Periodic Maintenance


The following information is presented as a guide for the tuner performing routine periodic maintenance of their carburetors.  It is assumed the tuner possesses basic familiarity of engine operation and associated auto mechanics.  The first time tuner will experience a steep learning curve but repetition over time will clarify the tasks and their purposes so reviewing the basic principles before each tuning session is recommended to refresh your total understanding of the processes.

Periodic Maintenance is defined as "routine, scheduled service of normally operating carburetors" and is performed during normal maintenance intervals.  Maintenance incorporating a gasket kit is considered to be a Routine Rebuild and is covered in that section.  Periodic Maintenance includes inspection and adjustment of the carburetors and is performed concurrently with setting valves, adjustment of the ignition timing, changing the oil, etc.

This Periodic Maintenance discussion assumes the carburetors are matched and properly jetted for your particular engine and are in good, operational condition. Also assumed is that maintenance is being performed on a regular basis and the carburetors are only subject to minor adjustments, more extensive tuning is discussed in Performance Tuning

Weber's do not have many moving parts but worn parts will cause drive-ability and tuning issues beyond what may be adjusted away by the processes addressed in this section.  Additionally, mechanical adjustments change as a function of usage: gaskets harden and leak, debris collects in the fuel bowls and air leaks begin to manifest themselves; all are good reasons to perform Periodic Maintenance to keep your engine running smoothly and efficiently.

 Recommended Tools for Periodic Maintenance

The following tools are recommended for routine maintenance and tuning your Webers.  It is possible to perform the tuning tasks without the recommended tools but with increased difficulty. 

  • Carburetor cleaner such as Berryman's, Gunk, Simple Green, Chem12 or similar
  • Shop rags (lint free) and paper towels
  • Type AB fire extinguisher
  • Small parts cleaning brushes (tooth brush and straight bristle brush)
  • Trays and containers for sorting/holding parts and for cleaning purposes
  • Large, medium and small flat blade screwdrivers with good condition tips
  • Small flashlight
  • Colortune tuning aid
  • STE synchrometer tuning aid
  • Float level vial
  • Fuel pressure gauge with adapter fittings
  • Mechanic's inspection mirror (mirror on a handle; for viewing Colortune)
  • Mechanic's magnetic wand (for retrieving dropped parts)
  • Dial calipers (measuring float needle valve shims)
  • Tweezers (for retrieving float needle valve shims from well)
  • 320 grit sand paper
  • Spark plug installation tools
  • Wrenches:
    • 7mm (wrenching flats on 8mm ball sockets on throttle linkage)
    • 8mm (idle air correction screw nut, throttle body and air horn nuts, drop link nuts)
    • 10mm (main jet holder and fuel bowl drain bolt)
    • 12mm (nuts on intake manifold)
    • 13mm (mounting fasteners for throttle cross bar)
    • 14mm deep socket, thin-wall, 3/8" drive (fuel float needle valve)
    • 16mm, 6 point (hex cap over fuel float needle valve)
    • 18mm, 6 point (banjo bolts)
    • 3/8" ratchet wrench for 14mm socket

Preparatory Tasks

Your engine must be in good mechanical and electrical condition prior to adjusting your Webers, specifically:

