Complete S&S Carburetor Schematic Diagram and Troubleshooting Guide
Start by locating the fuel inlet on the right side of the assembly–this is where precision adjustment begins. Use a 5/16-inch wrench to loosen the needle seat without fully removing it; over-tightening can strip the threads. The idle mixture screw should be turned 1.5 turns from fully seated for most Harley-Davidson Big Twin applications running S&S-style components. Deviating from this baseline risks lean condition on startup or excessive richness at cruising RPM.
Identify the accelerator pump circuit–this determines throttle response between 2,500 and 4,000 RPM. The optimal discharge nozzle size for air-cooled V-twins is #70, but swap to a #60 if hesitation occurs during mid-range acceleration. Verify pump arm engagement at 1/4 throttle; a delay exceeding 0.3 seconds indicates worn linkage bushings or incorrect diaphragm tension.
For altitude adjustments, reduce the main jet by one size per 2,000 feet above sea level. Colder climates require a leaner pilot jet (#35–#40), while humid environments benefit from a richer setting (#45). Always cross-reference flow bench data before finalizing changes–visual inspection of spark plug color is not sufficient for forced induction setups.
Venturi size governs airflow velocity, directly impacting torque curve shape. A 1-7/16-inch bore suits 96–110 cubic inch engines, while 1-1/2-inch suits stroked 120+ CI builds. Pairing mismatched components–such as a 1-3/8-inch venturi with a 32mm throttle body–creates turbulence that disrupts fuel atomization at high speeds. Verify alignment by inserting a 0.003-inch feeler gauge between the throttle plates and idle stop screw; interference here causes irregular idle.
Record all measurements before disassembly. Label each component–particularly emulsion tubes and check valves–as orientation affects performance even if parts appear identical. Store gaskets flat under 10 lbs of weight to prevent warping; silicone-based sealants cause fuel leakage when exposed to ethanol blends above E10.
Understanding the Illustrated Fuel Mixer Layout
Begin by identifying the throttle plate location near the intake bore–measure its clearance with a feeler gauge; 0.004–0.006 inches ensures optimal airflow without vacuum leaks. Adjust using the idle-mixture screw clockwise for leaner settings, counterclockwise for richer ones, verifying changes with a CO analyzer at 1.5–2.5% for most V-twin engines.
Trace the accelerator pump circuit: the diaphragm must snap back within 0.3 seconds when manually depressed–any lag indicates worn seals or contaminated fuel. Use a 0.015-inch drill bit to clean pump passages if hesitation persists during cold starts, ensuring no debris remains after disassembly.
Locate the main jet cluster beneath the float bowl; high-altitude tuning requires reducing jet sizes by 2–3 increments per 1,000 feet above sea level. For forced-induction applications, increase jets 3–4 sizes and install richer emulsion tubes (S&S part #36-4930) to prevent lean surges under boost.
Inspect the power valve diaphragm for cracks–test by applying 10 psi of compressed air; bubbles indicate failure. Replace only with Viton-coated versions for ethanol-blended fuels, as neoprene degrades within 6–8 months.
Calibrate the float height to 0.720 inches using a depth micrometer, adjusting tangs while holding the float at its highest point. Verify bowl drainage by tipping the mixer; fuel should cease flow within 5 seconds–exceeding this signals incorrect level or stuck inlet needle.
Check choke vacuum piston freedom by blowing through the air horn venturi–resistance should be minimal. Lubricate the linkage with graphite powder, not oil, to prevent grit adhesion that causes erratic enrichments during warm-up cycles.
Identifying Key Components in an S&S Fuel Delivery Blueprint
Begin by locating the throttle body–the central housing where airflow regulation occurs. This section houses the butterfly valve, directly controlled by the twist grip or push/pull cable. Verify its smooth movement; hesitation or sticking indicates worn bushings or misalignment.
