need a bolt on replacement carb
Le Mans Master
Joined: Oct 2007
Posts: 7,455
Likes: 977
From: RI, Now Franklin/Nashville TN
Yes, Lars you are the expert.
What I was saying and what is published in my quote above is that Qjet ACTS more like a vacuum secondary carb than a mechanical secondary. The Qjet is neither but closer to the vacuum secondary one.
What I was saying and what is published in my quote above is that Qjet ACTS more like a vacuum secondary carb than a mechanical secondary. The Qjet is neither but closer to the vacuum secondary one.
Melting Slicks
Joined: May 2011
Posts: 2,753
Likes: 119
From: Springfield Missouri
you haven't changed the fuel pump lately, really the "S" hose that feeds the fuel pump from the fuel tank line, using a regular piece of fuel line, it will pinch off when it gets warm enough and then no fuel to the fuel pump and then carb, ask me how I know.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
Check the inlet filter in the carb. They plug up very easily. Also Napa was making those inlet filters with a small check valve at the inlet of the filter to prevent drain back. They also restrict flow badly or jam up altogether. The engine starts, runs and idles fine but under any sort of heavy acceleration the carb runs out of fuel due to the restriction.See below. The one on the right has the check valve to prevent drain back and they do not work.
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
Yup... The one below is the same one as above with the check valve removed. You can see the ball inside is bigger than the inside diameter of the valve body pushing it out causing the ball to jam up. Run through the gears and it ran out of fuel at the top of second gear. Might have been a good idea but a really poor design and/or manufacturing standards.
Thread Starter
Instructor
Joined: Jul 2014
Posts: 163
Likes: 2
well I found the problem!!! a couple months back i pulled the carb because the front plug was leaking. not much but I wanted to fix it . so I drilled and tapped it and put the 1\4 npt plug in. no more leaks. but I did some how manage to put the filter in backwards. my fault. im not one to be hitting the wot very often so just easing through the gears there was no problem. I checked the sock in the tank and its pretty clean, but I have all new parts so im going to run most of the gas out and change it....
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
well I found the problem!!! a couple months back i pulled the carb because the front plug was leaking. not much but I wanted to fix it . so I drilled and tapped it and put the 1\4 npt plug in. no more leaks. but I did some how manage to put the filter in backwards. my fault. im not one to be hitting the wot very often so just easing through the gears there was no problem. I checked the sock in the tank and its pretty clean, but I have all new parts so im going to run most of the gas out and change it....
Changing the sock is still a good idea. They disintegrate after 40 years, and cause havoc. I changed the one in the 81 to beat the curve, and although it looked okay, it fell apart in my hand when I got it out.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
well I found the problem!!! a couple months back i pulled the carb because the front plug was leaking. not much but I wanted to fix it . so I drilled and tapped it and put the 1\4 npt plug in. no more leaks. but I did some how manage to put the filter in backwards. my fault. im not one to be hitting the wot very often so just easing through the gears there was no problem. I checked the sock in the tank and its pretty clean, but I have all new parts so im going to run most of the gas out and change it....
Burning Brakes
Joined: Mar 2016
Posts: 1,008
Likes: 163
Wrong!!
NO Rochester 4 BBL. carburetor ever had mechanical secondaries....Secondly, it doesn't matter what the secondaries do on a Q'jet when the engine if OFF, and there's no vacuum to the carb.
Let me edit this, as I haven't worked on a Q'jet since the early 90s.......the secondary throttle blades will indeed open, but the "secondary air valve" located at the top of the air horn, will not open, unless there's sufficient a sufficient air flow demand.
NO Rochester 4 BBL. carburetor ever had mechanical secondaries....Secondly, it doesn't matter what the secondaries do on a Q'jet when the engine if OFF, and there's no vacuum to the carb.
