We did add the zinc additive into the oil prior to intial startup. We put a little fuel in the carb for immediate startup. Once started quickly brought up to around 2300 rpm for 25 minutes. After that we shut it down changed the oil once again and added the zinc again. We did the total timing and took it down the road. At that point the edelbrock was too lean. Changed the metering rods and moved the linkage on the accel pump and it was running great. He took it out on a couple rides probaly 150-200 miles in all. Then as we felt comfortable we began giving it wot. Thats when we discovered the backfiring