Well, This is Odd (Engine Tuning Issue?), Changed batteries, and something strange happened. Options

[JDW914]

I'm not sure what's going on here, but I'm hoping somebody here will.

I've been driving my car ('74 2.0 all original engine and FI) for a few years now, I did some tuning last summer, and it's been running great since. But, having it out only once a week at most, this winter the battery had started to show signs of wearing out - the car was getting hard to start and often needed charging - so last week I bought a new battery and installed it.

Initially, this seemed to work great: I turned the key, the car started immediately, and it idled perfectly at 1200rpm. However, as soon as I touched the gas, the engine revved to 3000 and stayed there for about 20 seconds. Upon experimentation, it seemed that the engine would retain both power and speed of any throttle position for a ~20s period after the throttle was returned to idle. I was able to alleviate this to some extent by turning the idle-mixture adjustment to maximum and opening the idle bypass by 5 turns, but it still acts weird.

When I set the idle mixture to do anything besides flood the rpm down, the engine stays at 1500 rpm even with the idle bypass closed all the way and the AAV closed off.

My instinct is that it's a vacuum leak, but there's not a whistle to be hard or lose hose to be seen. Could it be (I hope not) something to do with the FI system?