troubleshooting

Overview

Before troubleshooting carburetor performance the following items must be in order since many times tuning troubles are ascribed to carburetor problems when actually they have been unrelated issues:

• Camshaft timing is assumed to be correct
• Valves have been recently adjusted
• Air leaks have been checked and corrected for pinholes, gasket leaks, worn throttle shafts, etc.
  • intake manifolds and gaskets
  • vacuum taps (power brakes, vacuum line to distributor, Sportomatic air lines, etc.)
  • exhaust headers, muffler and gaskets
• Engine cranking compression is high and even (above 150 psi and cylinder pressure differences within 10%)
  • It would also be good to perform a leak-down check to determine health of the engine.
• Electrical system components in good shape:
  • Contact breaker points (not pitted, gapped correctly and point grease applied to the rub block)
  • Spark plugs of the correct type and heat range
  • Ignition wires are in good shape including the rubber caps and plugs
  • Coil, condenser and ballast resistor are known to be in good health
    • A Capacitive Discharge ignition is strongly recommended
• Ignition timing is set
  • Distributor is of the correct type and the advance curve matches the engine design (camshaft & compression)
  • The mechanical advance mechanism is operating (advancing) smoothly
  • Timing set at idle and checked for full advance at 6000 RPM
• Air cleaner elements should be in like-new condition and element housings clear of oil film and road/engine grime
• Fuel filters are clear and fuel lines fresh (ethanol resistant fuel lines strongly recommended)
  • Don't forget the fuel strainer in the bottom of the fuel tank
• Fresh gasoline is in the tank
• Venturis and jetting have been correctly sized for the engine
  • Jets and venturis are sometimes resized from their markings so measurement is recommended
  • Jets, jet holders, accelerator squirter nozzles, etc. have been checked for damage from over-tightening, freedom from corrosion and checked for cleanliness in all aspects

Uneven idle, poor transition characteristics, excessive fuel consumption results from many issues including:

• unbalanced air flows
• ignition issues
• intake and exhaust leaks
• jet blockage by foreign particles
• incorrectly set float levels
• incorrectly set accelerator pumps
• worn throttle shaft/journals
• high fuel pressure

In addition, today's changing fuel formulations require jetting review: concerns about vapor lock and float bowl percolation with the resulting potential for engine compartment fire are very real.  Less scary are jetting revisions to accommodate our new fuels, typically idle and main jets are increased one size over what worked 20 years ago.

It is critical to have the throttle plates in the correct position at idle.  If retarded ignition timing (engine runs slower than for correct timing) by opening up the throttle plates then progression mixture strength will be affected with resulting loss in driving performance.  (See "Advanced Throttle Plate and Idle Adjustments for Special Situations" in Advanced Procedures for more detailed information regarding this topic.)  As throttle shafts wear, the extra clearance with their journals allows false air to be drawn into the throttle bores during idling and low speed operation, additionally the clearance allows the throttle shafts to move around in their journals with associated erratic idle response.  These are the root causes of erratic operation of worn carburetors.

Basic Troubleshooting Guide

Information in this section is provided as a quick guide to help in making adjustments to the jetting and tuning of your Webers. 

Black smoke from the exhaust (sustained operation, not during acceleration)

• Mixture screws set too rich (low speed operation)
• Fuel float level too high
• Fuel float rubbing on wall of float well
• Fuel float crushed from high pressure air or flooded (pinhole)
• Fuel pressure too high
• Fuel inlet valve leaking or sticking
• Idle jets too large or progression air correction jets too small (low speed operation)
• Main jets too large or main air correction jets too small (high speed operation)
• Blocked idle air correction jet (causes fuel to siphon into cylinder)
• Weak spark (use CD ignitionor resistor spark plugs if using standard, coil ignition (increases voltage to plugs over non-resistor plugs) or use hotter plugs

Sneezing up through intakes (At idle or under moderate driving conditions)

• One or more cylinders too lean (open mixture screws)
• Timing not advanced enough or is not advancing smoothly
• Idle jets too small
• Intake air leak (gaskets not sealing, uncontrolled air leakage past throttle shafts, cracked manifold, open vacuum tap on manifold or carburetor, etc.)