  1. Battery is fully charged with good, clean connections and all wiring in general is in good condition.  This includes verifying proper alternator functioning and fan belt adjustment.
  2. Valve clearances have been adjusted to factory specifications on a cold engine.
  3. Engine is in good condition with even compression readings in all cylinders.
  4. Spark plugs are clean, in good condition and are the correct type for the engine including:
    1. heat range
    2. reach
    3. correct gap
  5. Distributor and coil or CD ignition are in good condition including connections
    1. Advance mechanism has smooth operation and returns without sticking to idle advance setting
    2. Distributor advance and timing has been set correctly at idle speed and at 6000 RPM
    3. Vacuum advance/retard (if any) is operating and hoses and fittings to intake manifolds are in good condition
    4. Distributor type and advance curve are matched to engine specifications
    5. Distributor points (if any) are in good condition without pitting and have proper gap (dwell angle)...don't forget the point grease
    6. Distributor cap is:
      1. Clean of grease/road grime
      2. Correctly seated onto distributor body
      3. No cracks or other mechanical defects
      4. Internal electrodes are clean and free of corrosion
    7. Spark plug wires are:
      1. Clean of external grease/road grime
      2. In good condition without signs of age related deterioration
      3. Are firmly connected at each end
    8. Gaskets and vacuum lines are in good condition including:
      1. carburetors to intake manifolds
      2. intake manifolds to heads
      3. exhaust headers to heads
      4. muffler to exhaust headers
      5. power brake vacuum lines
      6. Sportomatic vacuum lines
      7. vacuum advance line to distributor
    9. Tachometer is working correctly
    10. Fuel delivery system is in good order:
      1. Fuel pressure has been verified to be 3.5 psi plus/minus 0.5 psi at the inlets of the carburetors and measured while the engine is running
      2. Fuel delivery rate has been verified to be sufficient for the horsepower of the engine.  See Advanced Procedures for more discussion regarding this topic.
      3. Fuel filters are verified to be in good order including:
        1. In-tank fuel screen (routinely overlooked for freedom from blockage due to debris)
        2. In-line filters both before and after the fuel pump
        3. Fuel screens in banjo fittings are clear
      4. Fuel systems incorporating a return-to-tank fuel line have been checked for fuel returned to tank is freely flowing
      5. Fuel in tank is fresh
    11. Exhaust system is in good order including:
      1. Connections between the exhaust ports on the heads and the muffler are tight and gaskets are in good condition and are not leaking
      2. Headers/heat exchangers and mufflers are free from holes due to corrosion
      3. Headers and muffler are correctly selected and sized for the engine
    12. Carburetors are in good condition including:
      1. Throttle shafts and throttle valves are not worn excessively
      2. Jetting is correct for the engine
      3. Carburetors are clean and any fuel leakage issues resolved
      4. Throttle return springs easily close throttles
    13. Throttle linkages are adjusted and in good condition:
      1. Full throttle is provided at the carburetors
      2. Linkages operate freely and return to closed throttle position when throttle is closed
      3. Wide-open throttle pedal travel is stopped at the throttle pedal and not stopped by throttle lever arm contact with the throttle body

Periodic Maintenance Procedure

Check for Fuel Leakage

The first task of periodic maintenance is to clean the carburetors of road film to allow a careful examination of components for condition and to facilitate the tuning process.  As long as a water shedding air cleaner housing is installed the use of standard de-greasing solvents such as Simple Green may be applied and then rinsed with water.  Simple Green may be applied in concentrated form with small brushes for cleaning the more stubborn areas.  Of course, dedicated carburetor cleaner fluids (Chem12, Gunk or Berryman's) are particularly effective when used properly but may cause secondary cleaning issues due to the rinse water carrying residues onto engine shroud and other downstream components.  Take note of areas of high concentrations of dirt accumulation as sources of fuel leakage to be investigated more thoroughly.

Following the external cleaning process the engine air cleaners may be removed to provide full access to the carburetors.  Stuff the air horns with clean, lint-free rags to stop nuts, screws, etc., loose debris and cleaning solutions from getting deeper into the throttle bores.  If a fastener or other object is dropped down a throttle bore: Don't Panic!  As long as the throttles of the carburetors are not activated then there is little chance of these parts falling further down and possibly into the cylinder through an open inlet valve.  Use the magnetic mechanic's wand to retrieve magnetic parts but non-magnetic parts will require carburetor removal to reliably retrieve dropped items.  For future reference: Get into the habit of inventorying all items as you remove them to be sure all items are accounted for during removal and re-installation.

Remove the rags from the intakes and turn the ignition on but no not start the engine; the first visual inspection for fuel leaks is performed at this time.  Check the fuel lines for seeping along the length of each fuel line and especially at the ends and where the lines make tight bends or rub against hard components.  Use your mechanic's mirror and flashlight to get a good look.  Remember to scrutinize those areas with heavy concentrations of road grime as indicators of seepage or pooling of dripping fuel.  Check all around the carburetors, front to back, side-to-side and the top cover for leakage past gaskets and the vent pipes installed into the tops.  All fuel leakage issues require attention to understand the nature of the issue and to make corrective actions.

