Was Missing With Headlights On

J

jg61hawk

Senior Member
Last evening I took the 29 Roadster for a spin. It ran great but coming home it was just getting a little darker so I put the lights on...the engine started to miss and backfired once or twice. I shut off the lights ran fine. I did this several times and same result.

This morning same thing. I checked the battery voltage at the starter it was 6.40 the voltage at the coil was 6.40 (both sides). I started it and voltage was 6.70 at battery and both sides of coil. I figured the coil was working properly so I cleaned both ends of the coil wire and removed the distributor cap and cleaned the top of the rotor and the cap with 400 sandpaper. The top of the rotor was pretty black.

Now the car runs fine, lights on or off. Is that possible the spark was just diminished slightly with the lights on and with the dirty rotor connection it started missing?
 
Did you check the voltages with the headlights on? You might be drawing enough to be on the ragged edge. Or you inadvertently fixed the real problem when you were messing with things. Maybe you moved a wire that is loose?
 
Was the springy part on top of the rotor pushing up tight against the distributor cap?
 
I did check voltage with lights on and off...it was good...that's the 6.70 reading. I assume the rotor and cap were making a good connection...I sanded the rotor and did give it a little push upward. My question, if I actually have one, is could the lights being on and the cap/rotor connection diminish the spark just enough to cause a miss? I ask to pass on information...my problem is solved whether by luck or process...
 
Hello, also check grounds , had similar problem turns out had a bad ground on engine, would run ok as long as there was no other electrical draw .
 
Do you have a fuse in you system? It could be that your headlights have an intermittent short and that's causing the ignition problems when the lights are on. A fuse would blow, but without a fuse you may not know it's shorting. It's common for the bullet connectors in the bulb socket to splay outward and touch the metal body of the socket. It could be that the bullet ends touch the socket only when the car is vibrating.
 
I had this problem for years. No one in my club would believe me. I did all the things suggested above, extra separate ground wires to each of the four lights (front/back), fused, checked voltages lights on and off, new cap, rotor and points several times over the last fifteen years, new coil (running 6v), new battery two years ago, new bulb sockets, new ammeter, blah blah blah.

I solved it by getting LED headlights. Clearly, the standard bulbs (not higher intensity, the low candlepower ones), were drawing too much. Car hasn't missed a beat, but when the lights are on, it is no longer a big discharge. Maybe an amp.

I hate it when I do not know what the problem was but it is now gone.
 
Jg61, I'd like to add a couple comments:

Check your coil polarity. Reverse polarity will bleed off several thousand volts of ignition spark. You can do this with a tool from Nu Rex or by placing a pencil between the plug strap and the top of the plug or with an old style analog VOM set on 1000 volt scale and briefly watching the needle jump up + or down -. (split second touch and untouch) red lead to ground, black to the top of the plug????

6.7 volts with engine running and lights on sounds like a generator. My memory says the voltage should be around 7.2??? You could adjust the third brush setting. My coupe has an alternator. It charges 7.36.

Test your distributor ground by black lead on ground side of points, red lead to the + post on the battery on low ohms setting (key off). My coupe was 0.xxx (less than 1 ohm) last time I checked it. Bob Bidonde has a sheet and it says 2-5 ohm max???

If you have Les Andrews blue "Troubleshooting and Diagnosis", chapter II has good instructions on how to perform VOLTAGE DROP testing. Maybe a local club member will have the book.


Good luck.
 
hangarb7 said:
...I solved it by getting LED headlights. Clearly, the standard bulbs (not higher intensity, the low candlepower ones), were drawing too much. ...

I hate it when I do not know what the problem was but it is now gone.
Click to expand...




Did you really solve it, or did you hide the symptoms of an ongoing problem? If everything is working correctly, a Model A will not start missing when you turn on the headlights. LED bulbs draw very little current, so I suspect whatever underlying problem that caused the miss when extra electrical load is put on the system is still there.
 
jg61hawk said:
Last evening I took the 29 Roadster for a spin. It ran great but coming home it was just getting a little darker so I put the lights on...the engine started to miss and backfired once or twice. I shut off the lights ran fine. I did this several times and same result.

This morning same thing. I checked the battery voltage https://journal.classiccars.com/2023/01/17/heres-why-a-classic-car-vin-lookup-should-be-on-your-to-do-list-when-purchasing-a-classic-car/ at the starter it was 6.40 the voltage at the coil was 6.40 (both sides). I started it and voltage was 6.70 at battery and both sides of coil. I figured the coil was working properly so I cleaned both ends of the coil wire and removed the distributor cap and cleaned the top of the rotor and the cap with 400 sandpaper. The top of the rotor was pretty black.

Now the car runs fine, lights on or off. Is that possible the spark was just diminished slightly with the lights on and with the dirty rotor connection it started missing?
Click to expand...

Had the same issue for years and couldn't do anything with it
 
Sort of an OH DOH solution - make sure the terminals on your terminal block are tight - the nuts inside, not the ones holding the cover on. Today I found one loose and it's amazing how much better everything works (and how much smoother the engine runs) when the wires connect properly. Going on a long drive tomorrow and I hope that cures the intermittent miss I've had.

JayJay
 
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