JEGS 15802 Carburetor Installation Instructions TCS5139 X 555

User Manual: TCS5139-X

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1-800-345-4545 jegs.com

Installation Instructions for 15800-05

Remanufactured Universal Performance Quadrajet Carburetors

15800, 15801, 15805 with Electric Choke

15802, 15803 with Divorced Choke

15804 with Climatic Choke

Version 3.0, Updated 07/01/05

15804

Table of Contents

1Thank You For Your Purchase

2A Word About Universal Carburetors

2Carburetor Fuel Curve Information

3Who To Call For Information

3Before You Get Started

3A Last Piece Of Advice

3Carburetor Removal

4Before You Start The Installation Procedures

4Fuel System Purge

5Carburetor Installation

5Electric Choke Information

6Idle Adjustments

6Parts And Systems To Consider

7Tuning Guide

8Troubleshooting Guide

10 Warranty Information

Thank you for purchasing a Jeg's Performance

Remanufactured Carburetor…

All of our performance carburetors are 100%

dynamically wet flowed before final packaging to be

shipped to our many customers. We wet flow our

carburetors to satisfy our final quality control reports

and standards. We also make the final fuel flow

adjustments to insure your carburetor is within a band

of adjustment that allows you to start and run the

engine in an effort to make the absolute final

adjustments according to the specific needs of your

engine package. Because we do not control

atmospheric conditions, altitudes, or vehicle package,

this carburetor will finally rest upon, all final

adjustments such as idle RPM, idle mixture, and

throttle “snap” response made by you.

A Word about Universal Calibrated Carburetors….

With this line of performance remanufactured

carburetors, we turn all of our expertise to designing a

carburetor assembly with a specific fuel curve that can

service the needs of a wide variety of engine packages.

However, with the universal application declaration of

this design there are engine packages in which this

carburetor assembly could require tuning and re-jetting

to properly service the fuel requirements of your

particular engine assembly. You may also own an

engine package that may require the same fuel

requirement but need custom tailoring of the dynamics

of the included fuel curve.

The vehicle that your engine package resides in has a

lot to do with the fuel curve that is in place within your

performance carburetor. For example, a 4000-pound

vehicle will require different tuning than a 2000-pound

vehicle with the same engine package using the same

style of carburetor. In the case of the remanufactured

Holley 4 BBL calibrations, please see the explanations

below for jetting and power valve selections associated

with the fuel curve calibrations of our carburetors.

Off Road Labeling and Emissions Standards

This carburetor assembly is not designed nor is it

represented to operate within the emission controlled

applications past 1974. We attach a label to the outside

of our carburetor container to insure that this carburetor

assembly is not used in violation of any or all of the

Clean Air Act. If you have any questions concerning the

applicability of this carburetor to your individual

application, please call our tech line at 1-800-345-

4545.

Carburetor Fuel Curve Information

NOTE: Please read this manual completely before

attempting to re-tailor your fuel delivery curve included

inside of this carburetor assembly. All aftermarket parts

and accessories manufactured for use in a Rochester

Quadrajet Carburetor will work in your Jeg's

Remanufactured Performance Carburetor.

The fuel curve information for the 15800 is as

follows:

This fuel curve will properly service the fuel

requirements for a small block engine up to 400 cubic

inch engines. It can supply enough fuel to support

engines up to 385 H.P. and RPM levels of 6000. The

carburetor can be jetted for higher fuel requirements to

reach higher horsepower and RPM levels. The vehicle

application includes vehicles from 2500 lbs. up to 3000

lbs. A heavier or lighter vehicle may require re-

calibration. Camshafts up to 228 degrees of duration at

.050” lift and dual plane or single plane manifolds are

also compatible. These carburetors are not designed to

be used in dual carburetor applications and will require

dramatic enrichment modifications when used with

nitrous. Main Jet .076, Metering Rods: Primary 130x49,

Secondary 130x102.

The fuel curve information for the 15801 is as

follows:

This fuel curve will properly service the fuel

requirements for a big block engine up to 460 cubic

inch engines. It can supply enough fuel to support

engines up to 430 H.P. and RPM levels of 5500. The

carburetor can be jetted for higher fuel requirements to

reach higher horsepower and RPM levels. The vehicle

application includes vehicles from 3000 lbs. up to 3600

lbs. A heavier or lighter vehicle may require re-

calibration. Camshafts up to 228 degrees of duration at

.050” lift and dual plane or single plane manifolds are

also compatible. These carburetors are not designed to

be used in dual carburetor applications and will require

dramatic enrichment modifications when used with

nitrous. Main Jet .078, Metering Rods: Primary 130x49,

Secondary 130x101.

