I recently replaced the mass air sensor on my '77 2.0 FI Westy. The previous sensor had a back pressure valve built into the butterfly valve. The back pressure valve broke off and stuck in the throttle body. After figuring out that problem, I replaced the sensor with a rebuilt one from Autozone. The butterfly valve doesn't have the back pressure valve, but everything else is the same. After replacing the sensor, the engine will turn over, idle (usually for less than a minute), then sputter and die. After it dies, I can hear a sucking sound coming from the engine, but I can't pin it down. I've double-checked all of the vacuum lines to make sure everything is hooked up properly. Today I tried again and it actually idled for about three minutes. When touched the gas to try to put it in gear, it immediately died. Any thoughts will be greatly appreciated.