Changed Exhaust Now Way Too Lean, 1975 1.8 L Jet Options

[motorvated]

I had my 1.8 L Jet motor with Bursch exhaust running well with immediate start up, smooth idle, and strong running up to redline. Removed the Bursch and replaced it with a tuned header with megaphone exhaust and now it's so lean that it won't run below 2000 rpm, and won't idle. It starts on the key with no pedal but dies immediately. If I work the pedal when cranking, I can get it to run, but only at higher rpm. With the Bursch I had it set to run with AFR of around 10-12 at higher rpm, and around 15 at idle. Now it's 15-16 at high rpm and it hits the max reading of 19 on the gauge as it drops from 2000 rpm and stalls.

Ignition and timing have been set and verified, and I have once again been trying to make sure that I have no vacuum leaks. Bursch had a fitting for EGR, and I had that hooked up when it was running well, but new exhaust has no EGR fitting so I have blinded it off for no flow. Tried it open to atmosphere but that didn't help or make things any worse. AAR and Decel valve both still hooked up. Tried disconnecting and plugging vacuum lines to EGR and Decel valves with no change. I'd like to remove them but I'm unsure of how best to plug them off so I don't create any new vacuum leaks. Could use some advice there.

Before I go back into the AFM box to start re-adjusting things, I'd like to know if others have had similar problems when switching to a free flow performance exhaust. Should I have expected to have to re-adjust the AFM settings to richen the mixture because of the more free flowing exhaust, or do you think I have a new problem that I need to find and address first.

All comments/suggestions welcome.