Dull popping from exhaust (at idle)

• One or more cylinders too lean (open mixture screws)

Backfiring from exhaust during acceleration

• Fouled spark plug

Flat spot in acceleration (up through 3500 RPM)

• Idle jets too small
• Progression air correction jets too large
• Floats set too low
• Try adjusting mixtures at 1800 to 2000 RPM
• Exhaust header too big
• Valve clearance set too tight (advances timing)
• Ignition over-advanced
• Weak spark

Flat spot in acceleration (near redline)

• Main jets too small
• Main air correction jets too large (Find largest air correction jet that just causes a high RPM misfire and decrease air jet size 10 to 20)
• Floats set too low
• Wrong emulsion tubes
• Exhaust header too big
• Valve clearance set too tight (advances timing)
• Ignition over-advanced
• Weak spark

Poor power throughout driving range

• Timing not advanced enough
• Main jets too small

Exhaust popping on deceleration

• Exhaust leak
• Intake air leaks

Detailed Troubleshooting Guide

Rich running issues

Symptoms

• Black smoke from exhaust
• Black, sooty spark plugs
• AFR readings below 11
• Sluggish response to throttle application
• Smells of unburned fuel, especially when hot

Possible problem sources & corrective actions

• Idle mixture screws (affects idle and slow speed operation)
  • Idle mixture screws set by "number of turns open" and not adjusted by "Lean Best" procedure (see Lean Best Idle Mixture Tuning in Standard Procedures)
  • First progression hole exposed (see Advanced Throttle Plate and Idle Adjustments for Special Situations in Advanced Procedures)
  • Mixture screw hole (orifice) enlarged or defective taper on mixture screw
• Idle jets (affects idle and slow speed operation through transition)
  • Sized too large for engine
  • Idle jets reamed larger than indicated size
  • Aftermarket idle jets delivered with orifices larger than indicated size
  • Cross-holes in jet body deformed or closed due to excessive installation torque on jet holder.  These holes allow air to mix with the fuel and if the holes are deformed then air cannot emulsify the fuel resulting in an overly rich fuel mixture.
  • Damaged interface between tip of idle jet and seat within the throttle body.
  • Incorrect idle jet holders.  There are two versions of idle jet holders: the early version did not use o-ring seals and the later version incorporated an o-ring that sealed with the port in the throttle body.  If the later version is used in a throttle body designed for the earlier version then the idle jet will not properly seat and will then allow a great amount of fuel to pass into the idle circuitry.
• Idle air correction jets (affects idle and slow speed operation through transition)
  • Sized too small for engine
  • Jets blocked by debris or by gasket
• Fuel level (affects all operational engine speeds)
  • Fuel floats adjusted incorrectly resulting in fuel level being too high.  (See Adjustment of Fuel Float Levels in Standard Procedures.)
  • Fuel delivery pressure too high resulting in high fuel level.  (See Measuring Fuel Pressure in Standard Procedures.)
  • Float geometry affected after proper setting of fuel level as the result of an off-road excursion damaging float geometry.
  • Floats have alignment issues resulting in rubbing on the fuel well walls
  • Float is binding on the fulcrum screw
  • Float has "divot" on tab causing fuel inlet needle valve to not close reliably
  • Tab on float is not perpendicular to inlet needle valve when fuel level is correct.  This causes the needle valve to side-load and thereby not seal reliably.
  • Float has leak or is damaged from high pressure air causing float to sink or otherwise result in a high fuel level
  • Fuel inlet needle valve leaking or sticking
  • Corrosion or debris stuck in fuel inlet needle valve and inhibits reliable closure
• Main jet (affects from transition to maximum engine speed)
  • Main jets sized too large for engine
  • Main jets reamed larger than indicated size
  • Aftermarket main jets delivered with orifices larger than indicated size
  • Jet is loose in main jet holder allowing fuel to pass through threads in addition to that passing through metered orifice
  • Loose fit of main jet holder with bung in throttle body.  There is no seal between the fuel well and the main jet, only the quality of thread fit between the jet holder and those in the bung.  A poor quality fit will allow un-metered fuel to enter the main circuit.
• Emulsion tubes (affects from transition to maximum engine speed)
  • Wrong type selected for engine
  • Debris blocking holes in body of tube
• Main air correction jets (affects from transition to maximum engine speed)
  • Sized too small for engine
  • Clogged with debris
• Main venturi (affects from transition to maximum engine speed)
  • Sized too small for engine
• Auxiliary venturi (affects from transition to maximum engine speed)
  • Tall auxiliary venturis designed to help boost signal to initiate the main circuit when large main venturis used.  If tall auxiliary venturis are fitted on an application where the main venturis are particularly undersized then the boosted signal to initiate the main circuit can cause a rich fuel delivery.
• Air filter (affects from transition to maximum engine speed)
  • A dirty air filter will restrict airflow into the carburetors and create a rich mixture.
  • Blockage of the air filter with a shop rag or similar will create a rich mixture.
• Synchronization of carburetors
  • Poor side-to-side carburetor synchronization of carburetors.  Un-synchronized air flow balance will cause one bank of carburetors to be advanced relative to the other and for a given total power output the throttles for the advanced bank will be more open than for what is optimum for the engine speed resulting in rich running conditions for those cylinders.
  • Sticking throttles may also cause a synchronization issue.
• Exhaust system
  • Exhaust system does not provide adequate back pressure.  If not enough back pressure is present then unburned fuel may be exhausted, especially at lower RPM operation.  If your muffler has dual outlets then the following is a method to check for this condition:
    • Block off one exhaust outlet, drive the car and see if condition improves
    • Block off only a part of one outlet and see if condition improves or degrades from previous test
    • If the second test results in improved performance than for the first then look to find reduced diameter exhaust tips to slip over existing tips
    • If the first test results in improved performance then try the OEM muffler again.
  • Racing camshafts used with street muffler will result in exaggerated intake reversion thereby diluting the inlet air with exhaust gasses resulting in enriched combustion due to lack of oxygen.
• Ignition issues
  • Inadequate spark intensity
  • Timing retarded
  • Spark plugs too cold
  • Spark plugs of wrong type (resistor plugs required for standard coil ignition to maximize spark efficiency)