Fuel Curve Information for Part Numbers: 15802,

15803, 15804, and 15805

The fuel curve information and application notes

for the 15802 is as follows:

750CFM, 1966 through 1973 327 through 454 engines,

Chevrolet bowl entrance and linkage style, divorced

choke, compatible with EGR, Main Jet .069, Metering

Rods: Primary -36B, Secondary -DR.

The fuel curve information and application notes

for the 15803 is as follows:

750CFM, 1974 through 1978 350 through 454 engines,

Chevrolet bowl entrance and linkage style, divorced

choke, compatible with EGR, Main Jet .069, Metering

Rods: Primary -36B, Secondary -DR.

The fuel curve information and application notes

for the 15804 is as follows:

750CFM, 1975 through 1985 305 through 454 engines,

Chevrolet bowl entrance and linkage style, climatic

choke, compatible with EGR, Main Jet .073, Metering

Rods: Primary -50M, Secondary -DR.

Not for use with computer controlled vehicles.

The fuel curve information and application notes

for the 15805 is as follows:

750CFM, 1980 through 1989 305 through 350 truck

engines, Chevrolet bowl entrance and linkage style,

electric choke, compatible with EGR, Main Jet .073,

Metering Rods: Primary -50M, Secondary -DR.

Not for use with computer controlled vehicles.

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Camshafts up to 228 degrees of duration at .050” lift

and dual or single plane manifolds are also compatible.

These carburetors are not designed to be used in dual

carburetor applications and will require dramatic

enrichment modifications when used with nitrous,

Who to Call With Your Questions

For more information please call the Jeg's tech line at

1-800-345-4545, or consult one of the many quality

books and manuals available today from your local

bookstore, or parts supplier. When calling for

assistance, please have your application information,

carburetor part number, and specific issue ready for the

technician. Having your information prepared is a big

help when interfacing with our technicians.

Before you get started

Caution: This Universal Performance Carburetor has a

fuel curve calibration associated with a mildly modified

engine. As with all universal calibrations, some

adjustments and/or tuning is required for ultimate

performance. Please read this manual in its entirety

before attempting the installation, tuning, and usage of

this carburetor.

The following precautions will reduce the chance for

mishap, property damage, vehicular damage, AND

PERSONAL INJURY. Please follow these installation

instructions and safety precautions before attempting

the installation of your Tomco Universal Performance

Carburetor.

1. Disconnect the negative terminal of the battery and

store it in such a way as to insulate it from

contacting any other metal surface until your

installation is complete.

2. Have a fire extinguisher ready at all times during

the installation procedure.

3. Do not smoke or perform this installation

procedure close to any open flame or fire source.

4. Perform this installation procedure in a well lit, and

well-ventilated area. Stay away from confined

spaces in which flammable fluids or fumes can

collect.

5. Always protect your eyes, and your person from

any and all automotive chemicals such as

gasoline, engine cleaners, etc.

6. Start this installation procedure only after the

engine and the engine compartment is cool.

7. Never attempt working on your fuel system without

the proper tools.

A Last Piece of Advice….

In so many situations related to fuel system

modification and service, the fuel-mixing device often

receives the blame for many ills that are not fuel system

related. In fact, more often than not, any number of

underhood systems when compromised create “fuel

system like” failure signals. Systems like the ignition,

valvetrain, carb spacers, and component compatibility

systems all create conditions in a primary failure

situation that also fail the carburetor's ability to function

properly. If you have modified any other underhood

system, please check to verify proper operations of

each of those systems before you install your new

carburetor. If you have modified any of these systems

and you do not know how they can or will affect the

performance of your carburetor, please call our tech

line.

If after the installation of your new Jeg's carburetor, you

find that your performance expectations are not met, or

your vehicle is running worse that it did before, please

stop and call our technicians.

Anew carburetor is the worst diagnostic tool in

your toolbox.

Carburetor Removal

Take the time to create a clean and safe working area

before attempting this removal procedure. Have some

containers ready to collect and properly store all

components and small parts during the installation of

your new carburetor. In case of trouble do not destroy

your original carburetor box. Also, do not attempt any

other modification before completing and verifying your

new carburetor is functioning properly.

