I've grounded my headlights to the frame on my 1916 runabout. They were grounded to the radiator, but I thought a better ground at the frame. When I turn on the headlights, the engine stutters a bit on the battery, and on the magneto it seems to run on 2 cylinders. Weak magneto? Ground back to the radiator? The frame ground is clean.
Grounding to the radiator or frame should make no difference. On a 1915/1916 the lights run off the magneto at all times, whether the engine is on BAT or MAG.
Here you can see that the wire to the light switch comes from the magneto terminal. This doesn't change when a battery is added to fire the coils.
If the lights are having any effect on how the engine runs, something other than the ground is amiss.
Robert, the stock headlights for 1915-16 do not use a ground at the headlights themselves, but only the single ground a the end of the series wiring between headlights, as shown in the above diagram. hat means the er goes in the right headlight and comes back out on the second contact, goes into the left headlight and comes out to be connected to ground. This way, the two headlights split the voltage making it possible to use the 9volt bulbs. Downside is if one bulb goes out they both go out.
Sounds like something is sucking power out of your system when you turn on the lights. Recheck your wiring and the current from from your headlight bulbs. Check for frayed wires and shorts to anywhere on the car. I once had a problem with the wiring on my brake lights such that whenever I used the brakes the engine faltered. Switch was fine but there was a bad wire making a short to ground. Sucked up all the power until I released the pedal. Took a doctor from Henry Ford Hospital to diagnose that for me!
Be very sure there is no way for the battery to get connected to the mag post. That would demagnetize hour magnets.
(Message edited by touringtom on August 19, 2017)
In the day, the headlights were designed to run off the magneto, with two 9 volt bulbs in series and a battery was supplemented to assist with starting. The battery would have been a 6 volt dry cell today called a "fence battery". 9 volt bulbs are no longer available. I am suspicious that you may have 12 volt bulbs in your had lights and that may be causing the stumbling. 6 volt bulbs are ok but a little more care is required to avoid a burn out. A small trick is to wire a third 6 volt bulb in the system under the car or display it as a third light to consume the voltage.