Get Your Carburetor Running Right

Factory jet sizes, float levels, timing baselines, and a repeatable tuning sequence for the most common vintage carburetors. No guessing, no wasted parts.

Find Your Specs

Select Your Carburetor

Choose your carburetor family and engine size. The table below fills in the factory baseline settings so you have a known starting point before making any adjustments.

Select a carburetor family and engine size above to see factory baseline specs.

Tuning Sequence

Follow this order every time. Skipping steps or adjusting the idle mixture before the engine is fully warm will give you inconsistent results.

  1. 1

    Verify the Basics

    Check that the fuel pump delivers at least 4 psi. Confirm the float level matches the spec table above. Make sure the choke plate is fully open when the engine is warm. Replace any cracked vacuum lines before proceeding.

  2. 2

    Set Initial Timing

    Disconnect the vacuum advance hose and plug it. Set base timing to the factory value using a timing light. For most small-block V8s this is 8–12 degrees BTDC. Reconnect the vacuum advance and confirm total timing reaches 34–36 degrees at 3,000 rpm.

  3. 3

    Warm the Engine Fully

    Let the engine idle for at least 10 minutes or drive for 5 minutes under light load. The choke must be fully open and the coolant temperature should be in the normal range. Tuning a cold engine wastes your time.

  4. 4

    Set Idle Speed

    Adjust the idle speed screw to the target rpm. Most street engines want 750–900 rpm in gear (automatic) or 800–950 rpm in neutral (manual). Use a tachometer. Do not trust the dashboard gauge.

  5. 5

    Adjust Idle Mixture

    Turn each idle mixture screw in slowly until the rpm drops slightly, then back it out 1/8 turn. Repeat for the other screw. The goal is the highest vacuum reading or smoothest idle. If you have a wideband O2 sensor, target 14.7:1 at idle.

  6. 6

    Check Accelerator Pump

    Snap the throttle open quickly. The engine should respond immediately. If it hesitates or stumbles, increase the accelerator pump shot by adjusting the pump cam or replacing the pump nozzle with a larger size.

  7. 7

    Road Test and Fine-Tune

    Drive the car through normal conditions: light cruise, hard acceleration, and deceleration. Note any flat spots, stumbling, or surging. Return to step 5 or check the symptom table below for specific fixes.

Symptom-to-Cause Lookup

Select the problem that matches what you are experiencing. Each entry lists the most likely causes in order of probability, so you can check the easiest fix first.

Select a symptom above to see likely causes and fixes.

Tuning Worksheet

Record your before-and-after measurements here. This helps you track what changed and return to a known good state if a new adjustment makes things worse. Your entries are saved in your browser and will persist between visits.

Parameter Before After Notes

Common Mistakes

Over-Lean Jetting

Dropping down two jet sizes at once to fix a rich condition often sends the engine lean. Lean mixtures burn hot and can damage valves in minutes. Change one size at a time and test drive between changes.

Choke Left Partially Closed

A choke that does not fully open when warm floods the engine with extra fuel. This causes rough idle, black smoke, and fouled plugs. Check the choke pull-off and bi-metal spring before touching the jets.

Ignoring Vacuum Leaks

A vacuum leak leans out the mixture and causes a hunting idle that no amount of mixture screw adjustment will fix. Spray carb cleaner around the base gasket and intake manifold while the engine idles. If the rpm changes, you found a leak.

Wrong Float Level

A float set too high acts like a rich jet change. A float set too low starves the engine under load. Always set the float level to factory spec before adjusting anything else.

Assumptions and Limitations

The spec tables show factory baselines for stock or near-stock engines at sea level. If you have a high-performance cam, headers, or a forced-induction setup, the factory numbers are a starting point, not a final answer. Ethanol-blended fuel (E10) typically requires jetting 2–4% larger than pure gasoline because ethanol needs more fuel for the same air mass.

Altitude matters. For every 2,000 feet above sea level, the air is roughly 3% thinner. You may need to drop one jet size for every 2,000 feet of elevation to maintain the correct air-fuel ratio.

This reference covers the five most common carburetor families. If you have a SU, Mikuni, Dellorto, or another type, the tuning sequence still applies but the specific jet sizes and float levels will be different. Check the manufacturer's manual for those numbers.

Last updated: January 2026 · Version 1.1