Strange vaccuum ??? - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
LinkBack Thread Tools Display Modes
post #1 of 6 (permalink) Old Jan 25th, 03, 01:49 PM Thread Starter
 
Join Date: Nov 2000
Posts: 84
Ok, here is a wierd one. I have been tinkering with my old buggy for a while now. Lot of different things doen mostly considered a major tune up. When I went to check timing a couple of onths after we set initial timing, this is what happened. When I removed the vaccuum hose to the distributor the engine accelerated instead of decreasing in rpm. I dont have a dwell meter but am going on what I hear. This just doesnt make sense to me. Anyone have any ideas why this would happen? I tried to connect the vaccum hoses to differetn ports and each time when all vaccuum hoses were connected the car would drop rpm's pull a vaccuum hose and the rpm's increase. Here are the specs. 73 350 bored .30 over. rebuilt Roch Q-Jet. major tune up all around.

------------------
Scott
73/74 mix. (Its a mutt) 73 350 bored 30 over, Th350 tranny. With the Lord willing, may we never have to drive a MiniVan!

The ole Mutt
http://photos.yahoo.com/bc/gateruner...otos.yahoo.com
gateruner is offline  
Sponsored Links
Advertisement
 
post #2 of 6 (permalink) Old Jan 25th, 03, 02:03 PM
DjD
Retired
Dennis
 
DjD's Avatar
 
Join Date: Apr 1999
Location: Fortuna, CA
Posts: 26,547
Garage
Scott - what you have happening is normal.
DjD is offline  
post #3 of 6 (permalink) Old Jan 25th, 03, 02:26 PM
Senior Tech
 
Join Date: Apr 2001
Location: Out here
Posts: 111
Scott,
To kind of explain what DJD said what you experiencing is a mild surge in terms of RPM from leaning out a bit. Nothing to worry about at all. Essestially, you are giving the motor just a little bit more oxygen but the amount of fuel ingested is the same. What happens is the extra oxygen increases the speed of travel of the flame front, thereby increasing power stroke speed at idle only. The end result is your motor speeding up a little bit. Hope this helps to explain what you are seeing a little better. Scott
psalm69 is offline  
 
post #4 of 6 (permalink) Old Jan 26th, 03, 04:29 AM
Senior Tech
 
Join Date: Mar 2002
Location: Austin, TX, USA
Posts: 5,277
To confirm what DjD and psalm69 are saying...did you experience the RPM surge with the vacuum port open or plugged. If open, then they are right. You just leaned the idle circuit out a bit causing the RPMs to increase. This is probably normal, but might indicate a rich idle. The important thing is that without vacuum advance, the timing is somewhere between 8 and 12 BTDC and with vacuum it's around 15-20 BTDC.

-dnult

[This message has been edited by dnult (edited 01-26-2003).]
dnult is offline  
post #5 of 6 (permalink) Old Jan 27th, 03, 11:26 AM Thread Starter
 
Join Date: Nov 2000
Posts: 84
Guess I was confused. I was lead to believe that when you take the vaccuum hose off it would actually bog down a bit. This was supposed to be the case because the vaccuum advance would not have a vaccuum on it at the distributor. Makes sense what you guys are saying anyway. I wont get concerned with it.

------------------
Scott
73/74 mix. (Its a mutt) 73 350 bored 30 over, Th350 tranny. With the Lord willing, may we never have to drive a MiniVan!

The ole Mutt
http://photos.yahoo.com/bc/gateruner...otos.yahoo.com
gateruner is offline  
post #6 of 6 (permalink) Old Jan 28th, 03, 07:30 AM
Senior Tech
Mark
 
Join Date: Sep 2000
Location: Shrewsbury, MA, USA
Posts: 6,513
If you have a stock advance system then there is no vacuum to your distributor vacuum advance at idle. The vacuum advance is ported vacuum, vacuum obtained above the throttle plates, not manifold vacuum obtained from below the throttles. At idle the throttles are closed and there is no ported vacuum. Leaving that hose uncapped at idle should have no effect on your idle mixture. Now if you have it connected to manifold vacuum, then you will affect your mixture as described above.

------------------
Mark Canning
1969 Indy Pace Car
350/300HP RPO Z11
My 69 L48 - 350/300HP Engine
Mark C is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user 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.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



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

 
For the best viewing experience please update your browser to Google Chrome