing something wrong, priming too much or not enough but it seems to be getting worse. The plugs I changed at about 100 hours. it was running ruff. I haven't touched them since. once running it runs perfect, smooth, maybe 20 or 30 rpm drop on mag check each side. Could it need new plugs or a plug cleaning? I was also thinking maybe the timing is getting out on the mags, I haven't touched them either. Any ideas?