WEBER CARBURETOR TROUBLESHOOTING GUIDE
This guide is to help pinpoint problems by diagnosing engine symptoms associated with specific vehicle operating conditions.
The chart will guide you step by step to help correct these problems.
For successful troubleshooting, please do the following:
1.Complete all steps suggested in the box, and any that follow in the guide.
2.Complete all of the suggestions in order, doing so will eliminate any secondary problems.
3.Follow diagnosis directions completely (Cleaning, adjustment etc.)
4.Do not do any other procedures other than the ones suggested.
5.CARBURETOR SET UP AND LEAN BEST IDLE ADJUSTMENT
This Weber carburetor has been manufactured using very high quality control standards.
Through years of experience we have found that most carburetor problems are caused by other engine components.
FOR YOUR SAFETY
Remember ! A Clean engine will make all work easier, accurate, and safer.
Use new hose clamps on fuel hoses.
Replace any cracked or missing hoses. (fuel, vacuum or coolant)
Label any hose or wire before removing.
Use an insulated connector for any wire.
Allow any dirt in the carburetor.
Run the engine without proper. filtration (Air and Fuel Filters).
Damage any gasket surfaces
Over tighten Jets or levers.
1. ELECTRICAL SYSTEM
To make sure that the ignition system is working correctly do steps 1 A - 1C with the ignition on.
Make sure that the battery is fully charged with no corrosion present on the terminals.
A. Test the fuse for the ignition/charging circuit:
If you do not have 12 volts at either side if the fuse, replace fuse.
B. Check static ignition timing:
Make sure that you do not have excessive free play at the distributor shaft.
C. See if the distributor is 180 degrees out of time:
Do a comparison to the cam timing or by reversing opposite pairs of ignition wires at the distributor cap.
2. FUEL DELIVERY
To make sure that the quality and supply of fuel to the carburetor is correct,
please so steps 2A - 2I . In our experience we have seen that fuel contaminated
with water, dirt or rust is the major cause of carburetor problems. Use fuel as fresh as possible.
On vehicles equipped with electric fuel pumps. Most are not powered with out the engine running or starter engaged.
A. Test the fuel flow at the carburetor by disconnecting the fuel supply hose and directing the fuel flow into a small dry,
clean can when the fuel pump is activated.
DO NOT ALLOW ANY CHANCE OF A SPARK DISCONNECT THE PRIMARY WIRE (s) usually a small 16 Gauge wire
THAT CONNECTS THE GROUND (-) SIDE TERMINAL OF THE COIL TO THE DISTRIBUTOR OR SPARK BOX, IF SOLID STATE IGNITION.
If fuel flow is adequate go to step 2E
If fuel flow is not adequate go to step 2B
B. Test the fuel flow at the fuel filter and then at the Fuel Pump: If fuel flow is not adequate go to step 2C.
C. Check to see if the fuel tank is empty or if the lines are blocked. They maybe blocked by dirt, ice, or other contaminants.
If you have fuel in the tank and the lines are not blocked go to 2D.
D. If the vehicle is equipped with an electric fuel pump, test the pump by applying 12 volts and a ground to the pump.
If you have a mechanical pump, crank the engine over. If you do not have any fuel flow. Replace the pump.
E. Check to see if the fuel is old or bad. If vehicle has been out of service for an extended period chances are that the fuel is bad.
If the fuel does not smell right: Try some fresh fuel a few drops at a time in the Carburetor inlet.
IMPORTANT NOTE AVOID FUEL CONTACT WITH SKIN
F. Check to see if the fuel is contaminated with rust or water:
To check remove and empty out fuel filter (from the inlet side) into a clean,
dry can , or if your vehicle has a drain plug on the fuel tank, empty some fuel
from the tank into a clean, dry can. Remember Water and Fuel do not mix.
G. Test the fuel pressure output with a fuel pressure gauge. If the fuel pressure
exceeds 3.5 psi, install a fuel pressure regulator.
H. See if the Fuel Return and the Fuel supply line are reversed. On some Weber
carburetors that have a fuel return, remove the fitting one at a time from the
carb. The Fuel return is restricted and the fuel inlet is totally open.
I. Make sure that none of the fuel lines are crimped or kinked.
To make sure that engine is mechanically sound. Please do steps 3a - 3C
A. Do a compression check on all of the cylinders. Compression should be over
125 psi per cylinder, and with in 10% of each other.
