Rough idle timing - 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 08-16-2002, 04:37 AM
66chevy2ss's Avatar
Member
 
Join Date: Jul 2002
Location: Des Moines
Posts: 202
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Post Rough idle timing

Supposing my problem isn't something else, consider the following (if you will):

My 350 has a "big" cam in it. It idles roughly. Now its difficult to get it to idle at around 700 RPM, which I'm told is approximately where the initial timing should be set.

So I've been setting the initial timing at 1000 RPM (where it idles fine). But the problem is (obviously) that the mechanical advance will upset this tuning.

Any ideas?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 08-16-2002, 11:24 AM
bullheimer's Avatar
NEVER use credit cards!
 

Last journal entry: car with tt2's, (stockers going back on)
Last photo:
Join Date: Jan 2002
Location: North of Seattle
Posts: 2,529
Wiki Edits: 0

Thanks: 0
Thanked 5 Times in 4 Posts
Post

no, but i'm dumb. <img src="graemlins/pain.gif" border="0" alt="[pain]" />
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 08-16-2002, 11:36 AM
66chevy2ss's Avatar
Member
 
Join Date: Jul 2002
Location: Des Moines
Posts: 202
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Post

Not as dumb as me, though. Trust me...
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 08-16-2002, 11:44 AM
willys36@aol.com's Avatar
Hotrodders.com Moderator
 
Last wiki edit: How to rebuild a Rochester Quadrajet 4MV carbureto...
Last journal entry: How to change auto shift timing on 200R4
Last photo:
Join Date: May 2002
Location: Bakersfield
Posts: 8,384
Wiki Edits: 21

Thanks: 1
Thanked 4 Times in 4 Posts
Post

Usual first culprit to look for is a vacuum leak. Also of the cam is too radical, idle transfer port is uncovered by throttle plate. Trick to fix the latter is to drill small (1/8") holes in throttle plates to allow the trottles to be closed down to let these transfer ports do their job. That is last resort though.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 08-16-2002, 11:48 AM
66chevy2ss's Avatar
Member
 
Join Date: Jul 2002
Location: Des Moines
Posts: 202
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Post

Could you tell me how to look for a vacuum leak?

I do have (I'm borrowing) a friend's hand-pump vacuum with guage, and a second guage with just the hose and pressure guage.

My car also has hooked up (and don't ask me why) a Summitt Racing vacuum guage by the tachometer.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 08-16-2002, 12:34 PM
willys36@aol.com's Avatar
Hotrodders.com Moderator
 
Last wiki edit: How to rebuild a Rochester Quadrajet 4MV carbureto...
Last journal entry: How to change auto shift timing on 200R4
Last photo:
Join Date: May 2002
Location: Bakersfield
Posts: 8,384
Wiki Edits: 21

Thanks: 1
Thanked 4 Times in 4 Posts
Post

It's not that kind of vacuum. What I am referring to is a leaking gasket somewhere on the intake manifold that is exposed to manifold vacuum while the egine is running. Only real way to do it is with the engine running. Take a can of WD40 with the red extension tube (if you are like me, you will need to go buy another can. I always lose the tube). Start the engine and methodically spray the WD40 on all the intake manifold joints (carb, head joints), a little at a time, everywhere you can reach. If you hit a leak, the engine will momentarily speed up. On a Chevy, you can also have a leak on the under side of the manifold that you can't reach - only alternative is reinstall the manifold w/ new gaskets. Bummer!
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 08-16-2002, 12:55 PM
aka Duke of URL
 
Last wiki edit: Finding vacuum leaks Last photo:
Join Date: Oct 2001
Location: Rockville, MD
Posts: 4,821
Wiki Edits: 1

Thanks: 0
Thanked 0 Times in 0 Posts
Arrow

Originally posted by 66chevy2ss:

[quote]My 350 has a "big" cam in it. It idles roughly. Now its difficult to get it to idle at around 700 RPM, which I'm told is approximately where the initial timing should be set.

So I've been setting the initial timing at 1000 RPM (where it idles fine). But the problem is (obviously) that the mechanical advance will upset this tuning.<hr></blockquote>

The way you are setting basic timing is correct for your application (hi-lift/hi-overlap cam).

You are also correct in your assumption that basic timing will be somewhat different with the 300RPM increase, but if the overlap in the camshaft doesn't allow a 700RPM idle, so be it.

The only way out of it is to know the advance specs on the distributor and adjust from there. Too much trouble. You are in essence power-timing the engine. If you experience detonation (through engine rattle-plug deposit check-replacing burned pistons/cracked rings ), back intial timing off a degree or two until the pre-ignition subsides.

If you are coming across a vacuum gauge, it is the best diagnostic tool available. Go to this <a href="http://www.users.bigpond.com/ergoff/OTHER.HTM" target="_blank">-site-</a> and look under Vacuum Gauge Interpretation.
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



All times are GMT -6. The time now is 03:54 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.