Correct Jetting on Holley 750 - 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 03-03-2009, 04:59 PM
Registered User
 
Join Date: Feb 2008
Location: Albuquerque,NM
Posts: 21
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Correct Jetting on Holley 750

After idling for about 15 minutes with new plugs, I pulled them out and they were pretty black. I had jsut went up 1 heat rang on the plugs but im thinking I need to go down a lot more in the jetting. I have dual 750 Holley carbs with vac sec. This is on top a 8-71 BDS blower with about 8-10 lbs of boost. Right now I have #68 jets in there. My elevation is about 5000ft, so i was thinking of going down to #65. Is that too much of a jump?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 03-03-2009, 05:21 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,284
Wiki Edits: 0

Thanks: 5
Thanked 399 Times in 393 Posts
Install a O2 sensor in the exhaust collector and read the afr with a afr meter or volt meter. The idle/low speed richness-leanness has little to do with the main jets.
Even a simple narrow band AFR gauge will be a huge help in dialing it in.

It will show you exactly what is going on at idle and off idle, and at part throttle cruise.
Blower motors like lots of initial spark advance at idle.
You'll probabily have to play with the idle air bleeds and or idle feed restrictions.
This is what controls the AFR at idle, not the main jets.

What is the carb model number?
What cam is in this motor?

Last edited by F-BIRD'88; 03-03-2009 at 05:28 PM.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-03-2009, 05:34 PM
Registered User
 
Join Date: Oct 2008
Location: Mora,MN
Posts: 422
Wiki Edits: 0

Thanks: 0
Thanked 8 Times in 8 Posts
If they are fouling on idle, fix that first, don't change main jets.
It could be that you are running rich on idle, then fine or even leaning
out in higher RPM's.
The idle mixture screws on the sides of the metering block are what
you need to adjust for idle.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-03-2009, 06:03 PM
tubbedz's Avatar
Registered User
 
Last photo:
Join Date: Jun 2007
Location: United States
Posts: 120
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I had the same problem. Dual carbs on my 6-71 Blower. Black plugs at idle and little response from my idle mixture screws. My fix was to lean out the idle feed restrictions in the metering blocks. My metering blocks are billet so all I had to do was go down a couple of idle feed restriction jet sizes. After this my idle mixture screws became responsive. If you don't have billet, adjustable metering blocks, I read somewhere that you can put a piece of wire in the idle feed restriction to lean it out.

If you have removable idle air bleeds, you can also try to enlarge the idle air bleeds to lean the idle mixture out. If you don't have removable/adjustable air bleeds, Holley says to leave the air bleeds alone. You will ruin your carb body.

Another issue I had was that my pcv valve was sucking oil into the blower which fouled my plugs. I had to get taller valve covers with better baffles.

Hope this helps......
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-03-2009, 06:10 PM
Registered User
 
Join Date: Feb 2008
Location: Albuquerque,NM
Posts: 21
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks everyone for the help, this gives me a really good place to start. I order a O2 sensor from summit a couple days ago and Im hoping it will be in by the weekend.


I dont have removable idle air bleeds so i'll have to look in to other options.

The cam is a solid roller Int .650 exh .660 235 ish duration. Its around that I havent looked at the cam card in a while. I know that info should be permanently burned into my brain.

I'll have to check the model numbers on the carbs too.
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
predator carb v.s. holley 750 dyno chart chevelle4542speed Engine 66 08-03-2013 04:11 AM
Right jetting for a sbc 383 Holley 4150 sanman Engine 2 12-21-2007 05:29 AM
Holley 750 problems buff Engine 9 04-02-2006 11:13 AM
Holley 750 Carb Leak tweeter400 Engine 25 02-17-2006 09:42 AM
750 Holley carberator 4 barrel double pump StressAngel General Rodding Tech 4 07-05-2004 10:55 AM


All times are GMT -6. The time now is 06:53 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.