Carbureted Classic Car Runs On After Key Off: Dieseling, Idle, Timing, and Heat Diagnosis

Separate combustion run-on from an ignition circuit that stays powered

True dieseling is irregular combustion after ignition power has been removed. The engine may shake, chug, reverse direction briefly, or stop after several uneven firings because hot chambers ignite mixture entering through an open throttle. Electrical run-on is different: the ignition, EFI, or controller continues receiving voltage and the engine may run relatively smoothly. Watch the tachometer and warning lamps, verify whether ignition voltage disappears, and note whether an electric fan, alternator, accessory circuit, relay, diode, or recently modified wiring can backfeed the ignition. The existing LS key-off guide covers that electrical condition; a carbureted classic should not be tuned around a powered ignition fault.

If the engine overheats, leaks fuel, backfires, makes mechanical noise, or continues running uncontrollably, shut it down safely and inspect it before another test. Do not reach into the fan, linkage, or belts, and do not place a hand over the carburetor. Record coolant temperature, hot idle rpm in and out of gear, timing, manifold vacuum, fuel grade, carburetor and distributor identity, choke position, recent work, and whether the problem follows a long idle, highway run, hot soak, air-conditioning load, or seasonal fuel change.

• Confirm ignition voltage actually turns off before changing idle speed, timing, jets, or fuel
• Check for overheating, fuel leakage, a sticking throttle, or a choke that does not fully open
• Use the factory hot-idle, timing, fuel, and shutdown procedure for the engine and carburetor installed
• Do not mask severe run-on by stalling an automatic transmission against the brakes or selecting gear after key-off

Establish the hot curb idle without exposing too much transfer slot

A throttle held too far open admits enough air and fuel to sustain combustion after the spark is removed. The cause may be a curb-idle screw set high, fast-idle cam still engaged, sticking linkage, tight cable, weak return spring, secondary plates held open, vacuum leak, incorrect PCV flow, or a large camshaft that was made to idle by uncovering excessive transfer slot. Holley's curb-idle guidance starts with full operating temperature, choke released, correct timing, and the specified idle adjustment rather than using the throttle screw as a universal cure.

Verify pedal and linkage return, choke and fast-idle release, secondary stop, transmission and accessory load, base gasket, vacuum hoses, brake-booster supply, PCV valve, manifold sealing, and throttle-plate position. An anti-dieseling or throttle-stop solenoid can hold the approved running position and let the throttle close farther with the key off, but it must be correctly specified, wired, adjusted, and mechanically safe. It is not permission to ignore overheating, excessive base idle, a vacuum leak, or an incompatible carburetor setup.

Check timing, mixture, fuel, chamber temperature, and deposits together

Retarded timing can increase exhaust and chamber heat and force the throttle farther open to maintain idle. Excessive advance, unstable advance, a sticking mechanical mechanism, wrong vacuum source, distributor wear, weak spark, or an incorrect firing order can also distort the idle adjustment. Establish dwell where applicable, base timing, mechanical and vacuum advance operation, and stable charging voltage using the correct factory or component procedure before trimming idle mixture. Holley's optimized idle procedure likewise treats timing, transfer-slot position, idle speed, and mixture as connected settings.

Overheating, restricted coolant flow, a hot intake, overly lean cylinders, excessive carbon, glowing deposits, sharp chamber edges, unsuitable spark plugs, low-octane or contaminated fuel, and a high compression ratio can create ignition sources after shutdown. Verify actual coolant temperature and cooling-system function, fuel pressure and float level, choke opening, mixture balance, plug condition, compression when indicated, and the fuel recommended for the engine. Do not pour water or chemical cleaners into a running carburetor or advance timing beyond a verified safe range to chase the symptom.

Correct the root cause and repeat the same hot shutdown test

The repair might involve an ignition-feed or alternator-backfeed correction, linkage and return repair, choke service, proper curb-idle and mixture setup, timing and advance repair, vacuum-leak correction, an approved anti-dieseling solenoid, cooling-system service, compatible spark plugs and fuel, or internal inspection when deposits or mechanical condition remain suspect. Validate cold start, choke release, hot idle in neutral and in gear, throttle return, coolant temperature, advance, fuel control, accessory loads, and several controlled key-off cycles after the same drive that originally caused the complaint.

Ironwake diagnoses carbureted classic cars and trucks for owners in King George, Fredericksburg, Stafford, Spotsylvania, Dahlgren, Colonial Beach, the Northern Neck, and nearby Southern Maryland. Send the engine, compression and camshaft details when known, carburetor list or model number, distributor and ignition, hot idle rpm and temperature, timing and vacuum readings, fuel and octane, choke and solenoid information, recent changes, and a safe video of the shutdown behavior. That evidence separates heat-driven dieseling from electrical run-on before calibration begins.