Lean running issues

Symptoms

• Dull popping from exhaust at idle
• Individual cylinders "sniffing" or "spitting" up through intake horns at idle
• Spark plugs have white insulator color
• AFR readings above 15
• Surging (slight acceleration and return to speed) during steady speed on partial throttle
• Stuttering or loss of power as throttles are slightly opened to accelerate from steady speed
• Engine speeds up momentarily when throttles slightly closed from steady speed
• Engine temperatures exceed 220F under normal operational conditions

Possible problem sources & corrective actions

• Idle mixture screws (affects idle and slow speed operation)
  • Idle mixture screws set by "number of turns open" and not adjusted by "Lean Best" procedure (see Lean Best Idle Mixture Tuning in Standard Procedures)
• Idle jets (affects idle and slow speed operation through transition)
  • Sized too small for engine
  • Aftermarket idle jets delivered with orifices smaller than indicated size
  • Tips of idle jets blocked with debris which decreases fuel delivery to idle/progression circuit
• Idle air correction jets (affects idle and slow speed operation through transition)
  • Sized too large for engine
• Fuel level (affects all operational engine speeds)
  • Fuel floats adjusted incorrectly resulting in fuel level being too low.  (See Adjustment of Fuel Float Levels in Standard Procedures.)
  • Fuel delivery pressure too low resulting in low fuel level.  (See Measuring Fuel Pressure in Standard Procedures.)
  • Float geometry affected after proper setting of fuel level as the result of an off-road excursion damaging float geometry.
  • Floats have alignment issues resulting in rubbing on the fuel well walls
  • Float is binding on the fulcrum screw
• Main jet (affects from transition to maximum engine speed)
  • Main jets sized too small for engine
  • Aftermarket main jets delivered with orifices smaller than indicated size
  • Jets soldered closed and resized smaller than indicated size.
  • Jet holder has internal blockage due to debris or corrosion which restricts flow through jet
• Emulsion tubes (affects from transition to maximum engine speed)
  • Wrong type selected for engine
  • Debris blocking holes in body of tube
• Main air correction jets (affects from transition to maximum engine speed)
  • Sized too large for engine
  • Installed loosely allowing un-metered air to enter emulsion tube
  • Aftermarket air correction jets delivered with orifices larger than indicated size
• Main venturi (affects from transition to maximum engine speed)
  • Sized too large for engine
• Auxiliary venturi (affects from transition to maximum engine speed)
  • Interface between hollow wing of the auxiliary venturi and the transfer port in the main throttle body is loose or poorly fitted allowing uncontrolled air to weaken fuel flow from emulsion tube well
  • Tensioning spring missing from wing of auxiliary venturi
  • Auxiliary venturi installed so hollow wing is not interfacing with fuel transfer port in the main throttle body
• Fuel galleries (affects idle and slow speed operation through transition AND accelerator pump operation)
  • Dirt, corrosion or old fuel will block these passageways and disrupt fuel fuel flow.  (See Fuel Gallery Cleaning and Lead Plug Replacement in Advanced Procedures.)
• Fuel filter (affects idle and slow speed operation through maximum engine speed)
  • Dirty fuel filters will restrict fuel flow into the carburetors and create a lean mixture.  This is more pronounced at upper engine speeds.  This symptom typically disappears when the engine is returned to idle operation and returns shortly after higher power is demanded.
  • Filters to be checked are: fuel screen in the fuel tank, in-line fuel filters and fuel screens around banjo bolts
• Air filter (affects from transition to maximum engine speed)
  • An engine tuned with the air filter removed or replaced with a more freely flowing air filter will experience a leaner fuel mixture.
• Air leaks (affects all operational engine speeds)
  • Throttle shafts and worn journal bearings allow false air to enter the engine and create tuning havoc along with erratic low speed operation
