Whether it is a 2-stroke or 4-stroke engine, whether it is a discontinued or current motorcycle, the ignition timing at which the spark plug ignites the compressed air mixture in the combustion chamber is critical. In the case of 2-stroke engines, ignition timing is often determined based on the height of the piston before the top dead center, and a micrometer headset in the plug hole can be used for this purpose.
It is important to know the correct top dead center position to correctly adjust ignition timing.
The micrometer head is shaped like the frame removed from a micrometer that can measure down to 0.01 mm. This gauge can be used like a micrometer, and turning the thimble one revolution moves the center spindle 1 mm. While the micrometer head synchronizes the movement of the thimble and spindle, this gauge features a spindle that can freely move in and out by loosening the locking screw.
Before setting the spindle on the cylinder head, align the 0 on the thimble with the scale line on the sleeve, and loosen the spindle locking screw to free it. An O-ring is attached to the top of the spindle to prevent it from falling.
In contactless-type engines with no wear elements in the ignition system, such as transistor ignition and CDI ignition, the rotor and stator coils determine ignition timing function without problems as long as they are fixed in the defined positions on the crankshaft and crankcase.
However, in the case of point ignition, which is common in discontinued and old motorcycles, once the contact breaker, which is opened and closed by the crankshaft and camshaft, is attached and removed, the user or maintainer must perform the opening and closing timing. Since the contact breaker is a physical part that opens and closes, the ignition timing may shift due to aging or wear, which must also be corrected.
Whether 2-stroke or 4-stroke, spark plug ignition timing is extremely important. The point of contact breaker must be opened so that the plug sparks at the appropriate time before the piston, which is moving up in the cylinder, reaches the top dead center of compression.
In the case of a 4-stroke engine, if the angle at which the points open = ignition timing is specified as 20 degrees before the top dead center, the T mark on the flywheel or points camshaft indicating top dead center and the F mark indicating ignition timing are relied upon to determine when the points open.
In contrast, in the case of 2-stroke engines, some models have a T or F mark, but some engines do not have such matching marks. In this case, what should be the standard for determining the ignition timing?
The reference point here is the piston position, which determines the ignition timing. Although we have not checked all models, it seems that most Yamaha air-cooled 2-stroke engines, whether single-cylinder or 2-cylinder, have a value of 1.8mm ± 0.15mm before top dead center. In other words, if the contact breaker is positioned so that the point opens when the piston rises from the bottom dead center to 1.8 mm ± 0.15 mm before the top dead center, the ignition timing can be adjusted without any matching marks on the engine or flywheel.
It is clear from this explanation that whether ignition timing is indicated by crankshaft angle or piston position, the reference point is the piston top dead center. This means that it is important to know the exact top dead center position in order to determine the ignition timing. If the engine has a T-mark on the flywheel or point camshaft, it can be used as a reference. On the other hand, if there is no clue to the top dead center, the top dead center position can be determined by measuring the piston movement. This is where a dial gauge or micrometer head type special tool comes in handy.
The cylinder head of an air-cooled 2-stroke engine, which does not have intake and exhaust valves like a 4-stroke engine, is simple in construction and can be directly touched to the piston top by inserting a dial gauge measuring tool or micrometer spindle through the plug hole.
When the gauge is fixed in the plug hole and the crankshaft is rotated, the dial gauge reading increases as the piston approaches top dead center, the needle stops moving at top dead center, and the reading decreases as the piston descends past top dead center, so adjust the dial gauge scale to zero at the point where the needle stops From there, reverse the crankshaft and adjust the position of the contact breaker so that the point opens when it drops to 1.8 mm ± 0.15 mm before top dead center.
- Point 1: To determine ignition timing with a contact breaker on a 2-stroke engine without a T or F mark, utilize the piston height before the top dead center as stated in the service manual or other documents.
Set the points to open when the piston contacts the spindle.
Screw the main unit into the plug hole, turn the crankshaft, and as the piston rises, the spindle sticks out at the top and stops when it reaches the top dead center. When the piston begins to fall past the top dead center, the spindle also falls, so near the top dead center, turn the crankshaft alternately forward and reverse to determine the position where the spindle stops, and tighten the locking screw securely. The zero of the thimble is now the top dead center.
Rotate the crankshaft backward to lower the piston before top dead center, and after lowering the spindle 1.8 mm based on the thimble scale, turn it slowly forward to raise it until the piston touches the spindle. If the locking screw is loose, the spindle will be pushed by the piston and move, so it must be tightened firmly. Even if the screws are tightened firmly, the spindle will move if the crankshaft is turned vigorously, so it is important to raise the spindle carefully.
In contrast to dial gauges, which read the scale on the dial, micrometer head type gauges are characterized by the fact that the operator sets the numerical value. A micrometer head is a measuring tool that removes the frame from a micrometer, which can measure lengths in increments of 0.01 mm, and uses only parts called a sleeve, thimble, and spindle.
The image is an example. The thimble at the top can measure 1 mm in one rotation, and the numbers on the thimble circumference are in increments of 0.1 mm. In other words, if the thimble is rotated clockwise one revolution from the 0 positions to the 3 positions, the center spindle will advance 1.3 mm.
The first step in setting the ignition timing is to screw the gauge body into the plug hole.
Align the scale on the sleeve with the 0 on the thimble, loosen the top locking screw to allow the spindle to move freely up and down, and the spindle tip will stop when it contacts the top of the piston, from which the spindle will gradually rise as the crankshaft is turned slowly. When the piston descends beyond the upper dead center, the spindle also lowers, and the locking screw is tightened when the spindle protrudes to the maximum. Since the scale of the sleeve and thimble are aligned at 0 in advance, the piston top dead center is at scale 0 when the locking screw is tightened.
Once the top dead center is determined, turn the crankshaft backward moderately and turn the thimble of the gauge clockwise one turn to the 8 scale, so that the spindle protrudes 1.8 mm above the top dead center position of the piston. Then, slowly and carefully rotate the crankshaft forward, and when the piston contacts the end of the spindle, the crank position is 1.8 mm before top dead center.
In measurement using a dial gauge, once the scale is set to 0 at top dead center, the piston is reversed while watching the gauge and stopped at 1.8 mm before top dead center to determine the ignition timing. On the other hand, the micrometer head type gauge adjusts the piston to the ignition timing set numerically. The majority of dial gauge types are described in manufacturers' service manuals, but the results obtained are the same for the micrometer head type, which is also used by a minority of manufacturers.
It may seem that a dial gauge that can measure down to 0.01 mm is more accurate, but the micrometer head type, which determines the ignition timing simply by bringing the piston into contact with the positioned spindle, has the advantage of being less prone to errors even if the user is not accustomed to handling dial gauges, whereas a dial gauge whose needle is easily shaken even by slight vibrations has the advantage of being more accurate. The micrometer head type, on the other hand, has the advantage of minimizing errors even if the user is not accustomed to handling dial gauges.
While the popularity of discontinued and old motorcycles continues to grow, maintenance is essential for point ignition in contrast to maintenance-free current motorcycles. There have been mistakes and problems with engines that have become impossible to start because the contact breaker was tampered with out of curiosity. It is difficult to understand completely from the beginning, but it is important to understand that the ignition timing of a two-stroke engine is often determined by the height of the piston, and the actual workflow is to find the top dead center and then measure the piston position before top dead center to determine the ignition timing.
- Point 1: In contrast to dial gauges that read piston position numerically, micrometer head type gauges are used by setting the piston height in advance.
