My Datsun 240Z runs rich and stalls at idle with SU carbs. How do I fix it?

Short answer: Start from the book settings, then lean each carb's mixture until it idles smoothly, and balance the two carbs with a synchronizer. Owners say many 240Zs need leaner than the book. If the plugs on one carb are black, clean that carb's float needle valve and check its float level. Put a thermostat back in, since one owner's hot-weather stalling started after removing it, and Nissan recommends a 170-degree unit for hot climates.

Symptom: Black, sooty spark plugs, smoke or a fuel smell, rough idle, and the idle dropping until it stalls at lights after the engine is fully warm.

What to check first

1. Pull the plugs: black on the front three or the rear three points at one carb.
2. Clean the float needle valve and its filter on the rich carb, and check float level.
3. Reset mixture from the book starting point, then lean it out a little at a time.
4. Balance airflow between the two carbs.
5. Make sure a thermostat is installed and the fan clutch isn't spinning too freely.

Likely causes

1. Mixture set too rich. SU carbs meter fuel with a tapered needle in a jet, and the mixture screw sets how rich it runs. Owners say the factory starting settings are only a guideline and many engines need a leaner setting.
2. Dirt in the float needle valve. The float and needle valve control how much fuel sits in the carb bowl. A speck of dirt holding the valve open floods the carb and makes it run rich.
3. Running without a thermostat or with heat soak. An engine with no thermostat runs at the wrong temperature, and heat in a hot climate can upset idle after a long drive.

What it usually costs

Owners in these threads didn't post prices. Most of this is adjustment and cleaning.

Is it safe to keep driving?

Stalling at intersections is a safety problem because you lose power steering assist and may block traffic. Fix it before regular driving.

DIY or shop?

Very DIY-friendly with patience and basic tools. A Z specialist can tune it if you'd rather not.

Recalls, bulletins and complaints in plain words

• Recalls: NHTSA's recall lookup returned no data for the Datsun 240Z for 7 model-year queries on 2026-10-31, so we couldn't confirm recalls here. Check your VIN at nhtsa.gov/recalls. If your car is part of a recall, the dealer fixes it for free.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect.
• NHTSA complaints: 0 keyword-matched complaints for Datsun 240Z 1970–1973 (of 0 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-31; keywords: stall, carb, idle, fuel) (NHTSA returned no data for 7 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In Tips and tricks for getting su carb throttle and mixture screws correct on Reddit r/240Z, a 240Z owner finishing a rebuild had it running rough and smoking with the SU mixture set to the book's starting point. Another owner said the book numbers are just a guideline and theirs needed to be leaner, even at sea level. They said a rich condition usually isn't a leaky float, that floats rarely need replacing unless they're the early brass type (you can test those by dunking them), and to clean the filter at the float needle valve.
• In 1972 Datsun 240z with stock SU carbs stalling at intersections on Classic Zcar Club, an owner who shipped a 1972 240Z to Panama removed the thermostat because the gauge ran past half. After about 25 minutes of driving, the idle dropped to about 600 rpm at lights and the car stalled. The front three spark plugs were black while the rear three were clean. A member said they run better with a thermostat, that Nissan recommends a 170-degree one for tropical climates, and to check whether the fan clutch spins too freely.

Quick tip

Don't fight it with screws alone. Read the plugs to find which carb is the problem, clean that float valve, and put a thermostat back in. Then tune.

Vehicle focus: Datsun 240Z with twin SU carburetors, 1970-1973.

Safety / mechanic note

A car that stalls in traffic is a hazard, and a flooding carb is a fire risk. Verify with a mechanic.