B. See if the intake manifold bolts or loose or missing.
C. If you have a heated intake. Make sure that the heat source is connected.
On most vehicles it is heated by water. Check with your Shop manual for
the heat source on your intake.
4. VACUUM SYSTEM
To make sure that you do not have any vacuum leaks, check all vacuum operated devices,
and connections to the intake manifold. Use the suggested tests to check.
IMPORTANT NOTE THE VACUUM DEVICES ARE CONTROLS AND VALVES THAT REQUIRE
SEALED VACUUM CONNECTIONS TO PROPERLY OPERATE
A. Testing the vacuum system with a handheld vacuum pump is very helpful
in finding leaking hoses and devices in the system away from the carburetor.
(Vacuum testers have an attached vacuum gauge to measure vacuum)
Start at the carburetor and work your way toward the engine, remove vacuum
hoses one at a time. Be careful not to mix up any hoses.
Plug the source of the vacuum and connect the vacuum pump to the source
and test for any malfunctions in the system.
TO TEST OTHER VACUUM CONTROLLED DEVICES:
-Distributor Advance look for a change in the engine timing.
-Distributor Retard Test with a timing light.
-EGR Valve Stalling will be normal when tested.
-Deceleration Valve engine will idle faster when tested.
-Canister Purge If the engine speed fluctuates with the hose disconnected, canister is fuel of fuel due to a problem in the tank vent or the fuel tank is overfilled.
Go to 4 B
B. Using the method of pinching off hoses is helpful when you have hoses that are leaking, such as air cleaner hoses and other devices.
-Install the air inlet duct or air cleaner.
-Start the engine and let it run at idle.
-One by one pinch off vacuum hoses with pliers and listen for a change in engine RPM.
-Recheck any suspected problems with the Vacuum pump.
-Hoses Choke stove air door ( thermostatic air cleaner)
-Power Brake Booster Heater Controls, in all positions
-Cruise Control Air Conditioning fast idle , ignition on A/C on
-Headlight Doors Vacuum Reservoirs and amplifiers
-PCV Valve (idle should drop 50 RPM)
Go To 4 C
C. Using Carburetor Cleaner to check for leaks. This will help you find defective Rgaskets and hidden leaks.
IMPORTANT NOTE AVOID SPRAYING YOUR SELF READ ALL CAUTION LABELS AND WASH AFTER USING
Remove air inlet duct or air cleaner and plug any vacuum sources removed.
With the engine idling, spray carburetor cleaner into the air inlet of then carburetor to determine the effect of running.
Wait until the engine returns to the previous idle. Spray carburetor cleaner on the outside of the carburetor and intake manifold.
If you have a vacuum leak the idle speed should have changed.
-Some vacuum leakage at the throttle shaft is normal.
-All gaskets and seals.
-Fittings and plugs.
Do the following steps only after you have completed 1A, 2A, 3A and 4A.
This will help you save time and avoid creating any secondary problems.
In our experiences most problems initially thought to be carburetor problems
are those caused by other engine components.
A. Testing the choke and the throttle linkage:
Check that you have free movement at the carburetor with an assistant operating the gas pedal.
In our experiences over tightened throttle shafts contribute to many carburetor problems.
B. Testing the Idle Speed Adjustment:
With the engine at normal operating temperature, adjust to manufacturers
specs: turn adjustment screw in (clockwise) to raise the speed and out (counterclockwise) to lower).
C. Testing the Fast Idle Adjustment:
It may be necessary to perform this test first thing in the morning or after the vehicle has been allowed to cool down.
With the engine cold, press the gas pedal to the floor once and release.
Start Vehicle. Engine speed should be 2000 RPM - 2500 RPM after about 10 seconds. Turn engine off.
To adjust turn fast idle screw in to increase fast idle speed and out to decrease. Do not disturb the carburetor linkage
during this adjustment. Without touching the gas pedal start engine again check engine RPM.
If you have to readjust use the procedure above.
D. Testing the Automatic Choke adjustment:
The choke is preset at the factory but in some cases it may be necessary to readjust.
The choke assembly must be cool ( below 68 F 20C) before attempting to
make any adjustments. First fully rotate the throttle lever and release;
Check for smooth operation; the choke plate should be all the way closed.