Step 1…. Negative battery Cable

Disconnect the negative battery cable before any work

on the fuel system starts.

Step 2…. Vacuum Lines

Carefully disconnect all vacuum lines from your

existing carburetor and mark each line with the

included labels.

Step 3…. Throttle Linkage and Cables

Use the proper tools and remove all clips, springs,

trans kick down assemblies, and cruise control cables.

Take a picture, create a sketch, or label each

component assembly during the disassembly period to

insure that you can properly re-install each of these

components and assemblies.

Step 4…. Disconnect the choke assembly.

There are several styles of choke mechanisms in use

on vehicles. Please consult your service manual if you

have questions concerning the style of choke you are

working with and familiarize yourself with any

specialized techniques associated with the removal of

the choke.

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Step 5…. Fuel lines.

NOTE: Before loosening any fuel line or performing

any work procedure on a fuel line, take time to

insure that your engine is cool. Validate that there

is no open flame, and the ventilation of your work

area is sufficient. Extinguish all sources of flame

and move the fire extinguisher into a ready

position that would allow easy access to it if a fire

were to break out.

5a… Place rags under the ends of all fuel lines that will

be disconnected to collect dripping and leaking fuel. Do

not allow for puddling or collection of fuel in, around, or

under the carburetor position upon or inside the intake

manifold.

5b... On applications with threaded fittings, use 2

wrenches and remove the fuel lines following proper

fuel fitting removal/tightening techniques. Please refer

to your shop manual for proper fuel line removal

techniques. When a barbed fuel line fitting has been

used, remove the fuel line clamp and disconnect the

fuel line.

Your new carburetor utilizes a threaded type inlet fitting,

therefore it requires some modifications to accept your

barbed fuel line system. NEVER use a barbed fuel

fitting in conjunction with an inverted flare inlet fitting.

5c... Remove the carburetor flange hardware, and lift

the old carburetor from the intake manifold. Once the

carburetor pad on the intake manifold is exposed and

open, please cover this opening with a rag therefore

keeping debris, and contaminants out of the inside of

your engine.

5d… With a gasket scraper and a cleaning agent,

remove all gasket material and sealant residue on the

carburetor-mounting pad of the intake manifold.

Note: Take care to insure no debris and/or small

parts fall into the intake manifold. Any foreign

materials or small parts allowed to enter the engine

can cause catastrophic engine failure.

Stuff to Look For Before Starting The Installation

Procedure Of Your Jeg's Carburetor…

You have now completed the removal procedure of

your old carburetor. Take this opportunity to double-

check the following areas of concern in preparation for

the installation of your new carburetor.

1. Check for any stripped or cross-threaded fasteners

on or around the carburetor-mounting pad of the

intake manifold.

2. Insure the carburetor-mounting pad of the intake

manifold is clean of old gasket material and

sealant residue.

3. Insure that the choke mechanism of the new

carburetor is compatible with the style of choke

mechanism currently in place on the manifold. Do

not install the carburetor if the choke mechanism

on the manifold cannot accommodate the choke

style of your new carburetor.

4. Check inside the plenum area of your intake

manifold to insure there is no debris or small parts

that could enter and damage your engine. Clean

all areas on and inside the plenum area of the

manifold before installing your new carburetor.

5. Inspect all rubber vacuum lines for signs of

cracking or old age. Replace these lines as

necessary.

6. If you are installing a new manifold at the time of

installation of your new carburetor, verify that the

linkage of the carburetor does not come in contact

with any portion of the manifold casting. Double-

check all fittings, fastener torque values, and

distributor positioning before attempting to run your

engine package.

Before You Run Your Jeg's Carburetor You Must

Purge The Fuel System To Insure A Clean Supply Of

Fuel To The Carburetor

It is very important to rid your fuel system of existing

dirt, rust and debris that has accumulated in your fuel

pump, fuel lines, and fuel delivery flow path over the

time you have been driving your vehicle. An empty fuel

bowl on a new carburetor is a prime candidate for

contamination on initial fire up once a new carburetor

has been installed. 80% of new carburetor failures are

due primarily to contamination failing some, or all of the

fuel flow (i.e. jet wells, needle and seat assy., etc.)

circuitry within the carburetor.

