Roadside Troubleshooting Printable Version (pdf)

Engine Fails to Start Although engine turns over freely

Culprits include:

Ignition switch is off
No gasoline is in the tank (Quick Fuel Check on this page)
Gasoline line valve is off (needs to point down for on)
If engine is cold, mixture may not be rich enough. (See Start/Drive opens a new page)
If the engine is warm-Mixture is too rich, over choking
Breaker points too close together. The correct adjustment is .018 to .022 (See Ignition opens a new page)
The spark plug gap is incorrect. The correct gap is .035. (See Ignition opens a new page)
Water in filter bowl or carburetor

Starter Fails to Turn the Engine Over

Culprits include:

Battery run down
Loose or dirty Battery Connections.

Missing at Low Speeds:

Culprits include:

Gas Mixture to rich or to lean (See Start/Drive opens a new page)
The spark plug gap is incorrect. The correct gap is .035. (See Ignition opens a new page)
Breaker points improperly adjusted, badly burnt or pitted.
Fouled spark plug-Spark plugs should occasionally be cleaned and the gaps checked.
Water in gasoline

Missing at High Speeds:

Insufficient gasoline, flowing to carburetor due to gasoline line or filter screen being partly clogged. (Quick Fuel Check on this page)
Gas Mixture to rich or to lean (See Start/Drive opens a new page)
Water in gasoline

Backfiring Under Load (throttle wide open at relatively low rpm’s) often indicates a problem in electrical/ignition/timing.

Culprits include:

Insufficient point gap (should be set at .018 to .020) (See Ignition opens a new page)

Timing

Bad condenser

Bad distributor body

Frayed wire between upper plate and lower plate on the distributor

Intermittent connection of the ignition switch

Backfiring With No-Load (example: shifting between gears and coasting downhill). Often indicates too little fuel in combustion chamber for proper ignition. (During the exhaust stroke, raw gas is expelled into the muffler where it ignites)

Culprits include:

GAV adjusted to lean (See Start/Drive opens a new page)

Intake vacuum leak (See Carburetion opens a new page)

Low carburetor float (See Carburetion opens a new page)

Backfiring Around 45 mph indicates insufficient fuel to supply to the carburetor.

Engine Stops Suddenly:

Gasoline tank empty (Quick Fuel Check on this page)
Dirt in fuel line or carburetor (Quick Fuel Check on this page)
Gas mixture to lean (Quick Fuel Check on this page)

Engine Overheats:

Lack of water
Lack of oil
Fan belt loose or slipping
Excessive carbon deposit on piston heads and combustion chamber
Incorrect spark timing (See Ignition opens a new page)
Gas mixture too rich (See Start/Drive opens a new page)
Water circulation retarded, by sediment/grease in radiator.

Quick Fuel Check:
Crank engine fully choke for seven or eight revolutions. If gas leaks from the back of the carburetor your line of fuel is not blocked.

If Fuel Line is Blocked

1. Use a stick to check the fuel level in the gas tank.
2. Repeat the above Quick Check for Fuel with the gas cap off.
3. Remove fuel line at carburetor. Test for fuel flow using the gas shut off valve for means of controlling flow. If blocked blow into fuel line to temporally clear blockage.
4. If gas flows from the fuel line reattach it to the carburetor making sure the line does not extend more than 1/8 of an inch from the ferrule into the carburetor, repeat the above Quick Check for Fuel.
5. Detach the choke lever and remove the center bolt of the carburetor and separate the two halves. Check for fuel flow.
6. Remove valve and check for fuel flow. If no flow check screen for blockage
7. Clean and blow through float valve

Quick Check: Ignition (Primary)
With ignition on, turn the engine over and observe the ammeter. If the needle jiggles the primary side of the electrical system is working. This includes: ignition switch, pop-out cable, points, condenser, primary side of coil, and the hidden wire in the distributor connecting the upper plate to the lower plate. (Paraphrased from Tom Endy’s article “Model A Ford Ignition Diagnostics”)

Note: For early Model A’s up until November 1929 the ammeter was not wired into the primary ignition circuit. To convert:

Disconnect the battery. Open the terminal box on the firewall. Locate the black wire leading from the coil to the terminal on the passenger side, move this wire to the terminal on the driver side. The Ammeter Needle will now jiggle to help with diagnostics.
(Paraphrased from Tom Endy’s article “Model A Ford Ignition Diagnostics”)

(See Ignition opens a new page)

Quick Check: Ignition (Secondary)
To check the secondary side of the electrical system, remove one brass spark plug connector from the distributor and position it approximately a ¼ of an inch above its terminal on the distributor. Using the key turn the engine over and observe the gap to see if a bright yellowish-blue spark, with a loud snapping sound is being created

No spark/poor spark; Culprits include:

Failed condenser
Lower plate wire broke or shorted
Point gap closing up
Pop-out cable to tight and grounding out in distributor

(See Ignition opens a new page)