For your hesitation and backfire, look for bad hoses, particularly idle air supply. These get hard and brittle with age, developing cracks and sealing poorly at fittings. Replace them all if in doubt. Note that any air leaks here will allow unfiltered air to bypass the airflow meter, causing a lean mixture. This will be only noticeable at idle and off-idle acceleration.

If your hoses are relatively new, then the airflow potentiometer regulates acceleration enrichment and typically develops a wear-thru spot at idle, with age. This usually also causes erratic idle.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7