The reason for this condition lies in the way the fuel

pump delivers fuel to the carburetor. During normal

operation, the fuel pump diaphragm moves only

enough to keep the supply line feeding the carburetor

full. When an empty fuel line and an empty fuel bowl on

a carburetor is sensed, the fuel pump will not only

deliver high amounts of fuel at a high rate, the

diaphragm within the fuel pump also stretches more

than it has in some time. All of the contaminates stuck

to the inside of the fuel lines, and across the face of the

fuel pump diaphragm break free and head right into

that new and clean of contaminates float chamber. This

is a very undesirable situation and can cause a failure

situation with the carburetor.

Fuel System Purge Procedure

1. Reconnect your battery cable(s).

2. Disable your ignition system so that you can crank

the engine without concern of spark energy going

through the ignition system.

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3. With a mechanical fuel pump, place the end of the

fuel line in an unbreakable container that is not

sensitive to gasoline and crank the engine for 7 to

10 seconds, or until approximately one (1) pint of

fuel has been pumped by the fuel pump into the

container. Repeat the procedure until the fuel

within the container appears to be clean.

When you have an electric fuel pump, no cranking is

required, just turning on the ignition switch should

be enough to cause the pump to come on. In

some applications, you may need to bypass the oil

pressure default switch to run the fuel pump.

4. Reconnect the ignition system and reconnect the

oil pressure default switch. Insure that all electrical

systems are connected properly.

Carburetor Installation

1. Position the included base gasket onto the intake

manifold carburetor pad. Verify a good fit, and take

care to inspect the mating of the plenum shape

and the perimeter contour of the gasket to insure

no open sections or potential vacuum leaks.

2. Install your Jeg's carburetor over the gasket and

onto the intake manifold again taking time to insure

a good match of perimeter shape of the

manifold/gasket and the carburetor base. Torque

the carburetor hold down bolts and nuts to 60 inch

pounds.

3. Reconnect the fuel line to the bowl fitting on the

front of the carburetor. It is very important to start

this fitting by hand first before attempting to use a

wrench for final tightening of the fuel line. DO NOT

use any sealant on the fuel line fitting. This fitting

uses an inverted flare to mate to the incoming fuel

line and proper seal is made only on the flare, not

with the threads of the fitting. DO NOT try to fix a

leak here by adding a sealant or by high torque

loads upon the fitting itself.

4. For those vehicles that have any form of inline fuel

filters, you must replace them at this time.

5. Reconnect all vacuum lines, and other connections

like choke linkage, choke electrical wires (if

applicable) and throttle linkage, which includes

throttle return springs at this time.

6. Verify all linkage connections by operating the

throttle linkage in it's full travel making sure that

the throttle plates move unobstructed to a full open

position.

Take care to insure there is no interference with any

portion of the intake manifold casting, or any other

components like carb spacers, etc that may take place

during the movement of the linkage. Manually operate

the linkage several times to insure that there is nothing

binding or interfering with throttle operation.

NOTE: The proper installation of a vehicles throttle

linkage is critical to the safe operation of your

vehicle. Failure to properly install your carburetor

and/or transmission kickdown linkage can result in

uncontrolled engine operation, which could cause

personal injury, property damage, or even death.

When Installing An Electric Choke On A Vehicle

That Did Not Come With One Originally…

The Rochester Quadrajet Carburetor electric choke is

a “single terminal, internal ground” style of choke cap.

This means that for your electric choke to function

properly a couple of very important procedures and

conditions must be in place to insure proper choke

operation.

If Your Vehicle Came With An Electric Choke From

The Factory…

ANY REMOVAL OF THE GROUND STRAPS FROM

THE ENGINE TO THE VEHICLE CHASSIS WILL

HINDER THE OPERATION OF THE ELECTRIC

CHOKE ON YOUR ROCHESTER QUADRAJET

FROM FUNCTIONING.

The reason for this lies in the fact that the ground

portion of the choke circuit is completed through the

housing of the carburetor through the ground strap of

the engine to the vehicle chassis. Many times an extra

ground strap is required to insure proper operation of

the electric choke when installing your carburetor onto

an aluminum intake manifold. This ground strap is

installed at one of the carburetor attachment studs and

run to the vehicle chassis or proper ground station.

If Your Vehicle Did Not Come With An Electric

Choke From The Factory…

To install the electric choke equipped carburetor onto a

vehicle that did not originally come with an electric

choke a “hot” wire (12 volts) must be run to the electric

choke cap. The source of this hot wire must be an

“engine crank only” source. This is to say that the

source of your 12 volts must only be “on” when the

ignition switch is in the engine cranking only position.

