I have a 72 ,350,4 speed all stock motor. I need the the number for a bolt on holley carb that fits the q-jet manifold. ive had the q-jet rebuilt and today I wanted to take a ride. I got going down the highway about 70 and kicked down the pedal and it just fell flat and wanted to die out. when I first got the carb back it ran great absolutely no problems. ive noticed that this falling flat bit is getting more frequent. the timing is set, all new plugs, the vacuum advance works. I replace the distributor with an hei. I can crank the car with no problem. it fires up and idles but has just a bit of bump in it . very very minor. no poping from the exhaust. plugs are burning clean. im pulling about 18-19 lbs of vacuum. maybe the fuel pump is bad? i just want to get some thing I can work on. thanks for any info peter

 

Have you ever replaced the fuel filter sock in the tank?

 

I would check the accelerator pump in the carb and the fuel filter.If you take the air cleaner off and look down the carb and pump the throttle do you see gas being squirted into the carb from the accelerator pump?

 

I'll 2nd the fuel filter.....Had about the same issues with mine except it just went dead....Found plenty of trash in the tiny filter....No problems since....If your tank has any trash in it at all and makes it's way to that tiny filter it will get stopped up quick.

Brian

 

ordered a new tank filter last night, plus the new rubber oring and retaining cam.

 

I was talking about the one in the carb itself....But changing the one in the tank is also a good idea.

Brian

 

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.

 

I was Chevrolet Line Tech in 1977 doing Corvette work.
We "REMOVED" the check valve from the fuel filter.
This was approved by Chevrolet at the time.
I've never installed one of these filters with that check valve since.

 

The Holley 4175 650CFM vacuum secondary carb is a direct Q-jet replacement...I have this carb on my 78 L-82 stock and now on my rebuilt L-82 355, roller cammed, AFR aluminum heads, 10.2:1 compression engine...work great..just had to up the primary jet size a bit to compensate for the big HP increase..runs perfect..on the car since 1986!

 

ive ordered the new parts for the fuel tank filter . so tomorrow im going to drain the tank and see what it looks like. im also going to check the carb filter as its brand new and may have a check ball. while im waiting for the parts to arrive im going to replace the fuel pump and I have the new lines that go from the fuel pump to the tank line. my very last resort will be to change the carb. I really like my q- jet and like I said it was working great when I got it back from the builder. hes already rebuilt 3 q-jets for me and they all worked great. ill post what I find. thanks peter

 

I have a spreadbore Holley 850dp that came off my vette. Will sell it to you for $100 +shipping if you are interested. Should bolt right up. Might be a bit big for your 350 though I suppose.

 

Holley 4175. Bolts right on. I have been running it for 4 years never an issue. I bought a remanufactured one from Holley. Looked brand new. You won't be disappointed

 

https://rspimages.holley.com/0-6210.jpg?width=583

The Holley spread bore is also known as an 4165 / 8555s, which works great. Get the electric choke.

The new number is Holley 0-6210 , a 650 cfm spread bore carb, that has all the ports for the car. You may need a bracket for the cable.

 

The early Qjets had a tiny fuel filter inside of a short inlet fitting. The 1977 or so version used a longer filter which flows slightly better. The long pattern inlet fitting was used on the full size cars. It can be swapped in with no other modifications other than a tweak to the fuel pipe. Dunno where you'll find one today. Maybe a boneyard. Or Lars Grimsrud.

 

The Holley 4175 and 4165 are both spread bore carbs but the 4175 has vacuum secondaries (direct Qjet replacement due to the vacuum secondaries) and the 4165 has mechanical secondaries.

 

I think you got that backwards. The QJET has mechanical secondaries.


If you push your throttle pedal all the way down, the secondaries open, even if the car is off.

 

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.

