Exhaust popping on deceleration (1.7 D-Jet), After changing MSP for a rebuilt one Options

[Jazzy]

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?

[Rand]

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.