4.0 head swap

[mike0914nascar]

I put the 4.0 head on the 258.I'm using HEI and a motor craft 1.21.I have to advance the timing to far for hesitation or bog to go away when getting on the gas.I had the same problem with the carb with the 258 head.Seems over carbed.I read 400 CFM was good with this swap.I used a 1.08 motorcraft with old set up cured the problem but with the head swap 300 Cfm seems a little light.What are you people using out there.Thanks!!

[stroked'73]

I know this post is pretty old, but i'll give my opinion for anyone reading...
I run a q-jet, that's about 178 cfm on the primaries and it's good. I ran the mc2100, supposedly a 1.08, before the q-jet and it was good, but I also struggled with an off-idle hesitation. Never figured out if it was carb related. I think when optimum cfm's are given for a setup, they're really talking more upper end - that's where you'd need the flow. For cruising around, not much flow is needed. That may be why the 1.21 isn't great off-idle but the 1.08 is. At the upper end, the 1.08 might run out of steam while the richer carb would be able to keep up. It's a trade off.
Just my 2 pennies.