I have a '73 1.7 with the stock D-Jetronic installed. The engine was lacking power, overall and especially around 3k rpm, so my mechanic and I checked everything from sparks to fuel. Started with the ignition and replaced the system with a new 1-2-3 ignition, coil, spark plugs and cables. Huge improvement, especially more torque at low rpm.

Next the D-Jetronic parts were checked and discovered that I had the wrong MPS and two 2.0 injectors. The injectors were changed with proper ones and all four tested. Fuel pressure was slightly off but everything else looked good.

When I got the car back from the shop I ordered a rebuilt MPS from Auto Atlanta and ordered this one: http://www.autoatlanta.com/detail.php?pn=B...E&user=Mike

With this MPS, my car drives much better and is definitely more powerful. Unfortunately the car now pops in the exhaust when decelerating. Replace the old MPS again, no more pops but also less power.

Any suggestions on next steps?

Fuel should be cut-off on decel, until down to about 1200 RPM, so perhaps the contacts on the throttle body are not making contact at throttle close position to send signal to brain, unless brain has got to sleep ??

Popping on decel is often a sign of running too lean, especially in your case where the swapped MPS makes a difference. I assume the AA MPS is tuned to spec, but was your old MPS not really holding a vacuum? If so, you were running rich (or at least richer) with the old one.

With the '73 you SHOULD have the idle adjustment knob on your ECU, right? I would turn it counter-clockwise and count the clicks until it's fully lean. Then go back clockwise plus an additional click to make the idle mixture richer. See if that helps. If not, keep turning the knob clockwise and try again.

If I'm mistaken and you don't have the adjustment knob, or if it didn't seem to help, check fuel pressure. Maybe bump it up slightly and see if that helps.

In general it sounds like you're fixing a lot of things that were wrong or mismatched, so keep checking for things like vacuum leaks and incorrect or unserviceable parts. Also make sure valves are properly adjusted. You can fix one thing in D-Jet only to find that it was masking another problem!

I had popping from the exhaust on decelleration when my heat exchangers were not secured tightly. AKA exhaust leak at the head/heat exchanger interface.

Yes, lean or exhaust leak. The old MPS is running rich, so it's masking the problem. I can't speak to the new one though, it's no piece of cake getting a replacement MPS right, so the new one could be wrong making it run lean.

Does the engine has the deaccelaration valve, if so remove it and plug the ports.
MPS may not be tuned for the engine displacement.

Say what? The decel valve protects the MPS diaphragm from premature failure, never remove it.