My engine and magneto was rebuild less than 50 kilometers ago. Today I was fixing fuel issues and suddenly magneto stopped working. Car starts to spitting and dies. What happened?
Check the contact. Might be dirty.
You can check the magneto output easily using the Regan Patterson Memorial test. So named after its inventors - a cheap easy thing to do:
To check the magneto voltage one connects a #1156 automotive bulb between the magneto post and ground. This provides a load so the magneto reading is known to be stable.
An analog (NOT digital) AC voltmeter is then connected to the same points. With the engine running at idle you should see maybe 6 volts at idle. With the throttle advanced to high engine speed a good magneto will read 20 - 30 volts. The light bulb will burn out rapidly if this is sustained.
I am curious, why would you not use a digital voltmeter?
A digital voltmeter won't give an accurate reading of the magneto. I can't recall all of the technical reasons why (excessive floonabs of recalcitrance), but you do have to use an analog meter or the test method which Royce describes above. Bill
Richard. No reason you can't hook both a digital and analog voltmeter to the mag at the same time and check to see how they read differently. Digital voltmeters might be deceptive based on their display up-date rate. And that will likely vary between different manufacturers as well.
If you have a high $ meter you might be able to but not with an off the shelf meter like most people have the reading gets scrambled.
An analog can compensated for the 16 pulses and give a output somewhat steady reading were a digital can't. It's trying to read the peaks and valleys and all along the wave at the same time so the reading gets scrambled.
RF interference caused by the ignition coils is why a digital meter is not useable.
Let us know how it goes and what you discover. Newbies like me are trying to learn as much as possible every day.
Charlie, your advice helped me just now and my car is running on magneto for the first time since I've owned it! The contact was dirty.
My analog voltmeter just jumps to the middle and I can't really tell how many DC volts it is reading, so I will post a picture in case anyone here knows how to read it. Not trying to hijack this thread since the info might still help the original poster. Also, I discovered that my engine is willing to idle at a slower speed on battery than it will on magneto. In other words, I have to give it a little bit of throttle to maintain idle on magneto because it dies if I reduce throttle to the top couple of notches.
(Message edited by Laughac on July 23, 2016)
Magneto is AC not DC. You need to check the magneto on the 50 volt AC setting.
You need to set your voltmeter on AC volts, only then you can follow the change in AC tension your magneto produce.
To get an equal Idle on magneto you need to advance your ignition 4 to 5 stops same as to start the engine on magneto.
During the ride on magneto you will find that your engine is not reacting the same way on the change of the advance lever as when you are driving on battery.
On battery, during the contact period of your timer, the ignition produce a arc to ignite the mixture.
On magneto, during the same contact period of your timer, the current produced by the magneto, change four time his polarity and the ignition produce only four sparks. There for you need to find the four position of advance lever where the engine is running the best.
The four sparks on magneto, if your coils are set correctly, are stronger as on battery due to the higher primary tension it gets from the magneto.
In both of my model T's I can only use three positions on the lever. When I pull the advance lever all the way down to find the fourth spot the engine start knocking due to way too much advance.
Hope this helps
Thank You, Andre
Chris when you add a photo type your reply then hit enter before you load the photo. Almost didn't see the the Ahhh! AC
My coils are not running as well on magneto as they do on battery. Looking forward to receiving rebuilt coils from Ron the Coilman next week.
Make sure to clean the underside of the magneto post where it leaves the tranny housing...Oil and band lint may be shorting the magneto out...remove the 3 screws and lift the post away from the magneto. Clean and reinstall...make sure gasket is good
I think (theory only)the reason to use an analog AC voltmeter is that the frequency and the voltage varies with engine speed and the digital meter would just keep flashing different readings.
Concerning the magneto failure. It could be caused by several different things: 1. first and easiest things to test are: the magneto connection at the top of the hogs head. Sometimes you will get band lint on it and that would make a poor connection with the magneto coil ring. Or the length of the plunger on an oiler type plug could be too short to make good contact. The contact itself could be shorted to ground. 2. If you have a good contact and get a reading of 6 volts or more, check the ignition switch. If the car runs on battery, you don't need to go beyond the ignition switch. If it won't run on either mag or battery, keep on checking the wires, the coil box, the coils and the timer. 3. If you don't have electricity to the magneto contact, you could have too much endplay in the crankshaft, which would be less likely in a rebuilt engine. 4. Lack of charge in the magnets. These can be charged using the "in car method of charging" which can be found elsewhere on the forum. 5. Worst case, could be defect in the magneto coils inside the engine. Either open or short. To fix this you would need to pull the engine and take off the flywheel and replace or rewind the coils.
On one of my cars, shortly after replacing the engine, the magneto stopped working and I ran on battery for about 10 years. Fortunately I had an outside oiler, because later when I finally found out what had caused the magneto to fail, was the magnets hitting the funnel on the inside oiler which broke off and hit the coils and cut the copper wire. A rewound magneto ring fixed that 10 years later. The outside oiler saved my bearings.