4 barrel Holley issues - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans Advertise
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Hotrodding Basics
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 01-31-2019, 08:40 PM
coltonbily's Avatar
Registered User
 
Join Date: Oct 2017
Posts: 41
Wiki Edits: 0

Thanks: 9
Thanked 0 Times in 0 Posts
4 barrel Holley issues

So in my c20 Iíve got a 750 double pumper on top of a big block thatís been bored .60 over and has a mild comp cam with the cast iron 781 heads ported with 2.19 inch valves and big valve springs. (Not sure of all the specs my engine guy didnít tell me). But basically the truck runs fine and moves pretty good on the 2 primaries but if I open the secondaries up on the low end the motor surges ALOT and sounds horrible, the RPMS will go up and down and up and down until you let off the pedal. Now if I open the secondaries once Iíve got a little speed say 30mph the motor acts fine and takes off pretty good. Iíve got a Holley red fuel pump so fuel pressure couldnít be an issue and the total timing is set to 35. Anyone got a clue as to whatís going on here?


Sent from my iPhone using Tapatalk

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 01-31-2019, 09:13 PM
70BossRanchero's Avatar
Registered User
 
Join Date: Jan 2018
Posts: 335
Wiki Edits: 0

Thanks: 14
Thanked 84 Times in 72 Posts
That's the thing about a double pumper. You have to control the rate of throttle opening to match the demands of the engine. Open it faster than the engine can take it, and the airflow slows down so much through the carb that it can't meter correctly and boggs down. Open the carb at a slower rate, so that the engine keeps pulling hard, and you should be able to seemlessly transition all of the way to wot. Many manufacturers recommend a vacuum secondary carb because you can mash the throttle to the floor and the vacuum secondaries will slowly.open at a controlled rate for you. Mastering a double pumper takes practice, but done right, is faster than a vacuum secondary carb.

Sent from my moto x4 using Tapatalk
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 01-31-2019, 09:21 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 52
Posts: 17,517
Wiki Edits: 0

Thanks: 268
Thanked 3,158 Times in 2,648 Posts
Yep, control your gas foot or else you need to add either a stall converter or deeper rear end gears(or maybe both).

Double Pumpers are race bred carbs, not really intended for street use at lower rpms, as you are finding out.
Reply With Quote Quick reply to this message
 
  #4 (permalink)  
Old 01-31-2019, 11:38 PM
Registered User
 
Join Date: Apr 2007
Posts: 12,399
Wiki Edits: 0

Thanks: 181
Thanked 1,142 Times in 912 Posts
Quote:
Originally Posted by coltonbily View Post
So in my c20 Iíve got a 750 double pumper on top of a big block thatís been bored .60 over and has a mild comp cam with the cast iron 781 heads ported with 2.19 inch valves and big valve springs. (Not sure of all the specs my engine guy didnít tell me). But basically the truck runs fine and moves pretty good on the 2 primaries but if I open the secondaries up on the low end the motor surges ALOT and sounds horrible, the RPMS will go up and down and up and down until you let off the pedal. Now if I open the secondaries once Iíve got a little speed say 30mph the motor acts fine and takes off pretty good. Iíve got a Holley red fuel pump so fuel pressure couldnít be an issue and the total timing is set to 35. Anyone got a clue as to whatís going on here?


Sent from my iPhone using Tapatalk
You have discovered the effects of stalling the mixture low inside the intake. What you're doing is providing more flow capacity than the engine can use at that moment. The gas laws tell us that when flow area is increased faster than demand, the flow looses velocity. When that happens the reversion forces become greater than the induction forces resulting in a net loss of cylinder fill of mixture. The burn slows to a point where you may get backfire, certainly enough that you get herky-jerky loading on the crankshaft.

The solutions are a vacuum secondary carb, this is exactly the condition they are intended to remedy, stiffer overall gearing to use the higher RPMs to get the flow velocity up to where reversion doesn't blow mixture out of the cylinders, or a manual gear box that can easily be dropped a gear to achieve that same end. Automatics are not big fans of being operated like that, not that they aren't but with it comes a much shorter life span; what's in your wallet?

Bogie
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 02-02-2019, 07:50 AM
Registered User
 
Join Date: May 2008
Location: ohio
Posts: 3,595
Wiki Edits: 0

Thanks: 267
Thanked 498 Times in 422 Posts
Are you running an automatic transmission? What rear gearing do you have and then on to your carb what are the color of your secondary pump cam and what size squirter are you using? Also your carb is a 4 corner idle and are all four of your idle mixture screws all even on all four sides?

I have ran a holley double pumper and know exactly what your talking about and with them you have to have a vehicle and engine that will pick up rpm very fast or they will be a pain to tune on the secondary side as they will end up bogging or spit and sputter till you pick up enough speed. Some people when then put them on wrong applications will end up putting a 50cc secondary pump shot to cover the bog but it ends up a band aid fix.

It might be able to be get rid of with the right squirter and pump cam.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 02-02-2019, 09:15 AM
coltonbily's Avatar
Registered User
 
Join Date: Oct 2017
Posts: 41
Wiki Edits: 0

Thanks: 9
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by eric32 View Post
Are you running an automatic transmission? What rear gearing do you have and then on to your carb what are the color of your secondary pump cam and what size squirter are you using? Also your carb is a 4 corner idle and are all four of your idle mixture screws all even on all four sides?



I have ran a holley double pumper and know exactly what your talking about and with them you have to have a vehicle and engine that will pick up rpm very fast or they will be a pain to tune on the secondary side as they will end up bogging or spit and sputter till you pick up enough speed. Some people when then put them on wrong applications will end up putting a 50cc secondary pump shot to cover the bog but it ends up a band aid fix.



It might be able to be get rid of with the right squirter and pump cam.


Yep th400 tranny and I have 4.10 gears in the back where can I find the accelerator pump cam??


Sent from my iPhone using Tapatalk
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 02-03-2019, 09:37 AM
Registered User
 
Join Date: May 2008
Location: ohio
Posts: 3,595
Wiki Edits: 0

Thanks: 267
Thanked 498 Times in 422 Posts
There are two pump cams. One on the primary side and one on the secondary side. I have never come across a 750 holley double pumper before so I need to ask what size shooters do you have and what color pump cams? They are on the throttle shafts at the side. Primary is on driver side and the secondary is on the passenger rear side of the carb. I need to know that to help you out more.
Reply With Quote Quick reply to this message

Recent Hotrodding Basics 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:
Insurance
Please select your insurance company (Optional)

Log-in



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
4 barrel has less explosive acceleration than my orig. 2 barrel drumharddriveanova Hotrodding Basics 16 11-20-2005 07:42 PM
2 barrel to 4 barrel swap super_silent Engine 18 12-30-2004 10:15 PM
Can I replace two barrel carb with four barrel on chevy 305 maverick19642001 Engine 6 02-20-2004 10:00 AM
Two barrel carb maifold with a four barrel on top 73 Nova Guy Engine 9 11-29-2003 08:28 PM
Difference in 2 barrel and 4 barrel Meangreen Engine 5 07-19-2003 09:31 AM


All times are GMT -6. The time now is 03:34 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.