Diagnosing Carburetor and Idle Problems: Rough Running (Misfire) and Stalling

Part numbers are from the manufacturer's original catalogue. Check them against your VIN before ordering.

• Set of 10-17 mm wrenches and sockets
• screwdrivers
• timing light
• multimeter
• vacuum gauge (recommended)
• can of carburetor cleaner
• tachometer
• spark plug socket
• feeler gauge

1. Check the wire order at the distributor and spark plugs. The firing order of the 4-cylinder GA13 is 1-3-4-2. Make sure the No. 1 cylinder wire is on the terminal that the rotor points to when the No. 1 piston is at TDC on the compression stroke. A single wire moved over by one position causes exactly this: rough running, popping and stalling.
2. Check the distributor installation. Set the crankshaft pulley marks to TDC for cylinder No. 1 (compression stroke), make sure the rotor points to the No. 1 cylinder terminal, and make sure the distributor body is fully seated in its bore. If the distributor went in one tooth off on the drive gear, the ignition timing will be off, and the engine will run rough or will not hold an idle.
3. Warm up the engine and set the ignition timing with a timing light. For idle, the base timing angle is given on the sticker under the hood (usually a few degrees BTDC). Disconnect and plug the vacuum advance hose while adjusting. Loosen the distributor clamp, rotate the distributor body until the timing mark lines up with the specification on the sticker, then tighten the clamp.
4. Inspect the vacuum hoses of the distributor, carburetor and valves. When a distributor is replaced, the vacuum advance hose is often removed and then reconnected to the wrong port or is cracked. Make sure the hoses are intact, have no vacuum leaks, and are connected according to the diagram on the sticker under the hood.
5. Check the fuel filter. The filter has a flow direction arrow: it must point from the fuel pump toward the carburetor. Make sure the clamps are tight and the hoses are not pinched and do not draw in air. If the filter is installed backwards, the engine stalls under load and at idle.
6. Check the fuel supply. Disconnect the hose at the carburetor inlet, point it into a container and crank the engine with the starter: there should be a steady stream. A weak stream means a clog, a failing pump or a pinched hose. Also check the fuel level in the float bowl through the sight glass (if equipped) or by the level marks.
7. Check the spark plugs: the gap is usually 0.8-1.0 mm (confirm against the sticker under the hood). Dark, fuel-soaked plugs should be cleaned or replaced. Inspect the wires: leakage to ground shows up as sparks in the dark. Check the distributor cap and rotor for cracks and carbon tracking.
8. Check for air leaks at the carburetor gasket and the intake manifold. With the engine running, carefully spray carburetor cleaner on the joints: a change in engine speed shows where the leak is. Replace the gasket if the joint does not seal properly.
9. Clean the carburetor. Remove the air filter, then use cleaner to flush the idle jets, the idle circuit passage, the throttle plate and the mixture limiter. Clogged idle jets are a typical cause of unstable idle and of stalling when you lift off the throttle.
10. Check the idle cut-off (anti-dieseling) valve, which is the solenoid valve of the idle jet. It should click when you turn the ignition on. If it does not click, check the power supply at the connector; a faulty valve makes the engine stall at idle.
11. Adjust the idle on a warm engine: use the throttle (idle speed) screw to set the speed, and use the mixture screw to get the smoothest running (on average about 700-900 rpm; use the exact value from the sticker under the hood). Adjust with all electrical loads off, with the engine oil warm and the choke fully open.
12. Check the operation of the choke plate and the automatic choke: with a warm engine the choke plate must be fully open. If it sticks, the mixture is too rich, and the engine runs rough and stalls.
13. If the engine still runs rough after all of this, check cylinder compression and the condition of the timing belt/chain, since incorrect valve timing after removing the distributor can cause the same symptoms. A difference of more than 1 bar (about 14.5 psi) between cylinders calls for separate diagnosis.

• Work on the fuel system away from open flames and sparks, keep a fire extinguisher nearby and avoid fuel spills
• Do not run the engine in an enclosed space without ventilation
• If the ignition timing and wire order have been verified and the problem persists, take the car to a shop experienced with carbureted Japanese engines