Ignition Timing 2 Strokes
by Bill Unger
Advance = the amount of angular measurement that the spark happens before the piston reaches top dead center (TDC) on the power stroke. .
Retard is less amount of advance .
Internal combustion engines need to start the combustion fire before the piston reaches TDC because it takes an amount of time to get the gases burning. The spark happens before TDC, this is called BTDC. If you don’t start the gases burning BTDC a lot of the energy will be lost. Obviously making the burn happen at the right time is important in the production of power. .
Many automotive engines have a mechanism to change the ignition timing as the revolutions per minute (RPM) goes up. The faster an engine turns the ideal time of ignition varies (the faster it turns the more advance is needed). Yamaha RDs do not have an ignition advance mechanism. Some automotive engines have mechanical advancers and some have electronic advancers. Your bike has neither. The specification from Yamaha is that the ignition happens at 2.00 MM BTDC. They could have said 19 degrees of crank angle but that is harder to measure. You would need some kind of degree wheel attached to the crank to measure degrees. With a dial gauge you can measure 2.00 MM BTDC, which is a very good substitute for angular measurement. .
So how do I time the bike?????? .
If you have a dial gauge you use it to make sure the ignition marks on the rotor and stator window line up at the time you want ignition to occur. If you do not have a dial gauge, I guess you have to trust Yamaha that they manufactured the bike with some quality controls in place and the marks are at least close to correct. Before you attempt timing you need to make sure the ignition points are set correctly. Why? Because the coils need a minimum amount of time to saturate (gain full charge). The manual suggests a point gap of 0.3mm. Make sure the points are all the way open before measuring and adjusting and make sure they are clean. Wipe them when they are closed with a white business card (you know the kind that a businessman has with his name and address on it). When the points are closed, open the points with your finger and insert the card (.25mm white card stock will work) close the points and slowly remove the card without tearing. Do this until the points leave no mark on the white card stock. Timing the bike you want the points to open at exactly the same time as the timing marks come into alignment. If you have timing light this is easy to do. But in case you don’t have a timing light you can use an ohmmeter to do the same thing. Hook up the meter and watch it and the timing marks at the same time. As the marks come into alignment the needle on the meter should swing from 0 ohms to infinity ohms. Oh you don’t have an ohmmeter?????????? Try this little trick; the ignition switch needs to be turned on for this trick to work. What you do is hook up a small light bulb (12V) to the ignition points as one connection and positive terminal of the battery as the other. The points are closed sending electricity through the condensers to earth, hence the bulb doesn’t light up. When the points open the coil fires and your bulb lights up at the same time. You want your bulb to light up as the timing marks align. Simple? .
Timing…the next frontier. Since you do not have an advance mechanism on your RD you have to chose a number and live with it. 2.00 MM will be OK both top and bottom. You have to decide what is best for you. Be careful about advance. Some gasoline works better than other gasoline. Some like advanced ignition timing(93 octane and more), other gasoline does not like advanced ignition timing(87 octane)..
Hence if somebody says retard your timing they mean make it fire closer to TDC or with less advance.