SBC 350 Vacuum Advance/ Idle issues - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 07-23-2008, 04:37 PM
Registered User
 
Join Date: Jul 2008
Location: Arizona
Posts: 2
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
SBC 350 Vacuum Advance/ Idle issues

I'm seeming to have quite a bit of trouble adjusting my timing/ idle speed. I pull and plug the vac advance line, set the timing to 12 BTDC. Adjust the idle speed and Air/fuel slightly to were i'm idling around 850 in park/neutral and idling at 650 in gear. When i hook the vac advance back up, idle speed in park shoots to around 1200/1300 rpm. I cant seem to keep the idle down low in park while still keeping it at about 650 in gear. some help would be great, thanks.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 07-23-2008, 06:11 PM
DoubleVision's Avatar
Not Considered a Senior Member
 
Last photo:
Join Date: Oct 2002
Location: Heart Of Dixie
Age: 40
Posts: 10,657
Wiki Edits: 0

Thanks: 15
Thanked 60 Times in 57 Posts
thats why you set the idle with the vac advance connected, if you set it with it disconnected your setting it to it`s base timing of 12 degrees, when you connect the vac advance your adding 12 more degrees which makes 24 and with that much more timing, the idle is going to go up but it shouldn`t go up that much which leads into the next chapter. You might want to check the timing at idle with the vacuum advance connected and see where it`s landing, it maybe giving more than needed resulting in a higher idle than desired, it sounds like the springs on the mechanical advance is too loose or worn out, when the engine speed increases with the vac advance connected, the additional rpm maybe forcing the mechanical advance to open more, resulting in the much higher idle. check the timing with the vac advance connected, it shouldn`t be more than 24 to 26 degrees, if it is, there`s a problem with the mechanical advance.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 07-23-2008, 06:24 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,095
Wiki Edits: 0

Thanks: 4
Thanked 541 Times in 457 Posts
Quote:
Originally Posted by afuller1
I'm seeming to have quite a bit of trouble adjusting my timing/ idle speed. I pull and plug the vac advance line, set the timing to 12 BTDC. Adjust the idle speed and Air/fuel slightly to were i'm idling around 850 in park/neutral and idling at 650 in gear. When i hook the vac advance back up, idle speed in park shoots to around 1200/1300 rpm. I cant seem to keep the idle down low in park while still keeping it at about 650 in gear. some help would be great, thanks.
This is the expected effect, there are several solutions:

- Use a timed vacuum port on the carb instead of a port that's always connected to manifold vacuum. If the timed port is showing vacuum on idle then the carb isn't adjusted correctly. The throttle plates need to be closed more such that they are below the fuel transfer slot and the timed vacuum port. On a big cammed motor this can lead to drilling air bleed holes into the throttle plates to let more air into the engine without opening the throttles. Some carbs allow adjustment of the secondaries to let more air in without opening the primaries too far as to expose the transfer slots and timed ports.

- The engine if modified from a mild factory configuration may need an adjustable vacuum cannister so you can dial in the amount of advance for the vacuum present. Also there are different factory vacuum cans that come on at different amounts of vacuum. A can set up for a large cammed engine with low idle vacuum will come on too soon with too much advance if used on a mild cammed engine.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 07-23-2008, 06:45 PM
Frisco's Avatar
Glad To Be Here
 
Last photo:
Join Date: Nov 2002
Location: Canton, North Carolina
Age: 73
Posts: 2,244
Wiki Edits: 0

Thanks: 0
Thanked 8 Times in 8 Posts
Quote:
Originally Posted by afuller1
I'm seeming to have quite a bit of trouble adjusting my timing/ idle speed. I pull and plug the vac advance line, set the timing to 12 BTDC. Adjust the idle speed and Air/fuel slightly to were i'm idling around 850 in park/neutral and idling at 650 in gear. When i hook the vac advance back up, idle speed in park shoots to around 1200/1300 rpm. I cant seem to keep the idle down low in park while still keeping it at about 650 in gear. some help would be great, thanks.


DOUBLEVISION & OLDBOGIE both answered your questions. I am just adding my comments in an attempt to simplify an answer to you.

You are setting the timing correctly.

You are setting the idle RPM at the wrong time.

Set the idle RPM after you have re-connected the vacuum advance.

You are using full manifold vacuum (which I recommend and prefer). When hooking it back up, the idle RPM will go up due to the vacuum advance increasing the timing. This is when you re-adjust the idle RPM.

If you are using timed (ported) vacuum source for the vacuum advance cannister, the idle RPM should not increase when re-connecting the vacuum hose. Some folks prefer using this connection for the vacuum advance source. Try it both ways and decide for yourself as to which performs best.

Last edited by Frisco; 07-23-2008 at 06:54 PM.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 07-23-2008, 06:55 PM
Registered User
 
Join Date: Jul 2008
Location: Arizona
Posts: 2
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by DoubleVision
thats why you set the idle with the vac advance connected, if you set it with it disconnected your setting it to it`s base timing of 12 degrees, when you connect the vac advance your adding 12 more degrees which makes 24 and with that much more timing, the idle is going to go up but it shouldn`t go up that much which leads into the next chapter. You might want to check the timing at idle with the vacuum advance connected and see where it`s landing, it maybe giving more than needed resulting in a higher idle than desired, it sounds like the springs on the mechanical advance is too loose or worn out, when the engine speed increases with the vac advance connected, the additional rpm maybe forcing the mechanical advance to open more, resulting in the much higher idle. check the timing with the vac advance connected, it shouldn`t be more than 24 to 26 degrees, if it is, there`s a problem with the mechanical advance.


Ok, well I read up abit more about ported vs manifold vacuum. I switched the vacuum advance over to the ported source through the carb and my Idle in park and idle in gear are just about where they should be, roughly 850 idle in park, and 650 idle in gear.

The springs i know are fine. the entire distributor assembly is brand new, and i have slightly heavier springs than original.

Now my question is, am I better off running it how it is? or switching it back to the manifold vacuum and then rechecking/resetting timing?
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 07-23-2008, 07:02 PM
Frisco's Avatar
Glad To Be Here
 
Last photo:
Join Date: Nov 2002
Location: Canton, North Carolina
Age: 73
Posts: 2,244
Wiki Edits: 0

Thanks: 0
Thanked 8 Times in 8 Posts
Quote:
Originally Posted by afuller1
Now my question is, am I better off running it how it is? or switching it back to the manifold vacuum and then rechecking/resetting timing?
Check your performance.

From a dead stop go to WOT. Is there a hesitation? Swap the vacuum source to FULL manifold and lower the idle RPM to where you want it to be. Repeat the same test without any other changes. Which way performed best?

Keep in mind that hesitation or bogging can also be caused by not enough timing and/or not enough pump shot from the accelerator pump. At this point you are only attempting to see whether full or ported vacuum source works best for you.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
350 Chevy off Idle Issues 84Caprice57 Engine 27 02-20-2011 05:43 PM
Port or Full time vacuum cool rockin daddy Engine 129 10-05-2010 04:00 PM
Edelbrock 750 tuning GCD1962 Engine 34 04-16-2008 07:03 PM
350 SBC doesn't idle, other problems too. ajax1 Engine 17 02-25-2008 08:33 PM
Chevy 350 Off Idle Issues! 84Caprice57 Engine 5 02-01-2007 10:50 AM


All times are GMT -6. The time now is 09:13 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.