Have u unhooked the automatic throttle control to see if it’s related to the inj pump or throttle control.?? 99% of the ones I’ve worked on are FULL OF WATER..(rust) U can adjust the screw to get it to low idle.. run/adjust it to about 1000rpm and wait to see what happens If it has a Stanadyne/RoosaMaster inj pump, try to get the # off the name plate..