Why do my lights make my motor run rough?

Desert Toad

Desert Toad

Member
I am very fortunate to own my grandfather's 1930 Model A Sport Coupe. It is a 20 year old restoration, but i have been "restoring" it as it has been sitting for many years.

I've rebuilt the carb, replaced the points adn condenser (boy, that was fun), rebuilt the starter, rebuilt the water pump, next up is replacing the float so I know how much fuel I have.

My current question is why does my engine run rough when I turn the lights on? It's understandable that power is being pulled, but shouldn't the generator keep up with it? At first I thought, it's old, just the way it is. But people used to drive at night in 1930. I am thinking about rebuilding the generator, unless there is a common problem I should be looking for.

This forum has been a treasure trove of info, so I thank you all. I farm for a living so fixing things is second nature, was hoping for a few opinions on this.

thank you all!
 
if you dont have your generator set to at least 10 to 12 amps when you turn on your head lites it wont be charging your battery, not allowing enough amps going to battery to run every thing, especially the ignition system, try winding the engine up to around 1000 rpm with lites on and see if the problem goes away, if so increase anps from generator and maybe increase engine idle so generator will put out more amps at slow speed
 
That happened to me when my battery was weak, no generator. If you still have 10 amps charge i guess you could try cleaning the earth connections. That seems to be a cure-all for electrics.
 
If the battery is low and the generator isn't putting out enough, then the spark could be weak when the lights are turned on. Make sure the nuts are clean and tight in the terminal box on the firewall and on the rear of the ammeter.

I'd restore the generator just so you know it's good to go. Also check the cutout to make sure the contacts close when the generator output reaches about 6.5 volts.
 
then what does it charge? it needs to be in the plus side, try adjusting the third brush to show 14 amps see if that helps
 
ford3 has a very keen question, i.e., number of amps indicated after lights are "ON".

Could also possibly have electrical resistance in lighting circuit for head & tail lights at switch below steering column or in light buckets or elsewhere stealing electrical power from ignition components.
 
Nobody is alarmed that he shows +10 amps charging "constantly [while] going down the road"?
 
I would charge the battery until you read 7v across the battery first. A simple conclusion is the longer it takes to get to 7v the better the battery (not a definitive test) as 6v batteries are current hogs. With a line charger and a voltmeter you can evaluate your situation. Bear in mind that every volt counts when you are trying to make thousands of volts to fire a plug.
OTOH when the charging system goes negative it is the obligation of the battery to maintain voltage. If it cannot your firing voltages will be reduced substantially. It needs some type of reserve capacity, i.e. a weak battery.

An anecdote that may help is on old analog systems customers would complain that their lights would dim at stop signs etc..Yet the system was charging fine. Early alternators produce little or nothing at idle unlike today. I would tell the customer to look at the battery as it should provide the voltage needed, like a flashlight, turn it on and it is bright and a minute later goes yellow...You replace the battery.
 
I replaced the battery about a month ago. Perhaps a generator rebuild is next on the list. Thank you all for the input. I'm in the process of installing an engine with a 12 volt system into my old international truck that originally had 6. Sometimes I'd rather rebuild an engine that search for why a taillight isn't working.....
 
I had too long of a screw in my headlite bucket and tightening it caused a short to ground when the lights were turned on. There was too much resistance to let any smoke out but the lights sure did act funny. I would guess this could also take a lot of juice from the system.
Just a thought.
Lee
 
How many amps discharge does the ammeter show when you turn on the lights? Sometimes the high and low beam contacts in the headlight connector plugs will touch, causing the high and low beams to both come on at the same time and causing about a 20 amp discharge.
 
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