Holley 750 vac sec Carb - 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 05-02-2004, 10:25 AM
onecoomofo's Avatar
Member
 
Last photo:
Join Date: Jan 2004
Location: Houston, TX
Age: 31
Posts: 155
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Holley 750 vac sec Carb

I have a new holley 750 vacuum secondary carb that I am going to put on my truck. My problems is that I know this carb will be way too big for the truck, making it run way rich.

I was wondering what size jets come stock on a 750 vac sec? Also, what jets come stock on a 750 DP?

Thanks
Sean

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 05-02-2004, 11:19 AM
Member
 
Join Date: Apr 2004
Location: Cassville,MO
Posts: 838
Wiki Edits: 0

Thanks: 22
Thanked 49 Times in 45 Posts
It's not the jet size that make a carb too big. A stock vac. sec. has 72's in the primary. My books show the primaries jetted 75's for a 4779 and 70's for 4779-1, 4779-2, 4779-3, 4779-4.
If a carb is too big it can actually go lean because the engine isn't pulling enough air through the carb to provide a strong enough metering signal to the boosters to pull fuel through the jets. This is why you can get a lean backfire through the carb when you mash the gas pedal on an over carburated engine.
You can get away with 3310 vac. sec. carb on a smaller than recommended engine by play with the spring in the vacuum pod that control the secondaries but then you end up making the carb a 2 barrel carb 95% of the time. A 350 is the smallest thing that I would put a 3310 on but even with that you not going to have great control of the idle and transition mixture unless it's a really hot 350.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 05-02-2004, 12:16 PM
MI2600's Avatar
Member# 3287
 

Last journal entry: Vortec Heads
Last photo:
Join Date: Jul 2002
Location: N. Muskegon, MI
Posts: 2,632
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
The stock jetting on a 750 vacuum secondary, like a 3310, is 72 and 78.
I would forget about DPs on the street.
Although you didn't specify what your engine is, on the assumption that it's a 350, I don't really see any problem with using a 750 vacuum secondary. The engine only demands as much as it can use. You may have to tune the vacuum module with different springs, but that's not that hard. Buy one of those quick change tops and save yourself some grief.
GM put 750 QJets on 305s and it's the same principle.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 05-02-2004, 01:17 PM
onecoomofo's Avatar
Member
 
Last photo:
Join Date: Jan 2004
Location: Houston, TX
Age: 31
Posts: 155
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally posted by tresi
If a carb is too big it can actually go lean because the engine isn't pulling enough air through the carb to provide a strong enough metering signal to the boosters to pull fuel through the jets. This is why you can get a lean backfire through the carb when you mash the gas pedal on an over carburated engine.
I get a small backfire every time i take off from a stop in my truck with the 750 DP on a 350. If I come off from a stop and give it half throttle, i get a big backfire and it almost dies, sometimes it does. All backfires are thru the carb. The idle mix screws are almost all the way in to so it wont idle so rich. When i go WOT and make a full pass and stop afterwards, all you can smell is gas, and you can feel it bogging the whole time.

I just pulled the carb off the truck and the nova so I can swap them. I've been told that stock jets on a Holley 750 Vac. sec. are 72 in the main. I just realized that the jets were 70s in the main. I have a pair of 68s that I am going to put in before i put this carb on the truck.

Think this will help the problem?
Thanks
Sean S.

Last edited by onecoomofo; 05-02-2004 at 01:46 PM.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 05-02-2004, 02:40 PM
Member
 
Join Date: Apr 2004
Location: Cassville,MO
Posts: 838
Wiki Edits: 0

Thanks: 22
Thanked 49 Times in 45 Posts
Quote:
Originally posted by onecoomofo
I get a small backfire every time i take off from a stop in my truck with the 750 DP on a 350. If I come off from a stop and give it half throttle, i get a big backfire and it almost dies, sometimes it does. All backfires are thru the carb. The idle mix screws are almost all the way in to so it wont idle so rich. When i go WOT and make a full pass and stop afterwards, all you can smell is gas, and you can feel it bogging the whole time.

I just pulled the carb off the truck and the nova so I can swap them. I've been told that stock jets on a Holley 750 Vac. sec. are 72 in the main. I just realized that the jets were 70s in the main. I have a pair of 68s that I am going to put in before i put this carb on the truck.

Think this will help the problem?
Thanks
Sean S.
Get a vacuum gauge and set your mixture for the highest vacuum that you can get. I'd start with the 68's unless you've got a really big cam. Due to all of your of your backfiring I'd start with a new power valve too unless you have a way of checking it. Make sure that the accellerator pump system is all ok. You might have to due some tuning with squirters and pump cams to control the bog with that double pumper.
Make sure that the throttle shafts aren't worn and cuasing a vacuum leak. You'll never get a good idle with a 750 if you've got much of a vacuum leak. Be aware that float height changes the jetting a little. With the same jets the higher the fuel level in the bowl, the richer it will be.
Some holleys have a check ball or needle in the hole that the pump shooter mounts to. If this check ball is missing it can suck fuel through the accelerator pump system at high rpm.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 05-02-2004, 03:39 PM
onecoomofo's Avatar
Member
 
Last photo:
Join Date: Jan 2004
Location: Houston, TX
Age: 31
Posts: 155
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I just put the 68s in the vacuum carb. and noticed that it doesn't have a secondary metering block. When I took off the second bowl, it had a big silver plate on it with weird secrews in it...
I guess on a double pumper the secondary metering block is the same as the primary right? Whats the silver plate on the vacuum secondary carb?

Sean S.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 05-02-2004, 04:54 PM
Member
 
Join Date: Apr 2004
Location: Cassville,MO
Posts: 838
Wiki Edits: 0

Thanks: 22
Thanked 49 Times in 45 Posts
Quote:
Originally posted by onecoomofo
I just put the 68s in the vacuum carb. and noticed that it doesn't have a secondary metering block. When I took off the second bowl, it had a big silver plate on it with weird secrews in it...
I guess on a double pumper the secondary metering block is the same as the primary right? Whats the silver plate on the vacuum secondary carb?

Sean S.
Only the 3310 no dash had jets in the secondaries. The 3310-1 and later have the plate that you discribed as does the 1850 600 vac sec. That plate is called the metering plate.
Holley did this to these carbs in order to make a mass produced cheap carb. You can get different plates but its a nightmare to get and to figure out what plate you need. There is no direct coralation to the number on the plate and how rich or lean the plate may be. The plate has the air bleeds to so just because you got a plate with a different number on it doesn't mean that the jet size well be different. If I were you I'd leave the secondaries alone until you get it running well and then get the conversion kit to put a metering block and jets in the rear. The secondaries never come in normal driving so worry about that later.

Last edited by tresi; 05-02-2004 at 07:17 PM.
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



All times are GMT -6. The time now is 10:04 AM.


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.