I have a couple of standard questions. I have a 1978 Anniversary Edition L82, automatic. I have had the carburetor (Rochester), gas tank, fuel pump, and filters replaced very recently.

However, since having this done when cold the engine idles incredibly fast and bogs down when giving it gas.

Once the motor warms up, the idle is fine. It still bogs when pressing the gas.

If I lightly step on the gas or push the pedal to the floor, it does not have this issue.

Other than replacing my current mechanic, what suggestions do you have for fixing this issue.

 

Choke coil is set too lean, fast idle set too high. Bog with moderate throttle is probably malfuncting accelerator pump, or out of adjustment.

 

The fast idle can be adjusted by the screw on the side of the carb.

The bog might mean the accelerator pump needs to be replaced.

 

Fast idle adjustment screw is on the right side of the carb.

The bog is often caused by the tension on the air valve spring being too low. Check the adjustment and it should be in the range of 3/4 turn to 1 full turn. Tighten in 1/8 turn increments and recheck.

Lars procedures are very helpful - email Lars for his Quadrajet papers...
V8FastCars@msn.com

 

First check your timing and vacuum advance and make sure they are working properly!

I wrote this for racers but it all applies.


Troubleshooting Tips


The following information is to help you resolve common problems that are often blamed on poor carburetor performance or fuel system problems.
More often than not the lowly carburetor gets blamed for everything, from poor idle and performance characteristics to a flat tire. Looking into this we’ll touch on all of the complimenting factors surrounding what it takes to make any four-cycle engine run at peak potential.


Before we get started
The area where the most ignorance lies is the area that is most likely to be blamed as faulty. When in reality the culprit is usually something else ninety-nine percent of the time.
This something else more often than not exhibits the same symptoms as fuel delivery or carburetor problems. So if you have a firm understanding of how all this stuff works together, you will be able to troubleshoot in the shop and at the track.

My Grandpa always told me “Experience is a hard teacher son but there is none better”

If you don’t have the experience to accurately troubleshoot some of these things don’t fret, there’s nothing in the world wrong with being ignorant on a given topic. Being ignorant just means you haven’t been exposed to the information needed to solve your particular problem, although Choosing to remain ignorant puts you in a whole different category.
The new category I speak of is called STUPID. As one of my favorite comedians Ron white says “YOU CAN”T FIX STUPID”. With the previous statement in mind …. The fastest way to the winner’s circle is to work smart, work hard and to educate yourself. This education can come in many forms and from many avenues. Some things you learn are not in any book you’ll ever read but come from the people you come in contact with. When you’re around people that have this type of knowledge and experience pay attention. Knowledge is the only thing you can take from another person that they will never miss, and never realize you have taken and in most cases are glad to share just for the asking.


No big Mystery
The carburetor is nothing more than a fuel-metering device. Metering in the sense that the carburetor allows a measured amount of air controlled by your right foot through the venturi’s, to a measured amount of fuel, which is metered by the jet’s and air bleed sizes. This is all based on the volumetric needs of your engine.
If your carburetor has been properly calibrated for air fuel ratio and dyno tested for maximum performance and you keep the exterior and all associated linkages clean and lubricated and keep your air and fuel supplies properly filtered. Your carburetor should perform flawlessly for several seasons before routine maintenance is required.

Carburetor basics
The following is a basic look at carburetor design and function. While the carburetor ap-pears to be a very sophisticated piece of racing equipment it contains the same basic fuel circuitry that the go-kart you rode as a kid did. This information should help any Racer improve his trouble shooting skills as well as his racing program. All carburetors
Posses in some form of design, the following components.



Idle Circuits
This portion of the carburetor feeds fuel to the engine when it’s at idle speeds, of say less than 1200 RPM for A lumpy camshaft. The reason I limit this example to a given RPM is because other circuits start to come into play above an acceptable idle RPM. Below these desired RPM’S, the carburetor should not draw fuel from any of the other following circuits if it does it will cause performance issues and or idle problems. If your carburetor has not been prepared for a longer than stock duration camshaft it will in most cases cause loading of sparkplugs and poor quality idle characteristics.

Air Bleeds
Air bleeds are present in ALL fuel delivery circuits in a carburetor whether it’s the Idle, Primary or Secondary fuel circuits. This excludes supplemental enrichment circuits, And transition circuits. Supplemental enrichment circuits, are circuits such as the accelerator pump that’s intended to deliver a full stream of un-areated fuel. Transition circuits are just what they sound like they supply fuel to your engine, when for instance the carburetor goes from running on the idle circuit to the primary venturi if there were no transition fuel circuits you would experience a flat spot or a stumble in between the ending of one circuit and the starting of the another because there would be no overlapping fuel to cover it up.
The functioning of air bleeds can be compared to drinking water through a straw that has an extra hole in it, besides the two holes it was originally designed with at either end. Trying to drink with our newly designed straw, makes quenching your thirst a little more eventful, in fact just getting the water to the top of the straw can be a challenge by itself. When you finally do manage to wet your lips, you don’t get a mouth full of High Quality H2O. Instead you get a frothy combination of air and water that will barely wet your lips.

