vacuum advance - 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 04-14-2011, 09:10 AM
Registered User
 
Join Date: Apr 2011
Location: California
Posts: 1
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
vacuum advance

I have a 454 Big Block-oval port heads-Crane cam (specs unknown)-edelbrock intake-holley 750 double pumper-hooker headers and a newly installed msd street fire hei distributor. the car has a muncie m21 4 speed.

Here is my problem;

Since I have installed the distributor-with the vacuum advance connected-I get an idle that goes up to 1500 rpms and then drops down to 600. It continues to do this no matter how I set the carb idle.

When I block the vacuum lines i get a consistent idle but have a noticable "miss" at around 3000 rpms. I have also installed a new set of AC Delco Rapif Fire sparkplugs and the wires are new MSD. The car also pre-ignites (keeps running slightly after shut off).

I have attempted to adjust the vacuum advance but am unable to locate the set screw within the vacuum cannister.

I have no problem running the car without the vacuum advance.

Help!!

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 04-14-2011, 09:17 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
The advance weight springs may be on the verge of advancing at idle, and when they do- the idle speed increases.

So, try one step heavier spring on ONE side of the advance mechanism. Hopefully adding that bit more spring will keep the advance from coming in at idle speed.

IF the vacuum can is adjustable, the screw should be inside the vacuum port. If you can, see if it's an allen type (usually 3/32") or possibly slotted for a screwdriver.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 04-14-2011, 10:14 AM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,691
Wiki Edits: 0

Thanks: 3
Thanked 403 Times in 348 Posts
Quote:
Originally Posted by the bullet
I have a 454 Big Block-oval port heads-Crane cam (specs unknown)-edelbrock intake-holley 750 double pumper-hooker headers and a newly installed msd street fire hei distributor. the car has a muncie m21 4 speed.

Here is my problem;

Since I have installed the distributor-with the vacuum advance connected-I get an idle that goes up to 1500 rpms and then drops down to 600. It continues to do this no matter how I set the carb idle.

When I block the vacuum lines i get a consistent idle but have a noticable "miss" at around 3000 rpms. I have also installed a new set of AC Delco Rapif Fire sparkplugs and the wires are new MSD. The car also pre-ignites (keeps running slightly after shut off).

I have attempted to adjust the vacuum advance but am unable to locate the set screw within the vacuum cannister.

I have no problem running the car without the vacuum advance.

Help!!
This sounds like one of a couple three things perhapes in combination.

If you're taking the vacuum signal off the carb's timed port and the throttle blades at idle are right at the threshold of exposing this port, it could be switching back and forth between off causing a low idle and on causing a fast idle.

Run on is usually a sign that the mixture is too lean at idle which runs the temps in the combustion chamber up to where something glows hot enough to keep the engine running. There are electric solenoids sold for the purpose of completely closing the throttle blades when the ignition is shut off which stops this habit. A colder range spark plug might help with no other changes, so could running the idle mixture screws out a little. A failed or disconnected EGR can cause this. The fuel mixture goes to lean unless adjusted when the EGR is failed or removed.

Curb idle needs to be set with the vacuum advance function unless this engine has a radical cam. For a mild cam use the full time vacuum port, for a radical cam use the timed port in this case it makes the engine easier to get started.

The throttle at idle must be below the transition slots, look in the carb's lower venturi bore you will see a rectangular slot, at idle the top of the throttle blade needs to be no higher than the bottom of this slot. If that doesn't provide sufficient idle air then the primary throttle blades can be drilled with a very small bit, if this is a Holley the secondary throttle blades have an adjustable closed stop that can be used to crack the blades a bit.

The 3000 RPM miss can be anything from a lean hole in the carburation to a poor transfer of timing advance such as the vacuum not going out quickly enough of the centrifugal coming in too soon or too late such that there is a moment of either too much or too little advance.

Bogie
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:
If you do not want to register, fill this field only and the name will be used as user name for your post.
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
Opinions on ported of full vacuum for vacuum advance ? y2k600f4 Engine 15 10-07-2011 09:38 PM
Timing... vacuum or no vacuum advance? 454ChevelleMalibu Engine 18 04-25-2010 09:37 PM
adj. vacuum advance ss396si Engine 2 09-29-2009 09:11 PM
ok to use same vacuum line for vacuum advance and tranny vacuum modulator? brian400ex Engine 4 02-19-2003 02:25 PM
computer advance or vacuum advance?? 1984camaroZ28 Engine 1 10-12-2002 02:57 PM


All times are GMT -6. The time now is 06:04 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.