Setting Valve and Ignition Timing by Tramming


| April/May 2002



Tramming

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.