z24 not getting fuel?

I have an '89 cavalier z24 with a 2.8l multiport engine. My problem is when i push the throttle there is a gap before the engine takes its fuel. I
can hear the air being drawn but no rev for the first 1/2 second. once i open throttle and hold it's fine, if I let go and rev repeatedly the engine bogs slightly then revs on every attempt.
EGR valve works. I just put a new exhaust system and gave it an electrical tune up (except o2 sensor which I'm going to try tomorrow), but It still behaves the same. I would love to hear any ideas that would save me some headache. Thanks.

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