Any other source of voltage will harm the choke and

possibly cause damage to your carburetor assembly, or

your vehicle.

Before You Drive Down The Street

Every effort has been made to insure that your Jeg's

carburetor will start and run through the adjustments

that we make on the flow bench during the quality

control check of the carburetor assembly. However,

due to altitudes, and specific climatic conditions

combined with engine package configuration, some

fine-tuning adjustments may be needed.

5

Idle Adjustments

Here are the idle adjustment areas that may require

some minor tuning for proper interface within your

particular engine package.

Idle Speed Adjustment Screw is the large screw on

the linkage side of the carburetor. Although we preset

idle speed here at the factory, take care to insure that

your idle speed (RPM) is set properly for your region

and engine package. Depending on the engine

package, engine idle speeds are best kept in the range

of 750 to 1000 RPM. This adjustment must be made

with the engine hot and all other systems normalized.

Idle Mixture Adjustment Screws are the screws that

face forward on the carburetor and are installed in the

base plate. The easiest way to adjust these screws and

your idle mixture is via the lean drop method. This is

performed when the engine is warm and all other

engine systems have normalized. Through 1/8 turns of

your screwdriver, find the adjustment point in which the

idle raises to the highest point before starting to

stumble. Once the engine begins to stumble return the

idle screw to the last position. Work between the two

idle adjustment screws to insure a balanced

adjustment and a smooth idle.

Fast Idle Adjustment is also set here at the factory

and is critical to engine performance during cold start

and normalization time periods. Proper fast idle speeds

should be in the range of 1600 to 2000 RPM. This

adjustment is made by setting the fast-idle linkage on

the highest position on the linkage and starting the

engine in neutral and when warm. All adjustments

should be made in 200 RPM increments until you are

satisfied with the RPM level.

Parts and Systems to Consider…..

This section is designed to cover some basic systems

and how they cause your carburetor to not always act

the way you would like it to. Also, we will discuss how

these systems can enhance the operation of your

carburetor and therefore your engine package as a

whole.

Fuel Pressure

The desired fuel pressure setting for ultimate

performance is 4 to 6 psi flowing. Quadrajet

carburetors can operate successfully at lower

pressures, however drivability can suffer under oblique

throttle positions such as snap to wide-open throttle

positions. The Jeg's Performance quadrajet is

designed to interface with stock style fuel pumps, stock

fuel tanks, supply lines, and pressures as low as 2 psi.

Spark Timing

Spark timing is a great way to tune an engine. With

octane differences and fuel additives, spark timing can

be an effective tuning method. As your octane rating

increases, so can your total timing increase. Not only

can you adjust the total timing figure, but you can also

tailor the timing curve to fine-tune the shape of the

timing introduction for your engine package. As your air

to fuel ratio is increased or decreased timing must be

adjusted. All jetting adjustments can be accompanied

with a timing adjustment.

Ignition System

The engine system that can best be described, as a

direct dependant sub system to the carburetor is the

ignition system. Any and all upgrades to the ignition

system can only help improve the ability of the

carburetor to operate properly. Remember, the

carburetor cannot process a fuel signal until it senses a

demand signal from the engine. If the ignition system is

weak and or in dis-repair, the fuel signal will always be

compromised.

Performance Camshafts

Many times the camshaft is called the heart of the

engine. The location of the timing events as

manipulated by the manufactured specifications of the

camshaft set many operational characteristics of your

engine. In relation to the carburetor, the camshaft

controls intake manifold pressure signals that can

dramatically affect the operation of a carburetor. Large

duration, or short lobe center camshafts drastically

affect idle characteristics. With the Quadrajet, the small

primary bores are less sensitive than other carburetors

with larger primary throttle plates however, there could

be some camshaft/intake manifold combinations that

could require a bleed hole to be drilled into the primary

throttle plate of the Quadrajet.

Remember…Before performing any modifications on

your carburetor please consult the manufacturer of the

camshaft, and call our tech line to discuss your

situation and or operational conditions that lead you to

believe you need to make changes to your fuel system

because of a camshaft choice.

Compression Ratio

The carburetor does not require much tuning as a

direct result of changing compression ratio. The fuel

curve does require some tuning, primarily due to the

possible timing curve manipulation to support a

compression change. Idle mixtures sometimes need

enrichment when going to a lower compression ratio. If

the mechanical efficiency increases with a

6

compression change (and it almost always does), then

more fuel will be required as a direct result of the

engines ability to produce more power.

