MTFCA Forum

Why does my engine labor when I honk my magnito horn?

Should be the other way around ... your horn should labor when your engines rpms are slower!

1- Horn has way too much amp draw.

2- Partial short between horn and switch.

3- Very weak magneto.

OK run some tests. Either you have a weak magneto or a misadjusted horn or both. Run the car on BATTERY so that nothing is pulling from the magneto and go for a ride. The pitch of the magneto horn will change with RPM but it will sound uniformly lousy all of the time if you have it setup about right The correct setting for a magneto horn is that it should draw 2 amps when tested on a horn test generator but nowhere does it say what that generator is but it is NOT your T generator that is charging the battery. I would simply say that if you have a St. Louis Magneto tester you can put that on the magneto to see if you have enough output and it should be at least above or at the 1914 mark. Then put an ANALOG AC ammeter in series with the horn to see if it is exceeding the 2 amp draw allowed. Adjust it down if it is above 2 amps at any RPM. Most stock magneto horns have rusted through holes in the diaphragm and they don't work very well without a new diaphragm and a readjustment. I rebuilt mine on my 1915 and it is very loud at all speeds and does not cause the engine to stumble. It also measures just under 2 amps AC at the higher RPM It draws different amounts of current depending on whether you are going fast or slow but a good magneto will not show any lack of performance when you blow the magneto horn unless the mag horn is adjusted to draw too much current which is not good for the horn nor the magneto nor ignition.

Hope this helps.

Mud dauber nest on the horn's vibrator plate.