What effects does low rpm's have on air fuel mixture? - 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 03-30-2013, 12:02 PM
Registered User
 
Join Date: Jun 2011
Location: USA
Posts: 44
Wiki Edits: 0

Thanks: 3
Thanked 0 Times in 0 Posts
What effects does low rpm's have on air fuel mixture?

(preface: 355sbc, 232@.50 480lift cam, decent heads, rpm air gap, holley 650 dp, t5 manual)

I put a wideband gauge in my Camaro and I've been having fun fine tuning this carb with it. Got it dialed pretty good so far, but wondering about something. It seems to stay lean until about 2k, 2.5k rpms. For instance, if I'm on the highway at 35mph in 4th gear and floor it, it shoots to about 15:1 and slowly lowers as the rpms raise. By about 2k-2.5k it drops to it's normal "way too rich" wot mixture of about 11:1.

I realize that an afr between 10 and 11 at wot is very rich, and I'm gonna tackle that next, but first I wanted to know if maybe this "lean until 2k rpms" thing is an indication of bad timing. I'm running about 21 initial and 36 all in with no vacuum atm. 1 medium 1 light spring.

Any ideas?
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-30-2013, 12:08 PM
vinniekq2's Avatar
Registered User
 
Join Date: Jun 2012
Location: BC,Canada
Age: 56
Posts: 6,740
Wiki Edits: 0

Thanks: 236
Thanked 619 Times in 602 Posts
11.9-12.2:1 is good for racing.maybe aim for 13:1 for street.
do you have remove able air bleeds?
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-30-2013, 12:18 PM
Registered User
 
Join Date: Jun 2011
Location: USA
Posts: 44
Wiki Edits: 0

Thanks: 3
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by vinniekq2 View Post
do you have remove able air bleeds?
Yes I do. Why?

Quote:
Originally Posted by vinniekq2 View Post
11.9-12.2:1 is good for racing.maybe aim for 13:1 for street.
What do you mean by this? That I should aim for 13 at wot because I DD it? If I idle at a nice happy mid-14, shouldn't 11.9-12.2 be fine at wot?
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-30-2013, 12:20 PM
Registered User
 
Last photo:
Join Date: Mar 2007
Location: Corinth, MS
Posts: 579
Wiki Edits: 0

Thanks: 0
Thanked 23 Times in 22 Posts
I've never used a double pumper, but could it be a bad power valve?
ssmonty
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-30-2013, 12:25 PM
Registered User
 
Join Date: Jun 2011
Location: USA
Posts: 44
Wiki Edits: 0

Thanks: 3
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by ssmonty View Post
I've never used a double pumper, but could it be a bad power valve?
ssmonty
I wouldn't think so. I actually just yesterday switched from a 4.5 power valve to a 6.5(got about 8 intake vacuum at idle). It REALLY helped it smooth out. With the 4.5 it would shoot VERY lean at part throttle acceleration. With the 6.5 it's extremely consistent and smooth no matter how much pedal I give it.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-30-2013, 12:32 PM
vinniekq2's Avatar
Registered User
 
Join Date: Jun 2012
Location: BC,Canada
Age: 56
Posts: 6,740
Wiki Edits: 0

Thanks: 236
Thanked 619 Times in 602 Posts
have you already advanced the accellerator cam? and or changed discharge nozzles?
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 03-30-2013, 02:39 PM
How fast is fast enough?
 
Last photo:
Join Date: Jun 2005
Location: St. Louis, MO
Age: 28
Posts: 8,236
Wiki Edits: 0

Thanks: 12
Thanked 220 Times in 205 Posts
Try a smaller air bleed, you'll need to lower your jet size a little after that too.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-31-2013, 12:41 PM
Registered User
 
Join Date: Aug 2006
Location: Alexandria, VA
Posts: 1,724
Wiki Edits: 0

Thanks: 4
Thanked 62 Times in 60 Posts
My SBC 350 with a rebuilt Quadrajet and a mild cam runs about 13.5 AFR in normal driving, and drops to about 12 at full throttle. Its probably not set up for optimum performance, but street driveability is great. It starts and runs well, and feels strong right off the line. However, my engine is limited by stock heads and iron exhaust manifolds, so I haven't tried getting more out of the carb tuning.

Bruce
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
air/fuel mixture strokin Engine 2 12-05-2011 05:01 PM
air/fuel mixture and air fuel gauges questions tomcraft Hotrodding Basics 10 08-20-2009 11:40 AM
Fuel mixture and air temperature SAATR Engine 1 04-01-2005 08:01 PM
Air/Fuel mixture and performance orian Engine 13 01-16-2005 09:24 AM
air/fuel mixture???? FenderGitars Engine 6 08-28-2004 07:59 PM


All times are GMT -6. The time now is 09:02 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.