I had a ZEX wet kit on my Cobra. It was running lean for the first 1000 rpm of the hit. I had a window switch on it that kicked it on at 3000 and it would spike to a 16 AF for the initial hit and come back down during the first 1000 rpm. I called ZEX and they said they never heard of it. I bought another ZEX brain (box of selonoids) and it made no difference. I replaced it with a NX 150 wet shot and no more problem. Besides the ZEX is 125 at the engine and the NX is 150 at the wheels, big difference. I would recomend the NX on a 4V.
The other thing was it looked ok on a DynoJet, just a slight notice of a lean condition but when I put it on a loaded Mustang Dyno the spike showed up. Sometimes they just need to be tuned on a true street loaded condition. The only thing that saved me was that I always mixed 101 and 91 50/50 when I filled it up.
My buddy put the ZEX on his 87 5.0 and it works fine with no lean condition. Go figure.