Vacuum Advance

ENGINE STUMBLES OR HESITATES ON ACCELERATION

Description
Upon accelerating, the engine stumbles, hesitates, backfires, or dramatically looses power.

Explanation
For proper acceleration additional fuel and ignition timing advance are required. If either is not increased, hesitation or backfire through the intake may result.

Causes

Accelerator Pump Problem
The accelerator pump is designed to shoot a stream of fuel into the intake when the throttle plates are opened. If the accelerator pump is not functioning, the engine will hesitate badly or backfire through the intake during acceleration.

Testing
- Remove the air cleaner assembly.
- With the engine "OFF", look down the throat of the carburetor while cycling the throttle lever.
- Each time the throttle is opened, a stream of fuel should be sprayed down the throat of the carburetor.

CAUTION:
- Do not perform this check with a hot engine.
- To prevent flooding the engine, minimize the number of times the throttle lever is cycled.

Lean Air/Fuel Mixture
Even with a properly functioning accelerator pump, if the initial air/fuel mixture is too lean there will not be a sufficiently rich mixture for acceleration.

Testing
- Verify there are no vacuum leaks present.
- Check the Air/Fuel Mixture and adjust if necessary.

Defective Vacuum Advance
The vacuum advance unit is designed to advance ignition timing upon accelerating. A ruptured diaphragm or defective vacuum line will prevent operation.

Testing
- With the vacuum advance hoses connected, check ignition timing at idle and 2500 rpm. The timing at 2500 rpm should be advanced.
- Using a hand vacuum pump, verify the vacuum advance diaphragm is not ruptured.
- Inspect vacuum lines leading to the vacuum advance and verify the routing in correct. Verify vacuum is supplied to advance unit at 2500 rpm.

Ignition Problems
Problems with the ignition system which are not noticed during normal operation may become evident during acceleration. The richer fuel mixture during acceleration requires a stronger spark to ignite than normal combustion. Existing weaknesses in the ignition system may be exposed during acceleration.

Testing
- Check ignition coil output. Spark should be thick and blue. Ignition Coil
- Inspect all spark plugs. Check for fouling, excessive wear, cracked insulator, and proper gap.
- Inspect ignition cables for signs for insulation damage or arcing.
- Inspect ignition cables for high resistance readings or corroded terminals.
- Inspect the distributor cap for signs of cracking or arcing.
- For additional testing see Ignitions System. Ignition System

Defective PCV
During acceleration, the PCV valve is designed to close. If the valve remains open, the engine may hesitate.

Testing
- Remove and inspect the PCV valve, verify it is not stuck open. Positive Crankcase Ventilation
- See Fuel Deliver and Air Induction for further testing. Fuel Delivery and Air Induction