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.
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.
Rich running symptoms:
Sluggish engine performance, black spark plugs, poor mileage, low AFR readings (10 and below), stinky exhaust:
Idle mixture too rich:
Perform air flow balancing with STE Synchrometer and Lean Best idle mixture tuning procedure
Fuel float level too high:
Check fuel levels with external gauge while engine is idling
Fuel pressure too high
Check fuel pressure with quality gauge while engine is running
Check jetting:
Compare main circuit jetting to information provided in Performance Tuning
Get jet gauges and measure actual jet sizes
55 idle fuel jets are typically used for 2.0 up to 2.7 liter engines and 60 idle fuel jets are for 2.7 and larger engines.
Blocked idle air correction jet (causes fuel to siphon into cylinder)
Check ignition system:
Spark plugs too cold or gapped too small
Check plug wires, plug connectors and distributor cap for cleanliness and aging
Check ignition timing and smoothness of advance; timing may be retarded
Lean running symptoms:
Sneezing or sniffing up through intakes, engine won’t take throttle, white spark plugs, engine runs hot (210F is maximum engine temperature to be acceptable), high AFR readings (14 and above):
Idle/progression mixture too lean:
Perform air flow balancing with STE Synchrometer and Lean Best idle mixture tuning procedure
Isolate which cylinder is sniffing and open idle mixture screw 1/4 to 1/2 turn
Check ignition system:
Check ignition timing and smoothness of advance; timing may be advanced
Check fuel delivery:
Test for adequate fuel delivery volume and pressure
Check jetting:
Compare main circuit jetting to information provided in Performance Tuning
Get jet gauges and measure actual jet sizes
55 idle fuel jets are typically used for 2.0 up to 2.7 liter engines and 60 idle fuel jets are for 2.7 and larger engines.
Check for air leaks:
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 during slow acceleration (up through 4500 RPM)
Idle jets too small
Progression air correction jets too large
Floats set too low
Try adjusting mixtures at 1800 to 2000 RPM
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
Axillary (booster) venturis installed incorrectly (rotated)
Exhaust header too big
Valve clearance set too tight (advances timing)
Ignition over-advanced
Weak spark
Inadequate fuel delivery
Poor power throughout driving range
Timing not advanced enough
Main jets too small
Exhaust popping on deceleration
Exhaust leak
Intake air leaks
Float bowls empty or engine hydro locks
Check idle circuit air correction jet for blockage
Hard Starting