Major RPM drop 79 chevy

[xjames14x]

My truck is a 79 Chevy K-10 with a 350, and turbo 400 transmission.  i am having an issue with a major rpm drop when i put my truck into gear. when it is warm the idle is fine, i just replaced the modulator on transmission today, the vacuum hoses seem to be working fine. Is this a transmission issue? my carb is an edelbrock 1405 model, but it seems to be working fine also. if someone can point me into the right direction that would be great.

[Lt.Del]

I'd say you have a vacuum leak causing too much air to enter your engine and when you put it in gear, the rpms go down anyway.

take some WD40 and spray around all vacuum hoses in engine while it is idling.  If the rpms shoot up more, you found a vacuum leak.   

[VileZambonie]

Check your timing, make sure the vacuum advance is working

[xjames14x]

Check your timing, make sure the vacuum advance is working

can you give me a example of the vacuum advance, because i believe my dad says that is working

[Lt.Del]

I'd pull the distributor cap off, then suck in on the vacuum advance hose and see if it moves the distributor parts inside (advances it). 
Suck in on it and put your finger over the hose to prevent air from escaping, see if the advance stays advanced---doesnt move.  If it moves, i"d say you may have a leaking diaphram in the vacuum advance (silver round thing the hose hooks to on distributor).
Make sure the hose is hooked to proper place fro vacuum. Mine, if you can follow, is from a 79 and hooked to carb port.

[VileZambonie]

How do you have it hooked up? Where is the vacuum hose running from the vacuum advance? Is there suction present at the hose going to it with the engine idling?

[xjames14x]

How do you have it hooked up? Where is the vacuum hose running from the vacuum advance? Is there suction present at the hose going to it with the engine idling?

from the vacuum advance to the carb, when the truck is in idle and you take the hose off from the carb that goes to the vac advance the truck will almost die

[beastie_3]

the advance shouldnt be hooked up to a port that has vacuum at idle.

[Lt.Del]

you take the hose off from the carb that goes to the vac advance the truck will almost die

it will die because when you take a vacuum hose off the carb, you then have a major vacuum leak.

[beastie_3]

Thats what I meant...

[VileZambonie]

Keep the vacuum advance hooked up to manifold vacuum. Sounds like the timing is retarded. Try advancing it. Do you have a timing light?

[xjames14x]

Keep the vacuum advance hooked up to manifold vacuum. Sounds like the timing is retarded. Try advancing it. Do you have a timing light?

yes we tried timing it, and on top of the manifold it looks like they plugged one hole for a vacuum i think, i will show you a picture of what im talking about

[xjames14x]

Sorry for the blue star but below it is what i think is blocked off for some reason, is that the manifold vacuum?  so that is supposed to go to the vacuum advance?

[Lt.Del]

is your heater hooked up?  if so, to what?  That should be a heater hose to your heater core going into the firewall.

[xjames14x]

no we need to get a new heater core, where is the manifold vacuum located