  • Vacuum lines can crack and leak air into the intake manifolds
  • Vacuum ports with rubber caps can easily blow the caps off and create a larger amount of air introduced into the manifolds
  • Missing gaskets at any interface between the intake ports on the cylinder head and the throttle shaft will draw in air and lean out the mixture
  • Engines using OEM intake manifolds and CIS heads can easily lose the seal at this interface due to poorly matched components and draw in air and lean out the mixture.
• Synchronization of carburetors
  • Poor side-to-side carburetor synchronization of carburetors.  Un-synchronized air flow balance will cause one bank of carburetors to be advanced relative to the other and for a given total power output the throttles for the advanced bank will be more open than for what is optimum for the engine speed resulting in lean running conditions for those cylinders in the retarded bank.
  • Sticking throttles may also cause a synchronization issue.
• Exhaust system
  • Exhaust system does not provide adequate back pressure.  If not enough back pressure is present then unburned fuel may be exhausted, especially at lower RPM operation.  If your muffler has dual outlets then the following is a method to check for this condition:
    • Block off one exhaust outlet, drive the car and see if condition improves
    • Block off only a part of one outlet and see if condition improves or degrades from previous test
    • If the second test results in improved performance than for the first then look to find reduced diameter exhaust tips to slip over existing tips
    • If the first test results in improved performance then try the OEM muffler again.
• Racing camshafts used with street muffler will result in exaggerated intake reversion thereby diluting the inlet air with exhaust gasses resulting in enriched combustion due to lack of oxygen.
• Gas issues (affects all operational engine speeds)
  • Modern pump gas typically demands slightly larger jet sizing than those from 40 years ago
  • Gas tank must be vented to atmosphere, either through the gas cap or by other means.  If the ambienttemperature drops then gas flow will stop if the tank is not vented.  Also, fuel consumption will create a vacuum in a non-vented tank and stop fuel flow to the carburetors.
• Ignition issues
  • Timing is advanced
  • Spark plugs too hot

Lean Idle (during tuning) cannot be adjusted

• Partial blockage of idle jet (internal debris or debris blocking tip)
• Fuel gallery supplying idle jet clogged (See "Clearing "Hidden" Fuel Gallery" in Advanced Procedures.)
• Check vacuum ports on intake manifolds and on base flanges of carburetors are plugged or correctly connected with fresh vacuum line and clamps
• Lead plug in fuel supply gallery fell out
• Idle mixture screw broke off tip and left it stuck in the 1.0mm diameter metering hole

Sneezing (Sniffing) up through Intakes at idle or during partial throttle Operation

• One or more cylinders are lean
  • Isolate cylinders and open mixture screws 1/2 turn
• Idle jets too small
• Idle jet(s) blocked with debris
• Side-to-side air flow balance needs adjusting at 3000 RPM
• Idle mixture leans out as RPM increases during progression - idle air correction jets too small
• Contaminated gas (water in float well/fuel tank)
• Loose throttle shaft/journal fit - uncontrolled ambient air drawn into carburetor with erratic fuel mixtures as the result
• Intake air leak (gaskets not sealing, cracked manifold, open vacuum tap on intake manifold, etc.
• Lead plug sealing top of "Hidden Fuel Gallery" is missing or loose
• Valves not sealing well in cylinder heads (set too "tight" or wear related issues)
• Engine is cold (cold engine will not vaporize gas mixture as well as for a warm engine)
• Air cleaner assembly removed or not used - open intakes are more prone to "sniffing", especially since they are frequently used with race -oriented camshafts with large valve "overlap"
• Timing retarded
• Spark plugs not of correct type for engine
• Weak spark
• Distributor advance not working correctly, smoothly
• Distributor driveshaft and bearings worn and allows erratic valve timing
• Distributor cap, ignition wires or plug connectors old/defective

