Ignition problems probably are probably the largest share of on the road problems with your Model A In a well maintained Model A with all parts of the ignition system in correct tolerance there should be no on the road failures.
Let us assume the car has stopped or is running poorly and you have checked for fuel and it is OK.
The next obvious thing to check is for spark. Remove one of the brass strips from a sparkplug and position it about 1/4 inch from the top of the plug. Turn on the key and crank the engine over. There should be a bright , fat, bluish-yellow spark that regularly jumps the gap. This spark should create an audible crackle or snapping sound. This would indicate that your spark is good but the engine could be out of time possibly due to a loose cam screw.
Remove the cap from the distributor, crank the engine over and watch to see if the rotor is turning. It is possible that the timing gear has lost teeth and is not turning the cam shaft which in turn operates the cam.
Check the point gap at the high point of each of the four lobes of the cam. They should be about .020 + or - .002. Put your finger on the cam and check for side play in the distributor shaft.
About .001 to .003 is normal play. Less play will mean a smoother running engine. Also check for side play in the upper plate. If the center hole in the plate is enlarged it will also allow for wobble in the point gap. All these things could add up to a poor or no running engine.
To time the engine, move the spark arm to retard (or up) position and remove the distributor body. Remove the timing pin, reverse it, and put back in the hole. Slowly crank the engine by hand until the pin drops in the hole. Sometimes this is difficult as it will want to go past the hole. If it goes past the hole , put the car in high gear & let off the emergency brake. While holding the pin in the hole, move the car back & forth a little. The pin should drop into the timing hole. Be sure and tak e the car out of gear and reset the emergency brake, Be sure to replace the timing pin in the hole as it was originally because oil will spray out of the hole if you dont.
At this position of the cam the next tiny bit of rotation should just start to open the points. The cam turns in a counter clockwise direction and there may be some backlash. Be sure to account for this. The notch in the cam should be near the number one cylinder post of the distributor body and the rotor should be about the 4 oclock position when looking down from the top.
Set the end of the coil wire where the spark can jump about a quarter inch to some metal part of the engine head. Turn on the ignition and pull the spark lever down slowly. The spark should crack or snap at about the third notch down from the top lever position. Loosen and adjust the cam until it makes at this spot. The car should be in proper time. Make sure the cam is tight. Replace everything.
The engine should start easily and run smoothly if each one of the items is working as it was designed.
From Evergreen Chapter Evergreen Echoes
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