Trace the fuel inlet path to the needle and seat assembly. The seat (a brass or stainless component) seals against the tapered needle when closed. Check for debris–tiny particles disrupting this seal cause erratic idle or flooding. Replace the O-ring annually if exposed to ethanol-blended fuels.
Examine the emulsion tube–positioned inside the main fuel well. Its calibrated holes mix air with gasoline before discharge into the venturi. Clogged holes result in lean conditions; use compressed air at 40 PSI to clear blockages without dislodging internal parts.
| Component | Material | Failure Signs | Inspection Tool |
|---|---|---|---|
| Needle valve | Stainless steel | Sputtering at cruise | Magnifying loupe (10x) |
| Float | Brass or Viton | Fuel leaks from overflow | Float height gauge |
| Idle jet | Copper alloy | Stalling when cold | Jet cleaning wire (0.3 mm) |
Inspect the accelerator pump circuit next. The diaphragm must hold vacuum; a torn unit drops power when opening the throttle rapidly. Test by depressing the pump lever–fuel should squirt from the discharge orifice. If absent, replace the diaphragm and check the check ball for freedom of movement.
Adjusting the Float Level
Remove the bowl and invert the assembly. The float should parallel the gasket surface; adjust the tang until a 0.060–0.080 inch gap exists. Use calipers–eyeballing risks fuel starvation or overflow. Ethanol corrosion necessitates annual adjustment.
Finally, verify linkage synchronization between dual throttle bodies. Both butterflies must open fractionally ahead of the other (0.010–0.020 inch difference). Use a feeler gauge blade between the throttle stop and butterfly edge–uneven gaps create surging at steady speeds.
Step-by-Step Guide to Decoding Performance Fuel System Blueprint Markings
Begin with the throttle bore identifiers–typically labeled *A*, *B*, *C*, and *D*–each corresponding to a quadrant of the airflow pathway. Locate the primary circuit jets (*Main Jet* near the base) and note their numerical values, which dictate fuel mixture ratios at cruising speeds. Adjacent to these, find the idle mixture screws (marked *Idle* or *Slow*), usually positioned just above the baseplate; turning them clockwise leans the mixture, counterclockwise enriches it. Cross-reference these with the pilot jet (often stamped *PJ* or *Pilot*), which governs fuel delivery at low RPMs–mismatched sizes here cause stalling or hesitation.
Trace the needle position (denoted *Clip* or *Notch*) on the metering rod–higher grooves increase mid-range richness, while lower settings trim fuel flow. Verify the accelerator pump linkage alignment (labeled *Pump* or *AP*) by cycling the throttle; improper synchronization causes flat spots. Finally, inspect the float level indicators (*Float Height* or *FH*), measured in millimeters from the gasket surface–deviations beyond ±0.5mm disrupt fuel consistency.
Key Tuning Adjustments for Performance Fuel Delivery Systems
Start by synchronizing the idle mixture screws–turn both clockwise until lightly seated, then back out 1.5 to 2 full turns. This baseline ensures proper off-idle response, but deviations of ±0.25 turns may be needed for optimal throttle transition. Use a quality AFR gauge to verify stoichiometric balance (14.7:1) at cruising RPMs (2,500–3,500), adjusting the main jets in 2-size increments (e.g., #74 to #76) until peak torque is achieved without hesitation.
- Fine-tune the accelerator pump shot volume by bending the pump lever: 0.020″–0.030″ travel ensures instant enrichment without bogging. Test by blipping the throttle; fuel should discharge within 1/10th of a second with no drips at rest.
- Adjust the float level to 0.500″–0.520″ from the gasket surface using a clear tube or float gauge. Higher settings risk rich stumble on deceleration; lower settings cause lean sputtering under load.
- For forced-induction applications, increase the power valve size by 1–2 numbers (e.g., #6.5 to #8.5) and pair with a 0.031″–0.039″ pump squirter to prevent mid-range lean-out.
