Swapping muffler
 

Since I went to my new muffler it seems I have a flat spot above 4k. Can I retune my carbs to compensate for the new muffler or do I need to move on to somthing else.

You will be able to advance your ignition curve too. Depending on your compression ratio and other factors....

You may need your distributor recurved (exreme) or simply advance it (start with 2 degrees). Listen for pinging. It it pings, retard it a little.

Keep in mind, even if you don't hear the pinging you may still be too advanced (detonation). My Butt Dyno works pretty good in this regard :D

Have Fun!