I did not read all the posts here but ....
have you read my (noah brinkman) 603 idles but no RPMS (or throttle) thread????

I got it resolved !!
find that post and read ALL THE STUFF I did to it to narrow it down to the problem

In short My 86 SDL 603.961 was fine started it the next mourning ran (idled) fine

step on the pedal RPM's would not come up more than ohh about 100 more than idle so 650-750

I will save you a week of ****** around

take off intake cross-over
start the car
spray WD-40 or diesel in a spray bottle hell you could use an un-lit propane torch if you want

raise linkage by hand while spraying fuel in
your RPM's will rise

after you read my thread you will say damn this guy tried everything
here is what it was!!! drum roll please!

Alda went bad..... yeah I know an un-molested alda on a rebuilt engine was fine one day and the next **** out on me to the tune of OHH maybe $800 after replacing other parts to find the problem
and shipping my IP off to detroit to get worked on

If you live near by (0-300 miles) I'll come help
I gotta pay it forward for all the help I got from Roy