 

The Qjet is an on demand carb that only allows the secondaries to operate if there is sufficient air flow through the carb-similar to a vacuum secondary carb. If there is not sufficient air flow through the carb the secondaries will open mechanically but there will no fuel metered through the carb unlike a mechanical secondary Holley: Please read here:

https://en.wikipedia.org/wiki/Quadrajet

"Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage; the primaries open before the secondaries, and use on-demand air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel.

Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. Thus the Quadrajet acts like a vacuum-secondary carburetor and only delivers more fuel as it is needed"][/B]

 

If the carb ran fine before, and is gradually starting to fall on its face under power, you have a fuel supply problem - not a carb problem. Change the fuel filter and check all your rubber fuel hose sections to make sure you don't have a kinked line.

Lars

 

The argument never seems stops on whether or not the Q-Jet is a Vacuum Secondary carb. Answer: It's not. Here is my explanation of the Q-Jet secondary system - just the facts - as contained in my Q-Jet Setup Paper:

Technical Explanation: The Q-Jet Airvalve and Secondary Operating System

Lots of misunderstandings on the operation of the secondaries on a Q-Jet carb.

To understand the operation of the Q-Jet, first you have to understand what a “vacuum secondary” carb is and how it functions.

A “vacuum secondary” carb is a carb whose secondary throttle blades are opened by the force created by venturi vacuum in the primary side of the carb. The vacuum created in the venturi of a carb is directly proportional to the mass flow of air passing through the venturi. This venturi vacuum is completely independent of manifold vacuum, which is non-existent at wide open throttle (WOT). A vacuum secondary carb has a little hole drilled right into the venturi on the primary side, and this venturi vacuum is fed to a spring loaded diaphragm attached to the secondary throttle shaft. Once airflow on the primary side approaches the maximum flow capability of the primary venturi, the vacuum will be high enough to overcome the diaphragm’s spring pressure, and the secondary throttle is opened by the primary venturi vacuum. This is a vacuum secondary carb.

The Q-Jet does not have any vacuum holes drilled in the primary venturi, and there is no vacuum diaphragm attached to the secondary throttle shaft. The Q-Jet is not a vacuum secondary carb – it is mechanical carb with a secondary airvalve control.

But vacuum sucks the airvalve open, and the airvalve is connected 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 front yard. As the wind starts blowing, the door gets pushed open. The harder the wind blows, the more the door gets pushed open. Do you have a vacuum on one side of your front yard sucking the door open..? Of course not – the pressure is the same all over your yard. The force opening the door is the mass flow of air pushing the door open. There may be a low pressure area in Texas that is causing the air to move, but Texas is not “sucking” the door open – mass air flow is pushing it open, and the door is responding to the actual total mass air flow being pushed through it. The Q-Jet operates the same way: At WOT, there is no vacuum in the manifold – the manifold is very close to atmospheric pressure (a correctly-sized carb will cause the manifold vacuum at WOT to be at about 0.5” Hg, which is nothing). So the force opening the airvalve is the same as the wind pushing your yard-mounted screen door open: 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” the secondary airvalve is actually the choke pulloff. It is also connected to the airvalve 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 to prevent excessively sudden opening of the airvalve: The longer the airvalve is delayed in its opening, the bigger “fuel shot” the secondaries get upon opening, thus preventing a secondary tip-in stumble. The pulloff merely allows a controlled opening rate of the valve, and is not a vacuum-operated control of the secondary throttle in any way. Think of the pulloff as the damper cylinder on the screen door: The damper cylinder does not open the screen door – it merely controls and dampens its opening rate.

Thus the Q-Jet is not a vacuum secondary carb. It is an airvalve-controlled mechanical secondary carb with a damper. The airvalve is not operated by vacuum – it is operated by mass flow. The airvalve’s opening rate is controlled and dampened by the loss of vacuum signal – not by the creation of any vacuum.

For some interesting reading on the 3 different types of engine vacuum, feel free to drop me an e-mail request for my “Engine Vacuum Explained” tech paper.