470 low speed question

[Lamont270]

I checked the choke that day and the flap was wide open and I looked for excess gas and it seemed ok. Also the flame arrestor is clean. Is there a low speed jet that could be out of whack? It is the Mercruiser carb. What brand of plug is recommended, I have Autolite plugs in it now and was thinking about switching to NGK. I will look into the Petronix kit I was going to put one in this winter when I had the engine out but ran out of spare cash after the other repairs.