MTFCA Forum

I need your help. I am new to Model T's and I have limited mechanical experience. I have a 1925 Model TT flatbed that I bought a few months ago. I have been learning to drive and even participated in a club "stroll". On a recent drive with my wife, at top speed, the truck began to back fire and slow down. After a struggle to restart, we got the truck running and returned home.

Last night, I drove "Hank" to the gas station (3 miles) and got there with no issues. On the way home, I noticed I had failed to switch to magneto. I slowed to a stop, then switched to magneto. I accelerated and made 2 stops, then after a right turn and accelerating, Hank back fired. Then backfired again and lost power. I found that when I ran on battery, Hank did fine. No backfires. But when I ran on magneto, back firing was a problem. So, I drove home on battery. What is happening and how can I correct this problem?

One of the first things to check would be the magneto contact post. It's located on top of the flywheel housing just back of the engine block under the firewall. It will have a single wire attached to it, which is the magneto output wire. Be sure that the wire is firmly connected to the post , then trace the wire as far as you can while looking carefully for any evidence of damaged insulation. The wire will lead to a terminal block and then to the switch and coil box. Any damage to insulation or loose connections can cause trouble. The switch itelf may have problems. I'd look first at the wiring. The engine should run better on the magneto than on the battery. A copy of the Model T service manual is a very good thing to have. You may also find good information by entering "magneto" in the search box at the upper right of this page.

The above suggestions are excellent, but in addition , I would s check the ignition switch. They have a habit or working irradically. When the backfiring occurs, try wiggling the key.
Art Mirtes

I agree with the above posts. Also if you don't find the source of the problem by checking the above, remove the magneto plug at the top of the hogs head and check for band lint or other debris causing a problem with the contact. Also, if you have a replacement type plug or a plug with an oil line attached to it, you might find that the contact is not quite long enough to make good connection with the button at the top of the magneto. If all the above finds no problem, try checking the magneto output by running on battery with a light bulb connected from the magneto post to ground. It should burn brightly. Use an analog AC volt meter across the light bulb and you should get at least 6 volts AC at idle with the output increasing to over 20 volts with higher RPM . If this is good, it is possible that the coils need to be adjusted to fire at the same amperage 1.5 amps. If the coils are not adjusted evenly, you could get one firing later than the others. Usually a backfire happens when unburnt fuel suddenly ignites at a time when a valve is open. If an exhaust valve the fire would be out the exhaust pipe or if an intake valve through the carburetor.
Norm

I had a bad ignition switch that did the same thing.

Thanks for the responses. We'll be getting a closer look this weekend. We need Hank to be ready for the fly-in at Ranger Airfield, Ranger, TX. I appreciate your help.

I don't think your engine is BACKFIRING it is most likely firing forward and the explosion is in the muffler. You most likely have an erratic ignition problem and the unburnt fuel is igniting in the muffler.

Or is it backfiring through the carburetor?