Intake Manifold Design

With the spread bore design, the Quadrajet manifold

can take some interesting shapes. There are many

manifolds to choose from in the marketplace today and

all may require some fuel curve adjustment especially

with the many different camshaft choices available.

Please consult your manifold manufacturer for any

advice as well as contacting our technicians. The Jeg's

performance carburetor is not designed to be used in a

multiple carburetion situation. We do not recommend

that you try to use any other than a 175 horsepower

increase with a nitrous system when used in

conjunction with the Jeg's quadrajet Carburetor.

Carburetor Spacers

Carburetor spacers are a great tuning aid especially for

spread bore carburetors. We recommend that you use

1/2” thick spacers and add or subtract them as your

engine package responds. Carburetor spacers are a

very good tuning tool when working through manifold

or cylinder-to-cylinder distribution issues.

Tuning Guide

The Quadrajet is one of the most sophisticated

carburetor designs ever produced. In the many years

that the Rochester Carburetor Company produced the

Quadrajet, several different versions were produced as

well as many subtle upgrades to each version of

Quadrajet. For these Jeg's versions we concentrate on

a non-computer controlled, 750-CFM style of

Quadrajet. For tune up information for the 15802,

15803, 15804, and 15805 please check the tune up

manuals for the years and applications of each vehicle.

15800 Carburetors

Adjustment Manual Trans Automatic Trans

Slow Curb Idle Speed in RPM 650 to 750 RPM 600 to 700 RPM

Fast Idle Speed in RPM 1500 to 1800 RPM 1500 to 1800 RPM

Float Level Setting .250” .250”

Accelerator Pump Rod Location Inner Inner

Accelerator Pump Adjustment .687” .687”

Choke Rod Setting .100” .100”

Air Valve lockout .015” .015”

Initial Timing 8 degrees 12 degrees

Max Timing 32 degrees 32 degrees

Plug Recommendations

Below 350 HP AC R44* AC R44*

350 HP to 385 HP AC R43* AC R43*

*Or equivalent in Champion, NGK, Motorcraft, Accell, Splitfire, etc.

15801 Carburetors

Adjustment Manual Trans Automatic Trans

Slow Curb Idle Speed in RPM 650 to 750 RPM 600 to 700 RPM

Fast Idle Speed in RPM 1500 to 2400 RPM 1500 to 2400 RPM

Float Level Setting .250” .250”

Accelerator Pump Rod Location Inner Inner

Accelerator Pump Adjustment .687” .687”

Choke Rod Setting .100” .100”

Air Valve lockout .015” .015”

Initial Timing 6 degrees 10 degrees

Max Timing 34 to 36 degrees 34 to 36 degrees

Plug Recommendations

Below 430 HP AC R44* AC R44*

430 HP to 470 HP AC R43* AC R43*

*Or equivalent in Champion, NGK, Motorcraft, Accell,

Splitfire, etc.

Notes…

1. Fast idle Cam second step should be close to

1500 RPM, last step should be close to 2400

RPM.

2. Disconnect vacuum advance feed line to distributor

before adjusting idle speed. Be sure to plug line to

insure no vacuum leaks.

3. Set idle in neutral with the air conditioning on (if

applicable).

4. Idle settings should be higher in speed with large

camshaft (228 or higher degrees @.050", or less

than 112 lobe centers).

Personal Tuning Guide and Notes

Use this area of the manual to keep track of your

calibration changes and tune up adjustments that

optimize your engine package.

Adjustment Description My Adjustment

Slow Curb Idle Speed in RPM

Fast Idle Speed in RPM

Float Level Setting

Accelerator Pump Rod Location

Accelerator Pump Adjustment

Choke Rod Setting

Air Valve lockout

Initial Timing

Max Timing

Plug Recommendations

Plug Choice #1

Plug Choice #2

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Troubleshooting Guide

Troubleshooting a carburetor can be a very difficult

procedure if a methodical and simple approach is not

used. Some simple tools and the use of your senses

can make diagnosing fuel system problems a snap.

Many times the use of a liquid that is flammable to

determine vacuum leaks is used and always with poor

results. We recommend the use of propane to verify

any carburetor and/or manifold integrity issues. Call our

technicians if you have any questions when using

propane to diagnose your engine.

