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Old 04-23-2018, 02:57 AM   #1
Rata Road
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Default Model A Ford back firing and jerking after lights turned on

A friend recently finished a restoration on a 30 Coupe and we went for its first decent test run yesterday.
Was buzzing along great about 45 mph when we turned the headlights on the engine started misfiring really bad with massive back firing etc. We managed to just limp home, jerk jerk backfire, dogs barking and people coming out of their houses....funny.
On inspection the aftermarket Fuse fitted above the starter is blown hence lights, brake lights etc not working.
It has a new wiring receiver thingie at the bottom of the steering column which has some issues as the horn can go all by itself and since the light switch comes down the same system perhaps the horn and this problem are related.
I can understand the fuse effecting the lights etc but what is the link to this and the running of the engine? The problem occurred exactly when the lights were turned on and stayed.
Any suggestions before we investigate would be appreciated.
Thanks in advance
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Old 04-23-2018, 06:19 AM   #2
tmiller6
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Default Re: Model A Ford back firing and jerking after lights turned on

Assuming your Model A is restored and wired stock, the headlights are probably causing your problem. The “new thingy” at the base of the column holds your light switch spider and the switch body. You mention the horn goes off when you turn the light rod. This is because the contact rivet in the center of the phenolic disc is going to earth inside the light switch housing. This is not a short since the six volts goes directly to the horn first. Disconnect the battery, remove the light switch body by flipping the bail and then look through the large hole in the end. On mine, I put an insulator like a length of large plastic tubing to ensure the rivet won’t contact the steel switch body. If your switch body is bright and shiny, you can suspect that it’s a reproduction. Find an original if possible. Bert’s and Bratton’s are two possibilities. The connections inside the headlight switch are all crimped and soldered, I doubt if your electrician did anything wrong inside there.

Now, the short. It’s probably in one or both of the headlights. I haven’t checked the suppliers recently, but most of the wiring harnesses in the headlights are total crap. If you disassemble your light and find three black vinyl wires, you probably have these harnesses. Where they fail is the white nylon insulators in the connector have large holes and the brass contacts slide off to the side and go to earth, shorting the system. Many restorers cut the three bullet connectors off the lighting harness and then make the connection inside the headlight bucket using wiring nuts. I did this for a while and eventually restored original parts and set it back the way Henry intended.

While on the subject, many of the late style brake light switches are also suspect. This is the one that fastens to the frame, not the transmission.

Finally, the fuse. I bought one and threw it in the tip. It became unsoldered and caused problems. I have yet to see a good reliable one marketed by the Model A suppliers so I made my electrics safe and don’t use a fuse.

One other bit of advice. Assuming your distributor is stock, carry a spare condensor.
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Old 04-23-2018, 07:10 AM   #3
BILL WILLIAMSON
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Default Re: Model A Ford back firing and jerking after lights turned on

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In short, when the fuse blows, the generator puts out REALLY HIGH Voltage & causes the car to run like CRAP! Once you find & fix the short, it'll be OK.
Bill W.
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