Once the static fuel seepage test is complete and those issues requiring maintenance have been addressed the engine may be started to allow a repetition of the same inspections to be performed.  The vibrations of a running engine will cause some seeps to disclose themselves that were otherwise "good".  In particular, look for leaks in the external fuel galleries and miscellaneous screws associated with the idle and accelerator pump circuits as these are not activated on a static engine.

The following are obvious fuel components requiring investigation for leakage:

  • Fuel needle valve hex caps
  • Banjo bolts & banjo fittings
  • Fuel lines
  • Accelerator pump bodies
  • Lead plugs (fuel gallery seals loosen over time, they may need to be reset or replaced)
  • Vent pipes in the top covers

Fresh fuel lines and gaskets rectify most of the above issues save for the lead plugs and vent pipes.  The corrections of seepage issues concerning these two are discussed in Advanced Procedures.

Reminder: All fuel leakage issues require correction before proceeding with Periodic Maintenance.

Air Filters and Intake Manifolds

Air filters are a must if you expect your engine to have any reasonable service life.  Open intakes look and sound fantastic but at a great cost, the abrasive dust in the air will wreak havoc with your piston rings and cylinder walls.  Additional trouble comes with debris in your engine compartment being drawn into your intakes or into your fuel well, debris commonly found in the recesses of your engine bay have entered your engine compartment and could have entered your engine with dire consequences.  Clean air filters do not significantly limit engine performance so use them!  Items to review regarding your air filters:

  • Clean filter elements
    • Inspect your air filter elements for cleanliness and use compressed air to gently blow them clean by blowing from the inside to the outside and be gentle about it!
    • After market air filters may have unique maintenance/rejuvenation procedures.  Refer to manufacturer's recommendations for maintenance procedures.
    • Replace dirty filters at recommended intervals (typically at 24,000 mile internals for normal environments and more frequently for dusty ones)
  • Gaskets and seals
    • Check for telltale dust accumulations to indicate areas of leakage
    • See Advanced Procedures for tips regarding augmentation of sealing efficiency
  • OEM air cleaner housings
    • Seal troughs in the OEM air cleaner housings are susceptible to rusting and subsequent piercing of the metal due to corrosion.  Pin-holes are a source of rust particles that may be drawn into the inner chambers of the air cleaner since they bypass both seals and filters.  Once inside the air cleaner these rust flakes are drawn into the cylinders through the air horns or into the fuel bowls where they end up blocking an idle jet.  Check the external portions of the air cleaner housings for evidence of rust such a bubbling of the painted surfaces indicating imminent piercing.  Gasket removal, rust neutralization followed by gasket replacement is in order.
    • Another hidden source of debris and rust inside the air cleaner is from the drain pipe that is located at the bottom of the main air cleaner housing.  This pipe allows condensed water to be drained from the air cleaner but since it is subjected to dirt and condensed water it will eventually rust.  Also, this is a point of unfiltered air being drawn into the engine, the hose that attaches to this pipe is a source of  debris so check it for cleanliness and replace as warranted.
    • Although the spark arrestor in the breather line to the oil tank and the intake air horn of the main air filter housing have their air filtered it is worth while to investigate them for rusting corrosion or dirt accumulation. 
    • Check the tension in the over-center spring clips securing the various components of your OEM air cleaners.  These need to be well tensioned to assure the gaskets are sealing with their interfaces as efficiently as possible.

Throttle Linkage Setup

Throttle linkages need investigation to determine suitability for continued usage and if deemed to be in good condition then cleaned of old grease and re-lubricated.  Components wear as pairs, the 8mm ball studs are more easily checked for wear but the ball sockets also wear and when the two are assembled they may be susceptible to disconnection while driving.  Lubrication acts as a wear preventative but only as long as it is clean, road grime will combine with the grease becoming a very effective grinding paste.  Cleaning and lubricating with good quality, water-resistant, lithium grease is recommended.

