You can also spray WD-40 around to try and find a vacuum leak, if propane is not handy. My shop manual says 4 turns out is the starting point for that carb, so if you think you have a vac leak just because you are that far out don't worry.
I'm sure you've got it down by now, but just in case here's what I found on adjusting idle on the 1ME:
You may want to do this with the throttle linkage disconnected, and see if it connecting it when you are done increases rpm and adjust the linkage as necessary....
A: Set "Curb idle" First1.) Adjust timing with vac. advance hose to distributor plugged (if equipped)
2.) Prepare carb per your emissions label that sgtdel was talking about. It will tell you what hoses to plug and what to leave hooked up.
3.) Engine should be warm, AC off, choke fully open, idle cam follower off steps of cam (see emissions label)
4.) Seat mixture screw and then back off 4 turns.
5.) Buy or borrow a tachometer. With selenoid energized (lead attached), turn the nut at the end of the idle stop selenoid using a hex wrench. Adjust until the rpm listed on the emissions label is reached for curb idle.
6.) Now adjust your idle mixture
screw to get the highest rpm at the idle setting you dialed in for the selenoid.
B: Now set "Fast idle"1.) Place cam follower tang on the high step of the cam.
2.) Support the lever with a pair of pliers and bend the tang to obtain the high idle speed specified on the emissions label.
A Haynes manual (mine is for 67-87 pickups) covers the 250 pretty well. You should be able to pick up a real life GM shop manual for your specific year of truck for around $40-50. I bought mine from Amazon.com, the seller was 'tacbook' I believe. It will have ALL of the wiring, vacuum diagrams, trouble shooting guides, correct procedures, etc. A good investment, and you get a good workout just carrying it around!
Hope this helps, good luck! Don't get too frustrated, you're almost there!
Jason
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