340 engine timing issue

First step, pull the #1 plug, and turn the engine by hand while you hold a finger over the plug hole. When you feel pressure on your finger, you are coming up to TDC on the compression stroke. Find the timing scale on the timing cover, and watch as the timing mark on the balancer comes around to reach "0". When they line up, take some brake cleaner and blow out the groove and the cover. I use white-out correction fluid to mark the slot to see it better. I agree, you have a timing issue. And you will have to tune the settings of the carb once the timing is set properly. I would start at 10° initial, then see what the carb is doing. You may not be on the idle speed screw if the choke is still "on". Let it warm up and see what she's doing.