Linkage ball joints to be serviced include:

  • Bell crank on intake manifold at cylinder #3
  • Bell crank on side of transmission side cover plate
  • Longitudinal connecting link from main throttle cross-bar to bell crank on intake manifold
  • Drop links from throttle cross-bar to carburetor throttle lever arms
  • 13mm ball sockets on each end of throttle cross-bar

Mechanical adjustment of linkages

  • Throttle pedal stop adjustment checked/adjusted
    • The throttle pedal must have a positive, adjustable stop to allow for full throttle opening without allowing the throttle lever arm on the carburetor from coming into contact with the mechanical stop on the throttle body.  If there is mechanical contact then there will be a greatly increased loading of the throttle shaft bearings which will accelerate wearing and contribute future tuning issues.
  • Check the linkages from the throttle pedal to the carburetor drop links for freedom of movement and that wide-open-throttle is achieved for both carburetors when the throttle pedal is depressed to its travel stop
  • The connecting links from the throttle shafts to the cam-levers that activate the accelerator pump need to be inspected for wear and replaced as necessary.
  • The throttle cross bar needs to be checked for misalignment issues within itself.  These are routinely out of alignment resulting in difficulty performing side-to-side balancing of the carburetors.  See Advanced Procedures for further information.


Tune-up Interval Carburetor Adjustments

Rough running and idle performance may be traced to several items but for this section only carburetor issues will be addressed.  Typically these running issues are the result of improper mixture settings or airflow balancing.  The sequence of steps is provided in simplified format with more complete discussions for specific items provided on separate web pages:

  • Have your fire extinguisher handy at all times
  • Remove air cleaner tops to gain access to the air intake horns
  • Connect cross bar linkage and drop links and adjust so they just snap onto the throttle levers without binding, these will be readjusted later but for initial starting it is handy to have these connected
  • Perform initial adjustments of throttle stop screws, idle mixture screws and idle air correction screws if they have been disturbed.  See "NOTES" below for initial settings.
  • Turn on ignition and allow float bowls to fill with fuel (ten seconds)
  • Actuate the throttle cross bar until fuel is observed squirting from the accelerator pump jet nozzles
  • Depress the throttle pedal to the floor and hold for a count of three and repeat (This allows the accelerator pump to deliver a full measure of fuel for each pedal depression.)
  • Hold throttle pedal at 1/8 throttle  and crank engine
  • Once the engine fires you should gently operate the throttles to allow fuel to enter the idle circuit which will result in a stable idle if all is well.  Note: Light revving of the engine with the throttles returned to the closed position will hasten the fuel delivery to the idle circuits, maximum intake manifold vacuum and subsequent filling of the fuel galleries is generated when the throttles are closed and with an engine spinning at elevated RPM.
  • If the engine will not idle when the throttle is released then adjust throttle lever stop screws in 1/4 turn increments until idling has been established
  • Allow the engine to come to temperature, 140 F is good for initial tuning.  Final tuning is best performed on a fully warmed engine which is 175 F.
  • Check fuel supply pressure is 3.5 psi plus/minus 0.5 psi.  See Standard Procedures for detailed information.
  • Disconnect and remove both drop links connecting the throttle cross bar with the carburetors.
  • Use STE air flow meter to measure air flows on cylinders #1 and #4 and adjust side-to-side balance to have equal air flow and to establish an engine RPM of 950 +/- 50 RPM if possible.  Use the idle speed stop screws (knurled-head screws with concentric springs that touch the throttle lever arms) for this adjustment.
  • Use STE to measure air flows on cylinders #1, #2 and #3 (STE reading of "5" is to be the standard goal for air flow balance.)
  • Use the idle air correcting screws (with the 8mm hex  lock nuts) to increase the air flow reading for the two throats with lower STE readings than that of the highest observed reading.
  • If the engine RPM changes as a result then use the idle stop screws to readjust engine speed to 950 RPM.
  • Repeat the STE measurement and adjustment for cylinders #4, #5 & #6.
  • If the engine RPM changes as a result then use the idle stop screws to readjust engine speed to 950 RPM.
  • Check that all six cylinders are flowing the same amount of air and engine is idling at 950 RPM.
  • NOTE: If there is spitting of "sniffing" up through the intakes during this air flow balancing then open the mixture screws for those offending throats in 1/4 turn increments to help tame the lean mixture issue.
  • Adjust idle mixture settings on all cylinders using "Lean Best" method (see Standard Procedures).  The adjustment of idle mixtures WILL affect engine idling speed upward with the result of all idle air correction screws and idle speed stop screws needing readjustment as before and all idle mixture screw settings will need to be optimized for Lean Best mixture again. 
  • Stop the engine and prepare to install the fuel level gauge by first opening the fuel bowl drain bolt and catching the spilling fuel in a small catch bowl.  Have some shop rags placed so any dribbles are caught.  Be sure the fire extinguisher is handy.
  • Install the fuel level vial by threading into the open fuel bowl drain port.
  • Turn on the ignition and allow fuel to enter the bowl, start the engine and allow it to idle for a few seconds allowing fuel to reach a steady-state of operation.
  • Switch the ignition off and observe the fuel level in the vial, a small flashlight is handy to use.
  • Adjust shims installed under fuel needle valve to adjust float level; adding shims raises the level and conversely
  • Repeat fuel level adjustments for remaining three needle valves
  • Adjust and install drop links between throttle cross bar and both carburetors.  See Standard Procedures web page for detailed procedure.
  • Adjust air flow balance at 3000 RPM.  See Standard Procedures web page for detailed procedure.
  • Install air cleaners and go for a test drive; it may be necessary to open the idle mixture screws a quarter turn to provide a rich enough mixture setting for actual driving performance


