One thing that can cause trouble with ignition tinning is that many old engines can be set over a wide range - and many times there is no information about what the manufacturer had in mind. Tramming, an old but accurate method of setting valve and ignition timing, can be a great way to set basic timing. The method outlined here is pointed toward hit-and-miss, single-cylinder engines, although with some thought it can be used with a wide variety of engines.
With most older engines, a good starting point is to set the ignition so that spark occurs about five degrees before top dead center (TDC). While this may not be the ideal setting for your engine (as factors such as engine speed, load, fuel, etc., have a direct influence on how your engine runs), just about any engine should run okay at this basic setting. Additionally, once you've established a base timing it is much easier to dial in your final tune.
On an engine with an atmospheric (vacuum operated) intake valve the exhaust valve must be set so there is no residual pressure in the combustion chamber at the end of the exhaust stroke. If, for instance, the exhaust valve were to close 10 degrees before TDC, the trapped exhaust would be compressed for the remaining 10 degrees and the combustion chamber would not return to atmospheric pressure until the piston had moved about 10 degrees the other side of TDC. This residual pressure will delay the opening of the intake valve, as it will not start to open until a vacuum has developed in the combustion chamber.
As for tools, tramming couldn't be simpler. The piece of 1-inch by 2-inch pine shown above is all that's needed, and nails driven through either end act as markers for setting up timing.
Early exhaust closure will result in a very small fuel charge mixture being drawn into the engine. It follows from this that the exhaust valve must close somewhat past TDC, and a good place for the exhaust valve to close is about five degrees after TDC. The piston will have only moved slightly on the intake stroke and the earliest possible opening of the intake valve is assured.
This is also a good time to check for when the exhaust valve starts to open. With most engines the exhaust valve will start to open about one quarter to one third of a rotation before bottom dead center (BDC) on the power stroke. If this seems to be way off it is possible the cam gearing may be wrong by several teeth and will have to be readjusted. Timing marks may have been put on by someone other than at the factory, and they might be just plain wrong.
The first thing you need is a tram. This is a very simple tool and can be made from a piece of 1-inch by 2-inch wood with a nail driven through each end. The distance between the nails should be the distance between the front of the hopper/cylinder and the vertical centerline of the flywheel. Nothing fancy needed here, you should be easily able to eyeball this within a half inch, which is good enough. The nails should be sharp and stick through the wood an inch or so.
Next you need to take a few measurements and do a little arithmetic. Wrap a tape measurer around the flywheel and measure its circumference. Divide the circumference by 360 so the distance around the flywheel per degree is known, then multiply the distance per degree by five so the distance for five degrees is known. Finally, divide the circumference by two so the distance halfway around the flywheel is known.
Now set the tram up. Make a center punch mark toward the front of the hopper so that when the nail is in the mark it will swing more or less over the vertical centerline of the flywheel. If a center punch mark bothers you use tape with an 'X' marked on it.
The next step is to establish BDC. Do this by placing a straightedge across the back of the cylinder, carefully moving the flywheel so that the piston just meets it. Swing a mark across the flywheel. This does one side of BDC and the process needs to be repeated with the flywheel coming from the other direction as the back of the bore is probably not exactly BDC. Now repeat the process on the other side of BDC. Mark a point equidistant between the two marks. This is BDC.
Measure half way around the flywheel from the BDC mark and scribe a mark on the flywheel. Do this in both directions and split the difference between the two marks, if any. This is TDC.
Now measure out five degrees in each direction, again scribing a mark. This establishes the timing marks for ignition and exhaust valve closure. Move the flywheel until the mark after TDC is lined up with the tram and adjust the rocker arm screw so that there is a slight pressure on the screw. This sets the exhaust valve. Make sure the engine is on the compression stroke and set the flywheel position to the five degree advance mark. Set the ignition trip at this point by pulling the flywheel through several times so that the tram point matches the trip point marked on the flywheel. You are done.
Tramming may sound a little complicated at first, but it is really quite simple, and after you have waded through it once you can easily do it in a half hour. You might find you need some help with some of the measuring as sometimes it takes three hands, but aside from that it is easy to do
My thanks to Rob Skinner for proofreading for me and also to Joe Prindle who got me straightened out on valve timing. Thanks also to many others of the ATIS Stationary Engine List for their many comments and thoughts on the subject.
Doing the Math:
Tramming Calculation Based on a 22-inch Flywheel
Flywheel Circumference of 59-1/4 inches.
59-1/4/360 = .192 or about 3/16-inch for 1 degree.
5 x 0.192 = .962 or about 31/32-inch for 5 degrees.
59-1/4/2 = 34.625, or 34-5/8 inches - the distance halfway around the flywheel