Finding the timing of my engine

The timing of the transfer and exhaust ports is a key factor controlling the performance characteristics of a two stroke engine. In order to optimize an engine and to find initial settings for a tuned pipe exhaust system, on must determine the timing of the engine.

The term "Timing" is somewhat misleading, as the values are expressed in degrees and not in seconds. These angles define how long the ports are open, expressed in crankshaft angle. Thus an angle of 150 degrees for a certain port means that the crankshaft turns 150 degrees, from the point when the port is just starting to open until the port is fully closed again.

Using angle units and not time has the advantage, that the numbers are easy to memorize and that they are independent of the actual RPM setting. If you are curious, you can easily convert the crankshaft angle into a time: for a given RPM of say 15'000 1/min the time required for one revolution is 1/15'000 minutes which is 1/250 seconds. The angle of 150 degrees is a fraction of 150/360 of one revolution and the port is thus open for a period of (150/360)*(1/250) seconds which is 0.001667 seconds only.

To determine the timing, two methods are usually used:

• measure the geometric dimensions of the port and the crankshaft, conrod and piston and derive the timing from trigonometric calculations. This method is quite accurate, but you need to dismantle the engine completely. Also a lot of measurements must be performed, which may introduce errors.
• measure the crankshaft angle and watch the port to open and to close while you turn the crankshaft. This method is simple and of sufficient accuracy if you do it carefully.
  In the paragraphs below you find a description of this procedure.

You can attach a disk with divisions in degrees to the crankcase and a pointer to the crankshaft (or vice versa).

Then you adjust disk and pointer so that the it indicates 180° when the piston is at its bottom dead center (BDC). Now you turn the crankshaft until the port of interest closes (when the piston crown arrives at the upper edge of the port). Note the indicated crank angle value. Now turn the shaft from bottom dead center into the opposite direction to the point where the port starts to close again. Take a second reading. Repeat the procedure to make sure that you have got the correct numbers. The timing angle is simply the difference between the two readings. If you have adjusted the disk as described above, both readings must be symmetrical to BDC. Non-symmetrical timing is only possible with controlled valves (e.g. for the carburetor inlet via a window in the crankshaft or on a four stroke engine) or when the cylinder is offset from the axis of symmetry..

You can download templates for disks with divisions in degrees:

in PDF format (clockwise and counter clockwise) DeegreeDisks.pdf

Example:

The animation shows, how to measure the timing angles of a port in the cylinder sleeve.

Reading the angles in both directions must be symmetrical to the bottom dead center, e.g. a port opening at at 100° must be closing again at 360°-100° = 260°, as shown in the animation. The port timing angle (shown in blue) is then 260° - 100° = 160°. This means, that port is open while the the crankshaft turns 160°.
Usually the transfer port will have a shorter timing than the exhaust port.

Some Tips:

• It is helpful to point a flashlight or a halogen lamp through the plug hole or to remove the cylinder head.
• If there is play in the conrod bushings, you should take all readings with the piston pushed downwards.
• For small engines, a Compact Disk makes a nice disk and you can print the divisions on a CD-label.

You should not try to determine the angles by sticking something into the ports. Even a piece of stiff paper may damage liner or piston on small engines.

By M.Hepperle