motorcycle flywheel

Most dirt bikes made before 1980 had points ignition ( but not all ! ) Most ( but not all ! ) of the two-stroke engines had fixed advance. That means there was no variable advance of the ignition timing. Most of the four-stroke engines had a variable advance. Our main concern is that the spark occurs at the right time when the engine is at full or maximum advance. Most four-strokes, namely Hondas, can be timed in the retarded position. But some, like Harleys, need to be timed in the advanced position. Look in your Shop Manual (you do have a Shop Manual, don't you ? ) to find out.

The timing marks are set up a number of different ways. Some have the advance and Top-Dead-Center (TDC) marks on the crankcase and one moving mark on the flywheel. Click on the picture to the right, and you will see two markes on the crankcase and the one on the flywheel. This engine rotates counter-clockwise, so the first mark is the full advance mark, and the second mark is TDC. You want the points to open when the moving mark, exactly matches, the full advance mark. This example is an old Hodaka 100cc engine. If you do not know the direction of rotation simply push on the kickstarter lever.

The example to the left has one mark on the crankcase and all the other markes on the flywheel. This Honda engine (50-70cc) is timed in the full RETARED position. This is because it has a centrifugal spark advance. The " F " mark is when the points open and the spark occurs. The " T " mark is TDC. The two marks on the left are the full advance marks. At 3000 or so RPM the spark will occur between these marks. To check this you must use a timing light with the engine running OR with the engine off, physically turn, and jam in position, the advance unit. Then check it. Most of the Japanese engines I time in the retarded position except for the Yamaha 2-stroke singles. They seem to have weak advance springs, making timing in the retarded position iffy. But that's OK...Yamaha was thoughtful enough to put a little hole in the flywheel which lines up with a hole in the advance unit at full advance (a lot of them, anyway ! ). Just put a small drill into the holes to lock it in full advance! Others have a lever you push to get full advance.

In the picture on the left, you can see " A ", the points and the screw that holds them in, and " B ", the adjusting groove. If you loosen the screw, you can open and close the point gap to set it. It sounds so easy,and it can be... sometimes...and then again, sometimes not. The point gap regulates how much time the coils have to build up the spark. Some systems charge the coil with the points open and some with the points closed. We won't go in to that here. We just want the engine to run good.
Not only must the points open at the right time for the engine, but they must open at the peak of the charging cycle of the coils. As the magnet in the flywheel spins around the ignition stator coils electricity is built up in the ignition coil. When the points open, it disrupts this electric field, causing the spark that fires the fuel/air mixture in the engine. This is usually engineered into the Mag unit and you can't adjust it. If you adjust the points to the right timing, but don't get a spark, it might be because the points cam follower,or heel ( the part of the point that the points cam pushes against ) is worn. Thus causing the points to be out of time with the rotating magnet. The cure is a new set of points. Most of the wear on magneto points is not on the contact points, but on the heel. This is because the voltage across the points is so low, around 1 to 3 volts. Battery ignitions have 12. (yes, some have 6 too) So points that look good can, in reality, be bad.
The spark occurs when the points open. We want them to open when the the piston is in the right position before TDC (BTDC). The mark on the flywheel and crankcase are just tools that allow us to set that opening point-of-time. It is hard to tell when the points open. We need some way to know exactly when they open. I use what is called a "Buzz Box". It makes a buzzing noise that changes tone when the points open. You can also use an ohm meter or a piece of cigarette paper. The paper is actually very accurate. Just put it between the points and pull gently so as not to tear the paper. When the points open the paper will come loose. Remember the timing marks must match when the paper pulls loose.
Another way to set timing is with a dial indicator put down the spark plug hole. Most Shop Manuals give how many thousands of an inch or millimeters BTDC, the piston should be. When the piston is at that spot, say 1.9mm BTDC, the points should open. If you time an engine with a dial indicator there is no need for timing marks, however, if the engine you are working on has no timing marks, you can make your own. Simply make two side by side marks, on the flywheel and crankcase. You can make these marks any where on the flywheel and crankcase but they must match each other when the piston is at the right spot BTDC and the points just opened. Some engines require the removal of the cylinder head to use the dial indicator. more tips