Why does my MGB lose power when hot or only start on ether?

Short answer: MGB fuel and ignition parts are old and sensitive to heat and modern fuel. If it loses power when hot, members suspected fuel boiling, an overheating coil, a weak fuel pump, or ethanol damage to old rubber. If it only starts on ether after sitting, check that the SU carburetor jets drop when you pull the choke. If it stalls with a gas smell and gas in the oil, check the float needle and seat and the mechanical fuel pump diaphragm, and change the oil.

Symptom: The car runs fine cold, then loses power once it's hot. Or after storage it only starts on ether. Or it stalls and smells of gas, and there may be gas in the oil.

What to check first

1. Pull the choke and check that the SU jets drop (one member said about 1/8 to 1/4 inch).
2. Look for fuel leaks at the float chambers, jet hoses and carb base.
3. Check the oil for a gas smell. Gas in the oil needs fixing and an oil change before more driving.
4. Check the coil when hot, especially if it's old.
5. Replace old rubber fuel hoses with ethanol-safe ones.

Likely causes

1. Choke linkage not dropping the jets. A member's explanation for a car that only starts on ether.
2. Leaking float chambers, needle and seat, or jet seals. Old cork and rubber parts wear and leak.
3. Mechanical fuel pump diaphragm. A leaking diaphragm can put gas in the oil, as members explained for a 1977.
4. Hot coil or other ignition part. Suspected for power loss when hot.
5. Ethanol damage to old fuel parts. Members asked about fuel separation and ethanol.

What it usually costs

No owner posted prices. Members recommended an SU carburetor tuning book for owners doing their own work.

Is it safe to keep driving?

Gas leaks on a hot engine can start a fire, and gas in the oil can damage the engine. Fix both before driving. Verify with a mechanic.

DIY or shop?

DIY. These cars are owner-friendly with a manual; tuning SU carbs takes patience.

Recalls, bulletins and complaints in plain words

• Related recalls: NHTSA lists these recalls touching this area of the MG MGB: 78V089000 (1975-1978 model years, fuel system, gasoline); 78V164000 (1977-1978 model years, electrical system). Check your VIN at nhtsa.gov/recalls. If your car is part of this recall, the dealer fixes it for free.
• Plain words: Recall 78V089000 covered carburetor fuel bowl screws on 1975–1978 MGBs that can loosen and seep gas. Recall 78V164000 covered an overdrive wiring harness on 1977–1978 cars that can chafe and short. If you smell gas under the hood, don't drive it until it's checked. Verify with a mechanic.
• NHTSA complaints: 0 keyword-matched complaints for MG MGB 1968–1980 (of 5 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-26; keywords: stall, dies, no start, power, carburetor, fuel, hot, choke) (NHTSA returned no complaint data for 2 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In 1967 MGB loses power when hot on Car Talk, members suggested fuel boiling, an overheated ignition part such as an original coil, a weak fuel pump, and ethanol damage to old rubber parts, and asked about the carbs, ignition and service history.
• In MGB won't start without ether on Car Talk, a 1968 MGB would only start cold on ether after storage. Members explained the SU choke pulls the jets down to richen the mixture, said to check that the jets actually drop, and warned that leaking float chamber seals and jet hoses are common.
• In 1977 MGB gas smell stalling on Car Talk, an MGB stalled after winter storage and had gas in the oil. It had a single Weber carb in place of the original. Members suggested checking for leaks at the carb and float, and said a leaking mechanical fuel pump diaphragm can dump gas into the oil, and to change the oil.

Quick tip

Most MGB fuel problems are old rubber and old cork. Rebuild kits are cheaper than chasing symptoms one at a time.

Vehicle focus: 1962–1980 MG MGB (1.8L B-series four with SU or replacement carburetors).

Safety / mechanic note

Raw fuel in the oil thins it and can wreck bearings. Change the oil once the leak is fixed. Verify with a mechanic.