Choke Service

Acommon problem with high mileage quadrajets is

gum deposits on the choke linkage hindering the

movement of the choke. Remember, the choke itself

has little torque producing capacity so maintaining the

proper choke operation by checking the free movement

of the choke linkage is very important.

Rough Idle

If after all base adjustments are completed, a rough idle

exists, you will need a vacuum gauge to perform some

simple diagnostic routines. The vacuum gauge should

be used to adjust the carburetor to the highest vacuum

reading that you can adjust into the carburetor without

creating a lean condition and experiencing a lean mis-

fire (which is different than a backfire!). The larger the

plenum area of the intake manifold, and/or the larger

the camshaft duration or the narrower the lobe centers

the lower the maximum vacuum reading will be able to

be achieved during the adjustment period.

If the vacuum reading is very low, check the vacuum

hose connections and check the vacuum lines for

cracks, splits, or flexibility to insure tight and proper fit.

Also check the base gasket to insure proper alignment

and tightness of all fasteners. The ignition system

components like the cap, wires, and primary trigger

mechanism can also contribute to a rough idle. If the

engine is equipped with an EGR valve, make sure the

EGR valve is closed at an idle.

High Idle Speeds

Afew different systems on the carburetor account for

idle speeds. As discussed earlier, we wet flow each

carburetor assembly before we package the carburetor

and during that flow testing, we adjust the idle speed on

all conditions which include idle mixture, idle speed and

fast idle speed. In a previous section of this manual we

discuss how to adjust each of these sub systems.

Again, any compromise in any sub system such as the

base gasket, vacuum lines, brake booster feed line,

distributor vacuum feed line, EGR valve, or intake

manifold seal can cause the idle speed to be high or

even erratic.

Another major source of idle or engine speed trouble

can result from a mis-aligned distributor.

Backfiring and Mis-Firing

The idea that a carburetor can cause a backfire is just

not accurate. A carburetor can cause a lean mis-fire,

but never a condition in which fire comes back up and

exits through the air horn of the carburetor itself. So,

lets talk about what happens when a backfire is

detected and how to diagnose its origin.

Abackfire condition exists when one of the major

components within the ignition system has failed.

Check the integrity of the ignition system secondary

components such as the coil, coil wire, coil interface

within an HEI distributor, spark plug wires, rotor, and

distributor cap. Timing is also a critical issue with

backfiring. Check rotor positioning in relationship to

number one (1) cylinder to insure proper rotor phasing.

Afailing camshaft can cause a backfire as well as

timing chain wear or mis-alignment to the crankshaft.

Many times the advance or retard adjustments of the

camshaft are outside of the operational window for a

particular engine package.

Detecting and fixing mis-fires is more a tuning issue

than a broken or worn component situation. A mis-fire

does not have fire coming from the air horn of the

carburetor. It is a pressure wave coming from the intake

manifold, which presents a loud “popping” sound, and

a shot of raw fuel from the air horn. If a choke comes

open too soon, or is not closing the choke plate

completely, a mis-fire can happen. A low float

adjustment or too low fuel pressure can cause a mis-

fire. A plugged bowl vent, bad EGR valve, or a mis-

aligned intake manifold gasket can also all cause a

mis-fire.

Flooding

As was discussed earlier within this manual,

contamination fails 80% of all new carburetors. There

are three (3) major reasons why carburetors

experience a flooding condition. The number one

reason a carburetor will flood especially during start up

conditions is due to some form of contaminates (dirt,

charcoal, etc.) interfering with the proper operation of

the float system within the float bowl. The needle and

seat can have a form of contaminate holding the

needle off of its seat and not allowing the float to

regulate the flow of fuel. If this condition persists, the

float itself can be forced into a sticking situation and

therefore lose its ability to properly move with the flow

of fuel again, resulting in unregulated fuel levels.

The fuel line and/or carburetor can be exposed to a

higher than normal heat source. This flooding condition

brings excess fuel into the venturi area of the

8

carburetor hindering starting of the engines and

excessively rich operating conditions. Many times this

condition is called percolation not unlike the way a

coffee pot works. This condition can be caused by a

fuel line running too close to a radiator hose, header, or

exhaust manifold.

This increased temperature causes the fuel to expand

and pressurize the needle and seat attached to the

float to the point of allowing fuel past the float system

and overflowing into the venturi area of the carburetor.

