The decel valve is also called the vacuum limiter. It's job is to limit or reduce the high vacuum that is caused by a closed throttle at high RPM.
Here's my theory for what it's worth. If you have disconnected the decel valve, then your WUR is seeing the very high vacuum levels caused by deceleration. This causes the WUR to lean out your mixture way too much, which can cause intake backfiring. A possible way to prove or disprove this would be to disconnect the vacuum line to the WUR and see if your condition improves. If the popping does go away, then fix the decel valve (don't just leave the WUR disconnected).
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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY