DO NOT VENT THE VACUUM LIMITER TO ATMOSPHERE!!!!
The vacuum limiter (called decel valve here) is wide open during the decels, and you are sucking unfiltered air directly into the engine.
Here is some data with no decel valve. Notice that the manifold pressure is around 8 kPa:
Here is some data with decel valve in place. The manifold vacuum is now around 21 kPa:
With CIS injection, the limiter prevents the mixture from burning in the exhaust manifold/exhaust ports by leaning in out so that it can't combust and make the annoying popping sound.
By keeping the manifold pressure up during decels, the manifold takes less time to refill when throttle is re-applied, so response is slightly better.
There is also less oil sucked down the intake valve guides.
There is no mysterious emissions function