This fuel eventually ends up lying as raw fuel on the

plenum floor of the intake manifold. This raw fuel

hinders start up performance and engine performance

due to overly rich mixtures.

Excessive fuel pressure can cause flooding as well.

The Jeg's carburetors are designed to work with low

fuel pressures but nothing under 2 psi or over 5 psi.

These carburetor assemblies use the long fulcrum style

of float and cannot control fuel levels at fuel pressures

above 5 psi. When fuel level control is lost, again we

experience overly rich fuel mixtures and a poor-

performing engine.

Hesitation

Hesitation is another operational condition that is

mostly associated with systems that are not adjusted

properly and not necessarily broken. Systems like the

EGR valve can cause hesitation when the EGR valve

itself is compromised in its ability to function and or if an

EGR equipped engine has had the valve removed

completely. The combustion chamber temperatures

can go up as high as 400 degrees between a non-EGR

and an EGR engine. A hesitation can be felt during

hard acceleration if some fuel curve or timing curve

adjustments do not compensate for the higher

combustion temperatures. Don't forget to double-check

the EGR valve or the block off plate for properly

torqued fasteners and gasket integrity.

Other systems that can cause a hesitation are exhaust

system restrictions including catalyst and muffler

contamination, compromised vacuum lines and

support lines like the brake booster connection. An

insufficient accelerator pump shot can be a major

contributor to a hesitation. With a Quadrajet, there are

several adjustments that can add or subtract pump

shot to overcome a hesitation or sag in acceleration.

Don't forget to verify that the timing adjustment is where

it needs to be and the timing advance curve is matched

to the fuel curve of the engine package. Like the fuel

curve, the ignition curve can be infinitely adjusted to

allow for vehicle and engine dynamics.

Hard Starting

When the engine is cold, the choke system restricts the

amount of incoming air to then enrich the air/fuel

mixture in an effort to help start the engine. Depression

of the gas pedal inside your car is enough to set the

choke to “on”. If the carburetor has an electric choke,

the minute you turn on the key, your choke goes to the

“on” position. As the vehicle warms up, the choke

mechanism pulls the choke plate to the open position

therefore allowing the entire potential air signal to enter

the carburetor. If the choke plate does not completely

close, the engine could be hard to start. In this case, a

simple adjustment on the rich side of choke operation

is required. Be careful to not over compensate, as

warm engine operation will be affected. If the choke is

adjusted to a rich position, the engine may be hard to

start when warm. Again, a simple adjustment of the

choke to the lean side of it's calibration will allow more

air to enter the engine when warm, therefore allowing

for easier starting when warm.

Many engines have vacuum advance. These engines

require that the vacuum line be connected to a “ported”

vacuum source on the carburetor. If the vacuum

advance line is connected to a “full” vacuum source,

the engine will see ignition advance settings that will

make starting difficult no matter if it is hot or cold. This

is due to a full vacuum signal over advancing the timing

during engine cranking time periods, which can make

starting difficult.

9

1-800-345-4545 jegs.com

Warranty Information

Conditions of Limited Warranty

1. Purchased carburetor must be returned to the original place of purchase within 90 (ninety)

days.

2. Purchaser must present the original sales receipt as verification of date of purchase.

3. The original sales box in which the carburetor is packaged is your warranty validation box.

This limited warranty is effective only if the warranty carburetor is returned in the Jeg's Sales

box.

Warranty Limitations

We do NOT warrant the proper application of the purchased carburetor and/or the competence of

the installer. This performance carburetor has been calibrated and adjusted as a universal

application and in many uses requires knowledge of adjustment and tuning of this carburetor to the

specific application for which the installer intends. We assume no liability and WILL NOT PAY for

LABOR, DAMAGE and/or TOWING claims arising from the installation or use of the purchased

carburetor. This limited warranty does not apply to any defects caused by negligence after

purchase, misuse, collisions or other accidents, or any unauthorized or unreasonable use

including, but not limited to, improper application and installation. This performance-calibrated

carburetor MUST NOT BE USED in any form of MARINE or AIRCRAFT application.

We neither assume nor authorize any person, agent, distributor, dealer, service facility or company

to change, modify, supplement or amend this Limited Warranty or assume for us any warranty or

liability in connection with the remanufacturing, sale, shipment or use of our products. Nor do we

authorize any other person, service facility or company to make any qualitative determination as to

the fitness of our products. Except and to the extent provided in the Limited Warranty, we make no

expressed or unexpressed warranty regarding this product.