2-Stroke Ignition Timing

I'll never forget watching a motorcycle reverse into a car at a stop light! The poor rider's bike had stalled; he went through the normal start procedure (check fuel, ignition on, out of gear) and gave the kick start lever a hefty kick. The bike fired up and sounded normal. As the lights changed he selected first gear and, much to his (and the car driver's) surprise, went backwards!

The cause of this problem with 2-strokes is the ignition timing.

If the timing is close to TDC (top-dead-center) it is possible to catch the piston just at the wrong time with the result that the engine runs backwards.

This problem can only happen on a 2-stroke because there are no valves to be operated in a set sequence, as in a 4-stroke. Typically, this problem would happen when the contact points became worn, or more precisely, when the contact point's heel became worn. The net effect of a worn contact point heel is the ignition timing progressively becomes retarded.

Checking the ignition timing on an early motorcycle is best done on a more regular basis than their modern counterparts; monthly if the bike is ridden daily such as a commuter bike. Not only will the potential for running backwards be avoided, but the engine's entire performance range will be optimized too.

Setting Ignition Timing

For the most part, setting 2-stroke ignition timing is simple.

The majority of classic 2-strokes have ignition systems that fall into one of two types: contact points inside a flywheel magneto (Villiers and early Japanese engines) and external contact points mounted on an adjustable plate with an internal flywheel.

Flywheel type ignitions with the contact points mounted internally are the more difficult to set up as the mechanic must complete the task through small inspection and adjustment holes in the flywheel which has magnets around its perimeter.

The difficulty is simply getting a feeler gauge into the contact points without too much interference from the magnets.

Ignition Setting Procedure

To begin the ignition setting sequence, the mechanic should remove the spark plug as this will make it easier to turn the engine over for piston positioning.  Next the crankshaft must be rotated to give the largest opening of the contact points—typically around TDC.  With the points open at their widest, the mechanic should set the required gap. However, if the points are badly pitted (see the article on checking for worn ignition contact points), the mechanic should replace the points—a flywheel extractor will be required for this job.

With the contact points’ gap set, the mechanic can turn his attention to the ignition timing. On all internal combustion engines, the ignition timing is set BTDC. This early ignition of the compressed gases inside the cylinder allows for the time it takes for the ignited gases to reach their fullest pressure.

To find the correct timing position, the mechanic should rotate the flywheel in the normal direction of travel when the engine is running. To find the direction of travel, the mechanic can use the kick-starter, or by rotating the rear wheel with the bike in gear.

(See note one.) After locating TDC, the mechanic should rotate the flywheel backwards (typically around 2.0-mm vertically of the piston) until marks on the flywheel align, this is the timing mark and the point at which the contact points should begin to open.

To get an indication of when the contact points are opening (ignition point) the mechanic can deploy a piece of paper (See note two.) A strip of paper fed between the points’ contact faces should have gentle pulling pressure applied as the flywheel is rotated toward the timing mark. As the points open, the paper will suddenly become loose. If the paper comes out before the flywheel mark for ignition (sometimes marked with an ‘F’ for fire), the internal mounting plate should be moved slightly in the direction of travel.

External Points

On some engines (most of the early Japanese multi-cylinder bikes), the contact points were mounted externally on a plate.

The ignition setting procedure on this type of ignition is the same for the most part as that of the flywheel type. The biggest difference is that the timing marks are located on an internal flywheel; these marks become visible through an inspection window as the engine is rotated.

Notes:

1) Use a plastic drinking straw in the plug hole to ease finding of the piston position. Metallic objects such as screw drivers should not be used for this procedure as it is possible to get them jammed against the plug threads.

2) To allow for the thickness of the paper, the contact points’ gap should be adjusted. For example, if the paper is 0.005” thick, the points’ gap should be reduced accordingly.