Well we got this one fixed . Customer called and said another customer had recommended us and could we put a set of injectors in his mixer truck . Truck showed up with driver and owner following in pickup said truck would start missing when warmed up and when they were pulling over the mountain it started smoking bad then cleared up . Later in the day we pulled in shop and hooked up ET and ran a check . no codes active or stored , went thru the cylinder cutouts , everything checked good . let it run for awhile at different Rpms while I was watching computer had man in seat hold throttle wide open , all good had hm let off , stab throttle while I was looking at readouts all the sudden a sputter and RPMs fell off about 400 and raw fuel smoke . Let come back to idle all returned to normal , eased back to full throttle , still good , came back to idle and stabbed back to wide open and the sputter , rpms dropping and fuel smoke came back . had him hold it there while I ran a cutout check and found it was really running off 3 and 4 and the other cylinders weren't really doing much , as it was quiting time we decided we would pull valve covers the next morning and have a look . puled covers next morning and found valve adjustments a little loose some more than others and injectors about 1/3 of a round looser than what we reset them at . cranked it back up and noticed right off that it sounded better , let it warm up and started working the throttle like before , the old gal was sounding good and everything checked on the computer , pulled it outside and let it run . 30 min. later went out and stabbed it and it started stutter and smoke , checked the computer again and surenuff only the 2 center cylinder were hitting . Checked the fuel pressure and it was a little high then checked after the filter and it was low , if you eased into the throttle it would stay low but wouldn't act up but if you stabbed it the instant it went below 20 psi you got stutter and smoke . Checked filter that customer had already changed and it looked good but tried another just to be sure . same thing . Puled filter housing and went thru it and found nothing but air wouldn't flow thru it good when reassembled then pulled priming pump back off and popped check valves out of priming pump and found a piece of rubber . Changed pump and seals and gaskets and reinstalled filter base . Cranked it up and checked pressure , all good , after letting it run a few hours and checking it ever so often looks like the problem is history . Raw fuel smoke was a low fuel pressure problem .