What do air bleeds have to do with your carburetor?
Well Physics 101 says, Air and liquids have different compression characteristics. We all know air is compressible but liquid is not. Thank goodness or we would all look like the Flintstones when we got to the end of the straightaway, due to the lack of brakes.
The air bleeds come in to play when we calibrate our fuel circuits for the desired air fuel ratio. All of the carburetor’s circuits can be tailored to work in conjunction with each other employing the use of air bleeds. This allows your carburetor to deliver a consistent air fuel ratio throughout the entire RPM range of your racing engine.
Now that you have an understanding of air how an air bleed functions and know that most all the circuits in a carburetor contains them. You can see how air bleeds can be used to tailor the air fuel ratio in each fuel circuit, thus giving us an air fuel balanced carburetor.

Accelerator pump
The Accelerator pump is designed to help the carburetor transition the engine from running on the idle circuit(s) to the engine running on the primary venturi and drawing fuel through the primary side main Jets.
Some carburetors may have only one pump circuit, but will have other circuits that will help with the transition of fuel from circuit to circuit, these other circuits are called enrichment circuits.
Not all Fuel flow from enrichment circuits, are powered by mechanical means. Like fuel from the accelerator pump circuit, but are powered by the vacuum generated by the venturi effect caused by air flowing by a strategically placed hole in the carburetor casting that is ported to a fuel supply within the carburetor.


Air fuel Ratio
The air fuel ratio is set by the JET size versus the size of the AIR BLEEDS. A carburetor has air bleeds in each of the four barrels as well as in the idle circuits.
The only way to properly calibrate (size the JETS versus the air bleed sizes) a carburetor’s air fuel ratio is with an O2 (oxygen sensor) and an Engine Dynamometer.
The reason I say you need both an O2 sensor and a Dynamometer is I have seen car-buretors exhibit the correct air fuel ratio and not make anywhere close to the power and torque numbers that the given engine combination is capable of.

This calibration can also be done through track testing and the reading of spark plugs but be for-warned unless you have experience in this area it’s a long and arduous process that requires some one with a good knowledge of plug reading.
If your faint of heart you’ll never get your carburetor lean enough to make maximum power.
I say this because maximum horsepower is made at the point the porcelain on the tip of the spark plugs are virtually clean (white in color) The real story reading plugs is at the base of the porcelain more on that later…….

Carburetor development is much more involved than what was previously stated but I gave you the meat of what it takes to do this in order to help you develop your trouble shooting skills.


FWIW, Jimbo

 

To everyone, thank you. The accelerator pump was also replaced with the new carb and gas tank. I found out the hard way that my mechanic isn't up to older cars. Many parts and thousands of dollars later, it is running worse. Now at least it seems to be adjustment issues more than part issues.

Found this one and got it mostly fixed. Will have to play with it a bit more for fine tuning, but definitely heading in the right direction.

How do I check/fix this?

 

Best way is to read a good carb book on the Q-jet. But, it's on the right side of the air valve (secondary flaps). Loosen the allen head screw slightly so that you can turn the common screw that it pins. If you watch the coiled spring closely (best to have the carb off and upside down, or else use a mirror), as you turn the screw CCW, the "j" end of the spring will come clear of the lever pin which moves the flaps.

Once you've figured out how it works, slowly turn the screw CW until the "j" of the spring barely contacts the pin and note the orientation of the screw you're turning. Now turn the screw an additional 3/4 of a turn and that's your first test setting. It sounds complicated, but isn't - it's just hard to explain without a nice picture.

 

Air Valve Tension:

In Lars' papers he discusses it in 4) determine secondary rod size.

If you have a 78 Chasis manual there is a pic on 6c-66

It's in the old Chilton book also. (74-81 repair manual)

Make sure the car is warm and the choke is off. The allen wrench you need is 3/32.

I found out about the tension adjustment when I went on a dyno last year. After two pulls, the tech said something didnt feel right, so on the 3rd pull we took off the air cleaner. At 5000rpm & 104 mph, the air valve barely opened. The tension was too tight.
Did some research and found the adjustment. Funny that I watched an 80 corvette do a pull after me and he had the same problem. The air valve did not open.

Once I made an adjustment I tried it out on the highway and got a kick in the butt response. Knowing what I read this was still not correct. Readjusted the air valve 1/8 turn and tried again. Just a gradual increase in power. Success

Here are two sites to read.They will give you an idea on what to look for.