Vacuum Secondary Calibration
Set the secondary diaphragm spring preload to 4–6 inches of Hg with a vacuum pump–this controls the rate of secondary opening. For aggressive cams (duration >230°), reduce preload to 2–3 inches to prevent hesitation. Verify with a road test: secondaries should open gradually at 60–70% throttle, not snap suddenly. If backfiring occurs under deceleration, richen the idle mixture by 0.5 turns or reduce jet size by 1 size (e.g., #78 to #76).
For alcohol-blended fuels (E85), increase all metering passages by 30–40%: main jets (#82–#90), power valves (#9.5–#11), and pump squirters (0.041″–0.047″). Replace viton-tipped needles with ethanol-compatible brass or stainless variants to prevent swelling. Monitor plug color after hard acceleration–healthy operation yields a light tan center with minimal soot or ash deposits.
Diagnosing Poor Engine Response with Reference Charts
Begin by verifying the float level against the manufacturer’s specifications. A needle valve stuck open or improperly adjusted causes flooding, leading to uneven idle or stalling. Use a clear fuel line temporarily to observe flow–consistent bubbles or spurts indicate air leaks or clogged inlet screens. If the engine bogs under load, check the main jet size; too small restricts power, while too large fouls plugs.
Inspect the idle mixture screws for corrosion or misalignment. Turn each screw in gently until it seats, then back out the specified turns–typically 1.5 to 2.5. If adjustment yields no change, the passages may be blocked; remove the screws and blow compressed air through the orifices. Listen for air escaping through the cavity–no hiss suggests debris.
Examine the accelerator pump operation. Depress the throttle rapidly and watch for a strong, immediate fuel spray from the pump nozzle. A weak or delayed spray points to a worn diaphragm, leaky check valve, or clogged passages. Disassemble the pump cover, rinse the components in clean fuel, and replace any brittle gaskets. Ensure the pump linkage moves freely–binding causes hesitation.
Common Fuel Delivery Issues
- Vacuum leaks at intake gaskets or cracked hoses draw unmetered air, leaning the mixture. Spray carb cleaner near suspect areas; RPM changes confirm leaks.
- Fuel filter clogs reduce flow, especially evident at higher RPM. Replace filters if fuel pressure drops below 4 psi.
- Deteriorated pump diaphragms fail to maintain pressure, causing surging. Test pressure with a gauge; readings below 3 psi require replacement.
- Corroded or pinched fuel lines restrict flow. Inspect full length for kinks, swelling, or greenish oxidation.
Check the choke mechanism for proper engagement. A stuck choke enriches the mixture, fouling plugs; a failed choke leans it, causing hard starts. Manually actuate the choke plate–it should close fully when cold and open completely when warm. Clean the assembly with solvent if movement is sluggish. Verify the choke spring tension matches factory settings.
Test the power valve with a vacuum gauge. Attach the gauge to the manifold; if vacuum drops below 5 inHg under load, the valve is likely leaking. Remove the retaining plug, inspect the valve seat for scoring, and replace if damaged. Ensure the valve activates at the correct load point–early opening wastes fuel, delayed opening causes hesitation.
- Remove the air cleaner and observe the venturi for dirt or varnish buildup, which disrupts airflow and fuel atomization. Clean with a dedicated solvent, avoiding wire brushes that can damage coatings.
- Reinstall jets finger-tight only; over-tightening distorts threads, altering flow. Use a torque wrench for precision–typically 4-5 ft-lbs.
- After reassembly, start the engine and verify baseline adjustments. Let it reach operating temperature, then fine-tune mixture screws in 1/8-turn increments until smooth idle is achieved.
Electrical and Timing Considerations
Weak spark due to fouled plugs or faulty ignition wires mimics carburetion issues. Replace plugs with the correct heat range and gap; inspect wires for cracks or resistance exceeding 5,000 ohms per foot. Verify ignition timing with a strobe–retard causes bogging, advance triggers pinging. Reset to factory specifications if off by more than 2 degrees.