I experienced the same tip in at cruz flat spot problem with my 340, I got rid of it.
Here's what it took to cure it.
Initial timing was to high causing it to idle to high this closed the throttle plate to far exposing to much of the transfer slot.
To much slot opening caused it to go lean on tip in.
I backed down the timing to 18* this lowed the idle speed down which allowed me to reposition the plates for less transfer slot and bring the idle speed back up to 850rpm.
Presto no more tip in flat spot.
In the end I had to re curve the distributor to get the total timing back to 34* all in.