Test Drive

Finalization of the tuning is best accomplished after the engine is warmed to working temperatures (175 degrees or more) to ensure efficient fuel atomization. The best time to take a test run is now and allow the engine to fully warm up and for you to take notes about its performance. Specifically you are looking for symptoms that will guide you in making detailed assessments based upon actual driving conditions rather than static adjustments in the driveway.  Is there surging between 2000 and 3000 RPM? Is it popping on deceleration? Is it transitioning seamlessly between idle and main circuits?  That is, no flat spots in the power delivery, right?

Return to home base. Make any final adjustments that you feel are necessary to the mixture screws. In other words, if it popped occasionally or surged, turn all mixture screws out an extra 1/8 turn. Repeat the test drive and perform any further adjustments that are necessary; always keep notes in case you need to return to a previous state of adjustments.

If the mixture screws are turned out more than three turns then the idle jets are too small. If the engine stumbles on acceleration or is slow to return to idle, the idle jets may be too small.  Another item to consider for stumbling on acceleration is adequacy of accelerator squirt.  See Performance Tuning for help in diagnosing tuning and jetting issues.



  • Mixture screws should be around 1 3/4 turns open from closed when idle jets are correctly sized.  If opened up beyond three turns then look to increasing idle jet size, checking idle jets for blockage from debris or to fuel gallery blockage issues.
  • Idle air correction screws may be opened up to three turns from closed to achieve air flow balance between throttle bores.  Larger openings are ineffective and indicate larger problems regarding wear of the carburetors or throttle valve alignment issues.  See Advanced Procedures for more information.
  • Idle speed stop screws typically open the throttle lever arms by 3/4 turns from their closed position
  • Be gentle when closing needle screws, especially the brass, after-market screws (use a magnet) as they can seize and break off in their metering hole
  • Do not over-torque the brass or other hollow fasteners as they may twist, distort or break.
  • Spitting or "sniffing" up through the intakes may indicate any of:
    • Retarded ignition timing
    • Lean idle mixture settings
    • Uncontrolled air leakage through worn throttle plates or throttle shaft journals
  • Carburetors with twisted throttle shafts or worn throttle shaft journals shafts will not respond to simple tuning adjustments.  Professional service is recommended as the best course of action to follow.
  • Carburetors that are worn may respond to performing Lean Best idle adjustments at a higher engine speed such as 1200 RPM.
  • Lean Best adjustment may cause the engine to suddenly increase as much as 400 RPM which is the result of the mechanical advance mechanism in the distributor activating.  If you cannot return engine speed to 900 RPM with the  idle speed stop screws then perform Lean Best adjustments at an elevated idle speed of 1200 RPM and when finished return the idle speed to 900 RPM and check side-to-side air flow balance before declaring the process completed.