Make a reference mark on the carburetor and the choke where the two faces
meet. Loosen the three 7mm hex head screws one full turn. While holding
the throttle about one third open, adjust the closing tension on the choke
plates just fully close; then slightly increase the tension on the spring by
continuing to rotate the thermostat 1/8" measured in the outer diameter of the
thermostat. Tighten the three 7mm hex head screws without distorting the
retainer ring; fast idle speed my need to be readjusted. If the choke plates
do not fully open after running the engine for three minutes go to step 5E
E. Testing the choke thermostat assembly:
The electric choke terminal must have a key hot 12 volt source.
The carburetor is grounded to the engine by a metal to metal connection.
If the vehicle was originally equipped with an electric choke and or an idle
solenoid use that wire. If you do not have a wire present , splice into the wire
leading from the ignition switch to the coil or ballast resistor.
Water chokes must have a free flow of engine coolant.
F. Testing the idle cut-off solenoid: (if equipped)
With the solenoid on the carburetor and ignition on and engine off,
disconnect the wire from the solenoid and listen for a click; then with the
engine idling, disconnect the wire leading to the solenoid. The engine should
stop. If the engine does not stop the solenoid is likely defective.
G. Testing the idle mixture adjustment:
Adjust with the engine at idle speed; turn the mixture adjustment screw in
(clockwise) by quarter turns until the engine speed drops; turn out
(counter clockwise) while counting quarter turns until the idle rises and
again drops noticeably; turn in again to middle setting.
If RPM does not change go to 5H
H. Cleaning the idle circuit:
Locate and remove the primary idle jet holder (this is also the idle solenoid
if so equipped). Remove the jet from the holder, hold up to a light and check for
roundness of the cavity. Remove the idle mixture screw from the the carburetor,
blow clean compressed air through the two open passages. Replace the jet
in the holder and install back in the carburetor along with the idle mixture screw.
I. Make sure that all of the vacuum lines are installed to the correct signal source.
Install a vacuum pump with a gauge. Start engine and operate throttle.
If vacuum is present at idle it is a manifold vacuum source. If vacuum is present
at partial throttle it is a ported vacuum source. If vacuum is present at a high
rpm it is a venturi vacuum source. Refer to your shop manual or any labels under
the hood that show vacuum hose routing.
J. Set the Float Level:
Float level problems are very rare with New Weber Carburetors. Refer to a
Weber Shop manual or contact your Weber Dealer for specifications or
instructions. Also check for dirt or other foreign matter in the needle and seat.
6. PERSISTENT PROBLEMS
The following problems are not listed in the guide because they are unique,
difficult to diagnose and are beyond the scope of the guide. We have listed
them because of past experiences.
A. Replace spark plugs; use only new spark plugs with the correct heat range.
B. Make sure that the ground strap is properly connected. It is necessary that the engine is grounded to the chassis and battery.
C. Double check or have any work coincidental to the problem such as body work or engine repairs, a tune up , installation of an alarm, stereo or
D. Check for any damage to the exhaust system or an internally collapsed muffler.
E. Check your sparkplugs. Most Shop manuals have a chart in color showing what the spark plugs should look like. Rejetting of the carb may be necessary.
F. See if the Transmission is slipping. This applies to both automatic and standard.
G. Make sure that the fuel tank vent hose is not plugged at the canister.
H. Some Vehicles , including Nissans, have two wires that go to the electric choke.
One is a ground wire and does not have to be connected.
I. Some original Electric Fuel pumps pressure exceeds 3.5 psi. Check with a pressure gauge or look in shop manual. A pressure regulator may be necessary.
J. On some water cooled VW's , vibrations caused by worn motor mounts may cause the carburetor top to fracture.
K. When the throttle return spring is located on the same side as the throttle lever, check to make sure that the first spring
coil does not get trapped between the lever and the carburetor. This will cause binding.
L. On progressive carbs incorrect installation of the throttle lever can cause both throttle plates to open at the same time.
This is wrong. The progressive is designed that the primary barrel opens up before the secondary.
M. On Multiple carburetor applications it is required that all of the carburetors be synchronized.
N. If you experience intermittent problems. Any diagnosis should be carried out when the problem occurs. Be prepared carry any necessary tools.
O. Go over the troubleshooting guide and see if any steps were missed or not understood.
It is important to follow all linkage and lever installation instructions. The number one and two reasons for tuning
errors are improper linkage installations and over tightened linkage nut, causing a binding in linkage assembly.
