250 engine drops 600 rpms from park to drive

[jfw432]

I recently purchased a 79 C-10 with a 250 straight 6 in it and the idle speed drops pretty significantly when I put it in gear. This truck has a Varajet II carb in it and I've been making some progress getting things working better. It idles around 1100rpms when fully warmed up and drops down to about 500rpms with my foot on the brake and the truck in reverse or drive.

1. First thing I did was fix the timing. Kind of a pain since the timing marks broke off but I've set the timing to about 8° at idle with the vacuum advance removed. (helped a lot but engine had to be semi warm before I could put it in gear and not have it die)

2. Next I inspected the wires, cap, and rotor. I only changed the plugs though. (no change)

3. Changed the dirty air filter. (no change)

4. Searched for vacuum leaks around the carb. (I found one leak coming from a dry rotted hose coming from a large bottle(?) near the drivers headlight. This made a large difference and the truck wouldn't die when cold and in gear but the rpms still drop a lot)

5. I've been searching for other vacuum leaks but it seems like there are about 30 vacuum lines going in and around the carb on this engine but haven't found anything else.


Additional information:
Vacuum reading on the engine is a little jittery. It only bounces +/- 1/4 in-hg. However, the rpms waiver from time to time which causes the vacuum to vary between 18-21in-hg. It may take a couple minutes for it to go from 18 to 21 though.

Compression numbers are as follows:
1 - 150
2 - 150
3 - 170
4 - 160
5 - 150
6 - 155

I'm still thinking vacuum leak but I can't find any others. Any advice would be appreciated.