You need to give a better discription of when ahd how it backfires...
A constant front backfire indicates a burnt or open valve..
If you stopm pn it and it backfires it is a sign og a power valve not doing it's job.
My first guess is you do not know how to set the timing! Vac disconnected? At idle? What is it set at? What year is the 4.2?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

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