PERFORMANCE. POOR RUNNING QUALITY DOES NOT MEAN A DEFECT IN THE CARBURETOR. AN
ADVANTAGE OF THE WEBER CARBURETOR - EASE OF ADJUSTMENT AND TUNING.
Start set up by confirming carb base line settings. Do not depend on the factory delivered settings. Check them before
the carb is installed.
1. All settings are done with choke disengaged or warmed up so that the choke is fully opened and disengaged. This is
done on automatic choke carburetors by first opening the choke butterfly by hand and inserting a wood block or wedge
of some kind to hold open while the linkage is cycled (linkage operated through its full movement) to clear the choke
cam. (You will hear a metallic click as the cam is released. You can check the fast Idle screw under the choke
2. Set the Idle stop screw by backing out the Idle speed screw until it is not in contact with the
throttle stop lever. Cycle the linkage again to be sure that the linkage comes to close without any assistance. (Checking
for linkage bind) Now bring screw back into contact with the lever and continue to open or screwing in 1 turn no more
than 11/2 turns.
3. Set the mixture screw by first screwing in until the screw stops, bottoms out. DO NOT FORCE OR
BIND AS THIS WILL CAUSE DAMAGE TO THE SCREW AND IT’S SEAT IN THE BODY OF CARBURETOR.
Back out the screw 2 full turns.
BE SURE TO FOLLOW THE NEXT INSTRUCTIONS IN THE PROPER SEQUENCE, DEVIATION WILL CAUSE
THE CARBURETOR TO NOT FUNCTION TO ITS IDEAL SPECIFICATIONS AND MAY NOT PROVIDE THE
PERFORMANCE AND FUEL ECONOMY AS DESIGNED.
4a. Start the engine, the engine will run very slowly more like a tractor. As long as the engine stays running idle speed
is not important at this point.
4b. The first thing to do is not set up the idle speed, but to set the idle mixture screw to lean best idle setting. First, turn
in the mixture screw until the engine dies or runs worse, then back out the screw (recommend turning ¼ to ½ turn at a
time). The engine should pick up speed and begin to smooth out. Back out ½ turn more, or until the screw does
nothing or runs worse then turn back to the point where it ran its best. Use your ear, not a scope or tuning instruments
at this point. You want to tune the engine by sound. Adjust to best, fastest and smoothest running point.
4c. Now that the mixture screw is at its best running location, you can adjust the idle speed the screw. The screw will
be sensitive and should only take ¼ to ½ turns to achieve the idle speed you like.
Check and set idle to your driving preference. Put the car in gear and apply slight load, (AC on) and set the Idle as
you like it. Don’t set it too high, as this will cause causes excessive clutch and brake wear. The Idle only needs to be 7
to 900 RPM with light load or AC on.
5. Recheck timing and vacuum hook ups. Recheck mixture screw to lean best idle again. If all is still best and
To confirm settings with the engine running. Start by screwing in the mixture screw and count the number of turns it
takes to bottom out and note if the engine dies. If Idle Mixture screws are with in ½ turn of base line setting then all is
well and have fun. Also check the speed screw and note how many total turns from initial contact. You may have
opened (turned in) the speed screw. Your final setting should be under 2 full turns. Reset the screws (back in) to the
best final settings (Per your notes) and go on a test drive and have fun. If the settings are other than described then you
may want to recalibrate the idle circuit (low speed circuit) to your engines needs. This is done by following the rule of
If the mixture screw is more than 2 turns out then the idle jet is too lean (too small). When the mixture screw is less
than 11/2 then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is
not opened more than 11/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of
a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it
is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please
understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition
holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are
available if needed.
With the speed screw set at no more than two (2) turns in after contact with the stop lever; and the best
idle occurring with the mixture screw set at 3 turns from bottom, indicates the need for a larger Idle jet. Achieving the
best idle at under 2 turns indicates the need for a smaller idle jet.
The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other
carburetors is the idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the
driving operation. This is the reason that the Weber should give a fuel economy improvement over most factory carbs
along with significant performance gains. In the worst case you should not see a significant fuel economy loss over
stock, while improving HP & Drivability.
The Weber Carburetor is a sequentially timed device to the motor like the distributor. Time taken in the setup will