Setting the air valve tension: http://www.73-87.com/7387garage/drivetrain/myqjet.htm
Dont release the screw driver from the slot and read the 2nd paragraph about the carb being on the car.

http://www.thirdgen.org/quadrajet



Good luck.

 

Just because the carb was replaced does not mean that the accelerator pump has not failed.
With the engine and air cleaner off look into the carburetor bores and briskly open the throttle valve, a full stream of fuel should emit from both of the pump jets and strike near the center of the venturi's.

When does your car bog? When you try to accelerate from idle or when you floor it at 30 MPH?

 

If I lightly push the accelerator down it is fine. If I floor it is fine. The problem comes in between.

When I press the pedal about 1/4 of the way down it bogs. Cold engine or warm engine doesn't make a difference. Whether just starting from an idle or traveling at 60 mph makes no difference.

The bog down always happens in the same spot.

If I press the pedal down or through that spot it runs fine.

 

Is the accelerator pump arm (green lever) connected to the short-side hole on the arm?

 

That's kind of odd and it may not be a carb problem. Take jimboscarbs advise (his first sentence) and check the timing and vacuum advance and let us know what you find.

There is a Sticky (at the top) if you need help with that.

 

Primary Bogs
you can experience Two differnt types of bogs when it comes to carburetors on the primary side this would be off idle (initial throttle application) as you described and can be caused by the following things.

Timing
Factory Chevy distrubutors are notorious for the vacuum advance siezing up, check static timing and then check the vacuum advance to make sure it is working properly if you can't find out how to check these let me know and I will write something up but I think there is a thread in regards to this already.

Acellerator pump not working or incorrectly adjusted
If there is no initial squirt when you first pick up the throttle ( you will be able to see this looking down the primary side of the carburetor)
The cup on the pump assembley may be torn or bad or the check valve adjacent to the acellerator pump well is leaking by. This is evident if you don't get a squiirt at initial throttle application but do start to get one at about 3/8' throttle movement. The squirt should be present the instant you pick up the throttle. If it's working and delivering fuel as it should you can adjust the amount delivered by moving the rod to the inner or outer hole.

Inner hole = more fuel for a shorter duration

Outer hole = less fuel for a longer duration.
There are some other things you can do to adjust this but you have to disassemble the carburetor to do it.

A flat stumble is like the ignition has been temporarily cut off this is lack of fuel

A blubber or studder type stumble is too much fuel

Float Level
A float level being to low will also cause the same problem because there is only so much vacuum generated by the primary booster, a low bowl fuel level will take longer to be drawn through the venturi because a low bowl level requires more vacuum to deliver the fuel thus taking it longer to start delivering fuel. (this is why fuel bowl level affects fuel curve) Therefore the accelerator pump circut will not deliver enough fuel to cover up the time before the primary booster starts delivering fuel and idle circuits become ineffective. In other words the fuel demand becomes too great before the primary boosters start delivering fuel and primary enrichment circuit cannot supply enough fuel by itself and you get a resulting stumble or bog.

The way you need to trouble shoot any carburetor problem is as follows because each circuit is an individual circuit but will always overlap the precedding circuit as to not cause any flat spots or stumbles.

So use this a as guideline and sequence in finding those gremlins. Fuel and ignition problems are hard to diagnose for someone who has not seen these problems before. I learned these the hardway by building a bunch of bad carburetors before I built any good ones LOL!
All carburetors works in the following order so this is how it should be diagnosed. Determining which circuit is at fault is the hard part, fixing it is the easy part.

Do these test with a warm engine because the engine relies on heat generated in the manifold to help put fuel back in suspension that has dropped out due to lack of velocity in the manifold under low engine demand and or poor or inefficient manifold design
.
1. idle circuit
2.primary transition circuit
3.primary booster circuit
4.secondary enrichment circuit
5.secondary booster circuit

Dagnose each one of these individually and know that each circuit overlaps the preceeding circuit.

I did not go into the secondary circuit but will if needed. It will take a little longer to type up so if the things listed above do not remedy the problem let me know

FWIW, Jimbo

 

I totally agree with you on this.

All of these problems began with the new fuel system (carb to gas tank). I believe that the parts are fine (as far as I can determine) and it is more of a mechanic problem. Now I will focus on finding a new mechanic and see if they can solve this problem.

I appreciate everyone's help on this.

 

I would suggest you try driving the car with the EGR valve disconected , if the problem goes away the EGR valve has too much flow for the engine. I would also suggest checking the air/fuel mixture, it could be too lean at part throttle. We are seeing a lean shift in the air/fuel mixture on many of the cars we tune since we have 10% ethanol in the gasoline we have in California. If you are interested there is a couple of tuning air/fuel mixture and ignition timing curves articles for today's gasoline at www.olescarb.com

I hope this helps Henry @ olescarb