Flat spot during acceleration out of corners

Symptoms

Difficulty in applying throttle at the end of cornering during road racing

Corrective Actions

• "Anti-percolation" vent holes drilled into top covers of carburetors: Fuel will "climb" the interior walls of the fuel well and pass through these holes with the result that fuel will flood the progression circuit and richen the progression on the carb on the outside of the corner; fix problem bu sealing these holes
• Fuel starvation: Fuel will move away from the inlets to the main jets on the inner bank of carburetors; fix problem by installing fuel well baffles (See "Install fuel well baffle plates" in Weber Performance)

• Tall auxiliary venturis loose or have poor mechanical interface with fuel transfer port in throttle body: Fix problem by improving fit of auxiliary venturi and prevent it from vibrating (See "Modify throttle housings to secure tall auxiliary venturis" in Weber Performance.)

• Accelerator pump squirt inadequate: Fix problem by increasing squirt amount or by reducing amount in case squirt amount is too much.

Sources of Debris in Float Wells

Most efforts to stop debris from entering the fuel wells revolve around installing new fuel filters and replacing old fuel lines with new, ethanol resistant hoses.  All these are good but there are three basic sources of debris in the float well to be aware of:

1. Dirt in the fuel getting past the fuel filter or internally decaying fuel lines downstream of the fuel filter.
2. Dirt bypassing the air seals on the air cleaner (or rust in the troughs where the air seals are installed) and entering through the holes and vent pipes in the top cover of the carburetor body.
3. Rusting of the inside diameter and internal bits of the vent pipes in the top cover. This is typically missed as a source of debris. Replacement of the vent pipes is recommended to correct corroding vent pipes; look for Weber Vent Pipe Replacement Kit.

Supplemental items to investigate to the above basic comments:

1. A rusty fuel tank will pass a fine silt of rust through your fuel filters and into your Webers until the filters clog so removal and professionally cleaned and relining is in order.  Be wary of the type of tank sealant used as some sealants will react with ethanol and create a goo that will really plug up everything from the tank to the jets in your carbs.
2. Remove and clean the fuel screen in the bottom of your fuel tank.  This screen is located to the rear and bottom of your tank and located near the steering rack.
3. The vent pipes in the top covers of your Webers and the steel washers used for their installation will rust and drop flakes directly into the fuel well, replacement is recommended.
4. Intake air horns if not plastic are subject to corrosion when the OEM nickel or zinc plating is damaged.  Rust is the result and can be ingested either by the engine or through the vent pipes and into the fuel wells.
5. The spark arrester in the OEM air cleaner housing (25mm diameter, right-angled tube in the front of the housing) is a good source of rust to be investigated
6. Check the hose that runs from the spark arrester pipe to the oil tank for cleanliness, anything inside can be sucked the air cleaner housing
7. The trough where the seals are placed in each of the two lower air cleaner housings ("boats") will collect water and rust through.  If the pinhole is to the inside of the gasket then flakes of rust will be sucked into your Weber...ask me how I know of this one.
8. Poor sealing of the air cleaner housings with their respective gaskets allow outside air and dust to bypass the seal.  OEM and aftermarket air cleaner housings both suffer this issue.  Augment the sealing efficiency by applying a strip of self-adhesive, closed-cell neoprene sponge rubber to each of the OEM gaskets to provide a very compliant seal.

Assuming you have achieved a well-sealed air cleaner along with rectifying the other potential debris sources you may still suffer crud entering your carburetors.  The smallest orifice in the Weber is that of the idle jet and thereby the most susceptible for blockage.  Anything smaller than the tip diameter of the idle jet will either stay at the bottom of your fuel well or be delivered to your engine to be spit out the tail pipe.  Remember that the air cleaner operates under a slight vacuum so any imperfect seal is potentially a source for dirt to enter your engine.