Let me edit this, as I haven't worked on a Q'jet since the early 90s.......the secondary throttle blades will indeed open, but the "secondary air valve" located at the top of the air horn, will not open, unless there's sufficient a sufficient air flow demand.
WRONG! No matter whether the engine is on or off, the secondaries will open. The only thing that won't open is the air door on top of the carb. I think everyone on this forum would agree that Lars is the guru on quadrajets. Below is an excerpt from his carb tuning paper:
"
Lots of misunderstandings on the operation of thesecondaries on a Q-Jet carb.
To understand the operation of the Q-Jet, first you have tounderstand what a “vacuum secondary” carb is and how it functions.
A “vacuum secondary” carb is a carb whose secondary throttleblades are opened by the force created by venturi vacuum in the primary side ofthe carb. The vacuum created in theventuri of a carb is directly proportional to the mass flow of air passingthrough the venturi. This venturi vacuumis completely independent of manifold vacuum, which is non-existent at wide openthrottle (WOT). A vacuum secondary carb has a little hole drilled right intothe venturi on the primary side, and this venturi vacuum is fed to a springloaded diaphragm attached to the secondary throttle shaft. Once airflow on the primary side approachesthe maximum flow capability of the primary venturi, the vacuum will be highenough to overcome the diaphragm’s spring pressure, and the secondary throttleis opened by the primary venturi vacuum. This is a vacuum secondary carb.
The Q-Jet does not have any vacuum holes drilled in theprimary venturi, and there is no vacuum diaphragm attached to the secondarythrottle shaft. The Q-Jet is not avacuum secondary carb – it is mechanical carb with a secondary airvalvecontrol.
But vacuum sucks the airvalve open, and the airvalve isconnected to a vacuum diaphragm, so it’s vacuum operated, right?
Not really. Imagine this: Take a spring-loaded screen door and set it up right out in your frontyard. As the wind starts blowing, the door gets pushed open. The harder the wind blows, the more the doorgets pushed open. Do you have a vacuumon one side of your front yard sucking the door open..? Of course not – thepressure is the same all over your yard. The force opening the door is the mass flow of air pushing the dooropen. There may be a low pressure areain Texas that is causing the air to move, but Texas is not “sucking” the dooropen – mass air flow is pushing it open, and the door is responding to theactual total mass air flow being pushed through it. The Q-Jet operates the sameway: At WOT, there is no vacuum in the manifold – the manifold is very close toatmospheric pressure (a correctly-sized carb will cause the manifold vacuum atWOT to be at about 0.5” Hg, which is nothing). So the force opening theairvalve is the same as the wind pushing your yard-mounted screen dooropen: It’s mass flow pushing it open.This is not a vacuum operated carb. There is no vacuum in the manifold at WOT,but there is plenty of mass airflow.
The diaphragm on the side of the Q-Jet “controlling” thesecondary airvalve is actually the choke pulloff. It is also connected to theairvalve to hold it firmly closed when manifold vacuum is high. When the engine is placed in a power condition(WOT or low manifold vacuum), the diaphragm relaxes at a controlled rate toprevent excessively sudden opening of the airvalve: The longer the airvalve is delayed in itsopening, the bigger “fuel shot” the secondaries get upon opening, thus preventinga secondary tip-in stumble. The pulloffmerely allows a controlled opening rate of the valve, and is not avacuum-operated control of the secondary throttle in any way. Think of thepulloff as the damper cylinder on the screen door: The damper cylinder does not open the screendoor – it merely controls and dampens its opening rate.
Thus the Q-Jet is not a vacuum secondary carb. It is an airvalve-controlled mechanicalsecondary carb with a damper. Theairvalve is not operated by vacuum – it is operated by mass flow. The airvalve’s opening rate is controlled anddampened by the loss of vacuum signal – not by the creation of any vacuum.
"
EDIT: Oops should have read farther down the thread to see that Lars had already responded with the same answer.
Last edited by mobird; Jan 19, 2017 at 08:12 AM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski