Help me understand why my 1915 Open Delivery occasionally backfires. It seems to happen when I am pulling a perhaps 3-5% hill in direct drive, with the throttle down and the spark advanced pretty much all the way. It will backfire, go for a hundred yards or so, then backfire again, etc. The motor and electrical system is totally stock, using 4 coils and a timer. Is this electrical or fuel related, and how can I analyze to fix the problem? It runs just fine most of the time such as when I am on flat ground or pulling slight hills.
Wes, a couple of thoughts. Pulling a 3-5% grade in high gear at full throttle and spark fully advanced is not good. That puts a lot of stress on the crankshaft. You should retard your timing in such a situation. The engine will tend to settle in at about 22-25 mph for a stock Model T. I would venture to guess that the back fire is due to a lean mixture if it is through the carb. My 15 runabout used to have a similar backfire when I was pulling a grade(although in low pedal) until I began enriching the mixture a tad before beginning the climb. The problem went away.
Why does timing need to retarded if the engine is not "pinging"? Retarded timing, particularly under load, leads to overheating.
How much petrol(gas) do you have in the tank?
Mine backfires when I only have 2 gallons in the tank while going up a three gallon hill. This is normally the signal to pull over fast and pour the spare can into the tank. I always carry spare gas on tour and always fill to the top first chance every 100 miles or so.
My '15 touring would bacfire occasionally when runing on mag back in 2005 at the All Ohio tour. Shortly thereafter the mag quit completely. Had to drive the final 400 miles / 4 days on the battery.
In this case the magneto coil had a solder joint come apart.
I would look close at the ignition system. Backfiring might be caused by a bad magneto post connection, or a faulty ignition switch, or faulty coil box connections. Have you rebuilt the coil box?
A mixture that is too lean will also cause a backfire for some reason that I don't understand.
There is also another possibility. Coils can randomly misfire when not properly adjusted. When the misfire occurs, raw gas is dumped in the exhaust system. This gas can then be set off by the next spark that comes along.
You may have several things going on here to make it backfire.
Carb mixture adjustment is one thought, if it is too lean or too rich it will not run smoothly. But what most people think are carb or fuel problems often turn out to be the ignition system. Have your coils been adjusted on a hand crank coil tester or strobospark?, Timer clean and oiled and not worn out?
From your description of the gas and spark lever setting it sounds like you are not moving the spark lever to the right position. The gas and spark levers do not get put in the same position necessarily. The spark lever is positioned to match the engine rpms you have, not the rpms you want. So lever all the way retarded for starteng on battery, advance it up to 1/2 after engine starts. As you drive, try to position the spark lever for the given rpms, so if you are accelerating in low gear (first gear, pedal down) as the engine speeds up the spark lever should be continually pulled down, as the engnie gets up to about max rpm the spark lever should be all the way down. To shift to high (2nd gear, pedal all the way up) the rpm will drop off so you match it with the spark lever about at half mast, as engine rpm increases you will continue to advance the lever. If you had a "modern" car with a distributor it will advance and retard automatically with little govoner weights or vacuum, but for the model T the driver sets the advance. So set the gas lever for what you want the engine to do in the near future, set the spark lever for the current engine rpms. When you are clmbing a hill, you have the gas lever advanced pouring on the coals, but because the engine rpm is in the mid range to low end the spark lever should be about 1/2 way down. Of course all engines are different and you will need to learn the sweet spots for the levers over time. A rule of thumb to prevent overheating is to not retard the spark beyond 1/2 mast.
The reason for retarding the spark going uphill is this: The time it takes for the fuel mixture to burn. When the engine is going fast, the spark is advanced so that the fuel will burn during the power stroke. If it were retarded when going fast, it would still be burning when it goes out the exhaust and would burn your exhaust valve and get the manifold red hot. However when the engine is going slow, the spark is retarded so that it will burn during the power stroke and not before the piston starts to go down. If it is too advanced while the engine is going slow it will start to burn before the piston gets to the top and will try to run the engine backward. This backward force will actually slow you down and put a strain on the crankshaft and bearings. Due to the low compression of the Model T you will not notice a "ping" unless it is very overheated and full of carbon.
All of your comments are very helpful and I appreciate the responses. I am going to go through the electrical system and clean it up again, and replace the current coils with another set of coils adjusted by Coilman. Also will try to follow the advice on the spark and fuel lever adjustments, as well as opening the needle valve a bit more when putting more demand on the engine. Thanks for the help.
I have a 22 Runabout which does not have a fuel pump. I also have a Ruckstell hill leading up to my house. Whenever the tank is below 1/2 full, it will run just fine downhill or on level but when I go up that hill I don't notice a backfire, just acts like it is running out of gas. Fortunately for me, there is a gas station near the foot of the hill so a U turn will get me more gas and I can climb the hill just fine.
From your description it sounds to me like an ignition problem. You might have an occasional misfire all the time but you only notice it when pulling hard. Sparkplugs can also be the problem. It takes more spark to fire under higher compression, and a full throttle will give you maximum compression.
I had a terrible backire problem in an old 49 Chev Pu. Ignition switch went bad and it actually split the muffler.