« previous next »
Pages: [1] 2   Go Down

Author Topic: Major RPM drop 79 chevy  (Read 12466 times)

Offline xjames14x

  • Newbie
  • Posts: 15
Major RPM drop 79 chevy
« on: April 19, 2011, 05:50:00 pm »
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.
Logged

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Major RPM drop 79 chevy
« Reply #1 on: April 19, 2011, 09:48:52 pm »
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.   
Logged

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 19192
Re: Major RPM drop 79 chevy
« Reply #2 on: April 19, 2011, 10:01:03 pm »
Check your timing, make sure the vacuum advance is working
Logged
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline xjames14x

  • Newbie
  • Posts: 15
Re: Major RPM drop 79 chevy
« Reply #3 on: April 19, 2011, 10:53:49 pm »
Quote from: VileZambonie on April 19, 2011, 10:01:03 pm
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
Logged

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Major RPM drop 79 chevy
« Reply #4 on: April 20, 2011, 12:59:32 pm »
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.






Logged

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 19192
Re: Major RPM drop 79 chevy
« Reply #5 on: April 20, 2011, 08:16:38 pm »
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?
Logged
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline xjames14x

  • Newbie
  • Posts: 15
Re: Major RPM drop 79 chevy
« Reply #6 on: April 21, 2011, 05:35:19 pm »
Quote from: VileZambonie on April 20, 2011, 08:16:38 pm
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
Logged

Offline beastie_3

  • Senior Member
  • ****
  • Posts: 3170
  • Josh
    • My truck pics
Re: Major RPM drop 79 chevy
« Reply #7 on: April 21, 2011, 05:38:29 pm »
the advance shouldnt be hooked up to a port that has vacuum at idle.
Logged

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Major RPM drop 79 chevy
« Reply #8 on: April 21, 2011, 08:09:17 pm »
Quote
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.
Logged

Offline beastie_3

  • Senior Member
  • ****
  • Posts: 3170
  • Josh
    • My truck pics
Re: Major RPM drop 79 chevy
« Reply #9 on: April 21, 2011, 08:21:25 pm »
Thats what I meant... ;D
Logged

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 19192
Re: Major RPM drop 79 chevy
« Reply #10 on: April 21, 2011, 08:56:30 pm »
Keep the vacuum advance hooked up to manifold vacuum. Sounds like the timing is retarded. Try advancing it. Do you have a timing light?
Logged
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline xjames14x

  • Newbie
  • Posts: 15
Re: Major RPM drop 79 chevy
« Reply #11 on: April 23, 2011, 09:18:06 pm »
Quote from: VileZambonie on April 21, 2011, 08:56:30 pm
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
Logged

Offline xjames14x

  • Newbie
  • Posts: 15
Re: Major RPM drop 79 chevy
« Reply #12 on: April 23, 2011, 09:26:59 pm »
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?






Logged

Offline Lt.Del

  • Andy aka:SgtDel
  • Senior Member
  • ****
  • Posts: 3864
  • DelbridgePhotography.com
    • www.delbridge.net
Re: Major RPM drop 79 chevy
« Reply #13 on: April 23, 2011, 11:33:19 pm »
is your heater hooked up?  if so, to what?  That should be a heater hose to your heater core going into the firewall.
« Last Edit: April 23, 2011, 11:35:53 pm by SgtDel »
Logged

Offline xjames14x

  • Newbie
  • Posts: 15
Re: Major RPM drop 79 chevy
« Reply #14 on: April 24, 2011, 02:59:05 am »
no we need to get a new heater core, where is the manifold vacuum located
Logged
Pages: [1] 2   Go Up
« previous next »