That will be the MAZDA M4-121G engine and an IMPCO fuel system....Model J Vaporizor and CA100 fuel mixer(carb).... CA100 might just need a good cleaning...possibly a new Diaphram put in it,and an adjustment of the A/F adjustment screw and the power valve adjustment that is under where the vapor hose comes in from the Vaporizor I have seen the same issue with other trucks and a small adjustment of the power valve at full governed RPM makes a world of difference as far as power under load.......
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