i have a 93 w250 6bt that im working on and a 4bt that does the same thing and it has a weird drop of rpms when you let off the throttle. When it gets to idel after you let of it acts like it misses for just a second. Then rpms go back up and runs regular. i have no problems any where else, no fuel leaks. Its like the pump cuts out for a second after you let out of it right before you shift or when you just revving it. What could cause this pump pressure, fuel pressure, idk. ill post a video later.
thanks