I think your issue is the Throttle Linkage not ADJUSTED properly. If you give it lotsa gas, it should keep going. If not a lot, it'll shift. However, if it is not aggressive enough, you'll never have enough throttle pressure to keep it from shifting. I had this issue on mine. Granted, my throttle valve linkage was, like, not there at all, but that solved things quite a bit. Try this...Shift manually. If it accelerates as you expect (for an Eagle
), then you need to adjust your TV linkage. If not, the next place I would look would be your timing. There are people here who an time by vacuum or ear. I can't. Therefore, I bought a badass timing gun at Autozone, the digital one $100. READ THE INSTRUCTIONS. The digital light gives you a tach right there and allows you to manipulate the timing so you time to the 0 BTDC mark on the engine. I am the world's "biggest" novice, and I have had these same issues.
***When you put the MC21xx on, make sure you did the ECM Test (that is N*tter if the blocker is still up). Also, you DO have to retime after that because if you did the ECM Test properly, the ECU no longer works. That thing actually adjusted timing based on what was going on in the carb. My timing was WAYYYYY off after I did the MC upgrade. BTW, I like the MC21xx. My car runs a HELLUVA lot better in every way after the upgrade.
Good luck!